// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
// Page heap.
//
// See malloc.go for overview.
package runtime
import (
"internal/cpu"
"runtime/internal/atomic"
"runtime/internal/sys"
"unsafe"
)
const (
// minPhysPageSize is a lower-bound on the physical page size. The
// true physical page size may be larger than this. In contrast,
// sys.PhysPageSize is an upper-bound on the physical page size.
minPhysPageSize = 4096
// maxPhysPageSize is the maximum page size the runtime supports.
maxPhysPageSize = 512 << 10
// maxPhysHugePageSize sets an upper-bound on the maximum huge page size
// that the runtime supports.
maxPhysHugePageSize = pallocChunkBytes
// pagesPerReclaimerChunk indicates how many pages to scan from the
// pageInUse bitmap at a time. Used by the page reclaimer.
//
// Higher values reduce contention on scanning indexes (such as
// h.reclaimIndex), but increase the minimum latency of the
// operation.
//
// The time required to scan this many pages can vary a lot depending
// on how many spans are actually freed. Experimentally, it can
// scan for pages at ~300 GB/ms on a 2.6GHz Core i7, but can only
// free spans at ~32 MB/ms. Using 512 pages bounds this at
// roughly 100µs.
//
// Must be a multiple of the pageInUse bitmap element size and
// must also evenly divide pagesPerArena.
pagesPerReclaimerChunk = 512
// physPageAlignedStacks indicates whether stack allocations must be
// physical page aligned. This is a requirement for MAP_STACK on
// OpenBSD.
physPageAlignedStacks = GOOS == "openbsd"
)
// Main malloc heap.
// The heap itself is the "free" and "scav" treaps,
// but all the other global data is here too.
//
// mheap must not be heap-allocated because it contains mSpanLists,
// which must not be heap-allocated.
//
//go:notinheap
type mheap struct {
// lock must only be acquired on the system stack, otherwise a g
// could self-deadlock if its stack grows with the lock held.
lock mutex
pages pageAlloc // page allocation data structure
sweepgen uint32 // sweep generation, see comment in mspan; written during STW
sweepdone uint32 // all spans are swept
sweepers uint32 // number of active sweepone calls
// allspans is a slice of all mspans ever created. Each mspan
// appears exactly once.
//
// The memory for allspans is manually managed and can be
// reallocated and move as the heap grows.
//
// In general, allspans is protected by mheap_.lock, which
// prevents concurrent access as well as freeing the backing
// store. Accesses during STW might not hold the lock, but
// must ensure that allocation cannot happen around the
// access (since that may free the backing store).
allspans []*mspan // all spans out there
_ uint32 // align uint64 fields on 32-bit for atomics
// Proportional sweep
//
// These parameters represent a linear function from heap_live
// to page sweep count. The proportional sweep system works to
// stay in the black by keeping the current page sweep count
// above this line at the current heap_live.
//
// The line has slope sweepPagesPerByte and passes through a
// basis point at (sweepHeapLiveBasis, pagesSweptBasis). At
// any given time, the system is at (memstats.heap_live,
// pagesSwept) in this space.
//
// It's important that the line pass through a point we
// control rather than simply starting at a (0,0) origin
// because that lets us adjust sweep pacing at any time while
// accounting for current progress. If we could only adjust
// the slope, it would create a discontinuity in debt if any
// progress has already been made.
pagesInUse uint64 // pages of spans in stats mSpanInUse; updated atomically
pagesSwept uint64 // pages swept this cycle; updated atomically
pagesSweptBasis uint64 // pagesSwept to use as the origin of the sweep ratio; updated atomically
sweepHeapLiveBasis uint64 // value of heap_live to use as the origin of sweep ratio; written with lock, read without
sweepPagesPerByte float64 // proportional sweep ratio; written with lock, read without
// TODO(austin): pagesInUse should be a uintptr, but the 386
// compiler can't 8-byte align fields.
// scavengeGoal is the amount of total retained heap memory (measured by
// heapRetained) that the runtime will try to maintain by returning memory
// to the OS.
scavengeGoal uint64
// Page reclaimer state
// reclaimIndex is the page index in allArenas of next page to
// reclaim. Specifically, it refers to page (i %
// pagesPerArena) of arena allArenas[i / pagesPerArena].
//
// If this is >= 1<<63, the page reclaimer is done scanning
// the page marks.
//
// This is accessed atomically.
reclaimIndex uint64
// reclaimCredit is spare credit for extra pages swept. Since
// the page reclaimer works in large chunks, it may reclaim
// more than requested. Any spare pages released go to this
// credit pool.
//
// This is accessed atomically.
reclaimCredit uintptr
// arenas is the heap arena map. It points to the metadata for
// the heap for every arena frame of the entire usable virtual
// address space.
//
// Use arenaIndex to compute indexes into this array.
//
// For regions of the address space that are not backed by the
// Go heap, the arena map contains nil.
//
// Modifications are protected by mheap_.lock. Reads can be
// performed without locking; however, a given entry can
// transition from nil to non-nil at any time when the lock
// isn't held. (Entries never transitions back to nil.)
//
// In general, this is a two-level mapping consisting of an L1
// map and possibly many L2 maps. This saves space when there
// are a huge number of arena frames. However, on many
// platforms (even 64-bit), arenaL1Bits is 0, making this
// effectively a single-level map. In this case, arenas[0]
// will never be nil.
arenas [1 << arenaL1Bits]*[1 << arenaL2Bits]*heapArena
// heapArenaAlloc is pre-reserved space for allocating heapArena
// objects. This is only used on 32-bit, where we pre-reserve
// this space to avoid interleaving it with the heap itself.
heapArenaAlloc linearAlloc
// arenaHints is a list of addresses at which to attempt to
// add more heap arenas. This is initially populated with a
// set of general hint addresses, and grown with the bounds of
// actual heap arena ranges.
arenaHints *arenaHint
// arena is a pre-reserved space for allocating heap arenas
// (the actual arenas). This is only used on 32-bit.
arena linearAlloc
// allArenas is the arenaIndex of every mapped arena. This can
// be used to iterate through the address space.
//
// Access is protected by mheap_.lock. However, since this is
// append-only and old backing arrays are never freed, it is
// safe to acquire mheap_.lock, copy the slice header, and
// then release mheap_.lock.
allArenas []arenaIdx
// sweepArenas is a snapshot of allArenas taken at the
// beginning of the sweep cycle. This can be read safely by
// simply blocking GC (by disabling preemption).
sweepArenas []arenaIdx
// markArenas is a snapshot of allArenas taken at the beginning
// of the mark cycle. Because allArenas is append-only, neither
// this slice nor its contents will change during the mark, so
// it can be read safely.
markArenas []arenaIdx
// curArena is the arena that the heap is currently growing
// into. This should always be physPageSize-aligned.
curArena struct {
base, end uintptr
}
_ uint32 // ensure 64-bit alignment of central
// central free lists for small size classes.
// the padding makes sure that the mcentrals are
// spaced CacheLinePadSize bytes apart, so that each mcentral.lock
// gets its own cache line.
// central is indexed by spanClass.
central [numSpanClasses]struct {
mcentral mcentral
pad [cpu.CacheLinePadSize - unsafe.Sizeof(mcentral{})%cpu.CacheLinePadSize]byte
}
spanalloc fixalloc // allocator for span*
cachealloc fixalloc // allocator for mcache*
specialfinalizeralloc fixalloc // allocator for specialfinalizer*
specialprofilealloc fixalloc // allocator for specialprofile*
speciallock mutex // lock for special record allocators.
arenaHintAlloc fixalloc // allocator for arenaHints
unused *specialfinalizer // never set, just here to force the specialfinalizer type into DWARF
}
var mheap_ mheap
// A heapArena stores metadata for a heap arena. heapArenas are stored
// outside of the Go heap and accessed via the mheap_.arenas index.
//
//go:notinheap
type heapArena struct {
// bitmap stores the pointer/scalar bitmap for the words in
// this arena. See mbitmap.go for a description. Use the
// heapBits type to access this.
bitmap [heapArenaBitmapBytes]byte
// spans maps from virtual address page ID within this arena to *mspan.
// For allocated spans, their pages map to the span itself.
// For free spans, only the lowest and highest pages map to the span itself.
// Internal pages map to an arbitrary span.
// For pages that have never been allocated, spans entries are nil.
//
// Modifications are protected by mheap.lock. Reads can be
// performed without locking, but ONLY from indexes that are
// known to contain in-use or stack spans. This means there
// must not be a safe-point between establishing that an
// address is live and looking it up in the spans array.
spans [pagesPerArena]*mspan
// pageInUse is a bitmap that indicates which spans are in
// state mSpanInUse. This bitmap is indexed by page number,
// but only the bit corresponding to the first page in each
// span is used.
//
// Reads and writes are atomic.
pageInUse [pagesPerArena / 8]uint8
// pageMarks is a bitmap that indicates which spans have any
// marked objects on them. Like pageInUse, only the bit
// corresponding to the first page in each span is used.
//
// Writes are done atomically during marking. Reads are
// non-atomic and lock-free since they only occur during
// sweeping (and hence never race with writes).
//
// This is used to quickly find whole spans that can be freed.
//
// TODO(austin): It would be nice if this was uint64 for
// faster scanning, but we don't have 64-bit atomic bit
// operations.
pageMarks [pagesPerArena / 8]uint8
// pageSpecials is a bitmap that indicates which spans have
// specials (finalizers or other). Like pageInUse, only the bit
// corresponding to the first page in each span is used.
//
// Writes are done atomically whenever a special is added to
// a span and whenever the last special is removed from a span.
// Reads are done atomically to find spans containing specials
// during marking.
pageSpecials [pagesPerArena / 8]uint8
// checkmarks stores the debug.gccheckmark state. It is only
// used if debug.gccheckmark > 0.
checkmarks *checkmarksMap
// zeroedBase marks the first byte of the first page in this
// arena which hasn't been used yet and is therefore already
// zero. zeroedBase is relative to the arena base.
// Increases monotonically until it hits heapArenaBytes.
//
// This field is sufficient to determine if an allocation
// needs to be zeroed because the page allocator follows an
// address-ordered first-fit policy.
//
// Read atomically and written with an atomic CAS.
zeroedBase uintptr
}
// arenaHint is a hint for where to grow the heap arenas. See
// mheap_.arenaHints.
//
//go:notinheap
type arenaHint struct {
addr uintptr
down bool
next *arenaHint
}
// An mspan is a run of pages.
//
// When a mspan is in the heap free treap, state == mSpanFree
// and heapmap(s->start) == span, heapmap(s->start+s->npages-1) == span.
// If the mspan is in the heap scav treap, then in addition to the
// above scavenged == true. scavenged == false in all other cases.
//
// When a mspan is allocated, state == mSpanInUse or mSpanManual
// and heapmap(i) == span for all s->start <= i < s->start+s->npages.
// Every mspan is in one doubly-linked list, either in the mheap's
// busy list or one of the mcentral's span lists.
// An mspan representing actual memory has state mSpanInUse,
// mSpanManual, or mSpanFree. Transitions between these states are
// constrained as follows:
//
// * A span may transition from free to in-use or manual during any GC
// phase.
//
// * During sweeping (gcphase == _GCoff), a span may transition from
// in-use to free (as a result of sweeping) or manual to free (as a
// result of stacks being freed).
//
// * During GC (gcphase != _GCoff), a span *must not* transition from
// manual or in-use to free. Because concurrent GC may read a pointer
// and then look up its span, the span state must be monotonic.
//
// Setting mspan.state to mSpanInUse or mSpanManual must be done
// atomically and only after all other span fields are valid.
// Likewise, if inspecting a span is contingent on it being
// mSpanInUse, the state should be loaded atomically and checked
// before depending on other fields. This allows the garbage collector
// to safely deal with potentially invalid pointers, since resolving
// such pointers may race with a span being allocated.
type mSpanState uint8
const (
mSpanDead mSpanState = iota
mSpanInUse // allocated for garbage collected heap
mSpanManual // allocated for manual management (e.g., stack allocator)
)
// mSpanStateNames are the names of the span states, indexed by
// mSpanState.
var mSpanStateNames = []string{
"mSpanDead",
"mSpanInUse",
"mSpanManual",
"mSpanFree",
}
// mSpanStateBox holds an mSpanState and provides atomic operations on
// it. This is a separate type to disallow accidental comparison or
// assignment with mSpanState.
type mSpanStateBox struct {
s mSpanState
}
func (b *mSpanStateBox) set(s mSpanState) {
atomic.Store8((*uint8)(&b.s), uint8(s))
}
func (b *mSpanStateBox) get() mSpanState {
return mSpanState(atomic.Load8((*uint8)(&b.s)))
}
// mSpanList heads a linked list of spans.
//
//go:notinheap
type mSpanList struct {
first *mspan // first span in list, or nil if none
last *mspan // last span in list, or nil if none
}
//go:notinheap
type mspan struct {
next *mspan // next span in list, or nil if none
prev *mspan // previous span in list, or nil if none
list *mSpanList // For debugging. TODO: Remove.
startAddr uintptr // address of first byte of span aka s.base()
npages uintptr // number of pages in span
manualFreeList gclinkptr // list of free objects in mSpanManual spans
// freeindex is the slot index between 0 and nelems at which to begin scanning
// for the next free object in this span.
// Each allocation scans allocBits starting at freeindex until it encounters a 0
// indicating a free object. freeindex is then adjusted so that subsequent scans begin
// just past the newly discovered free object.
//
// If freeindex == nelem, this span has no free objects.
//
// allocBits is a bitmap of objects in this span.
// If n >= freeindex and allocBits[n/8] & (1<<(n%8)) is 0
// then object n is free;
// otherwise, object n is allocated. Bits starting at nelem are
// undefined and should never be referenced.
//
// Object n starts at address n*elemsize + (start << pageShift).
freeindex uintptr
// TODO: Look up nelems from sizeclass and remove this field if it
// helps performance.
nelems uintptr // number of object in the span.
// Cache of the allocBits at freeindex. allocCache is shifted
// such that the lowest bit corresponds to the bit freeindex.
// allocCache holds the complement of allocBits, thus allowing
// ctz (count trailing zero) to use it directly.
// allocCache may contain bits beyond s.nelems; the caller must ignore
// these.
allocCache uint64
// allocBits and gcmarkBits hold pointers to a span's mark and
// allocation bits. The pointers are 8 byte aligned.
// There are three arenas where this data is held.
// free: Dirty arenas that are no longer accessed
// and can be reused.
// next: Holds information to be used in the next GC cycle.
// current: Information being used during this GC cycle.
// previous: Information being used during the last GC cycle.
// A new GC cycle starts with the call to finishsweep_m.
// finishsweep_m moves the previous arena to the free arena,
// the current arena to the previous arena, and
// the next arena to the current arena.
// The next arena is populated as the spans request
// memory to hold gcmarkBits for the next GC cycle as well
// as allocBits for newly allocated spans.
//
// The pointer arithmetic is done "by hand" instead of using
// arrays to avoid bounds checks along critical performance
// paths.
// The sweep will free the old allocBits and set allocBits to the
// gcmarkBits. The gcmarkBits are replaced with a fresh zeroed
// out memory.
allocBits *gcBits
gcmarkBits *gcBits
// sweep generation:
// if sweepgen == h->sweepgen - 2, the span needs sweeping
// if sweepgen == h->sweepgen - 1, the span is currently being swept
// if sweepgen == h->sweepgen, the span is swept and ready to use
// if sweepgen == h->sweepgen + 1, the span was cached before sweep began and is still cached, and needs sweeping
// if sweepgen == h->sweepgen + 3, the span was swept and then cached and is still cached
// h->sweepgen is incremented by 2 after every GC
sweepgen uint32
divMul uint16 // for divide by elemsize - divMagic.mul
baseMask uint16 // if non-0, elemsize is a power of 2, & this will get object allocation base
allocCount uint16 // number of allocated objects
spanclass spanClass // size class and noscan (uint8)
state mSpanStateBox // mSpanInUse etc; accessed atomically (get/set methods)
needzero uint8 // needs to be zeroed before allocation
divShift uint8 // for divide by elemsize - divMagic.shift
divShift2 uint8 // for divide by elemsize - divMagic.shift2
elemsize uintptr // computed from sizeclass or from npages
limit uintptr // end of data in span
speciallock mutex // guards specials list
specials *special // linked list of special records sorted by offset.
}
func (s *mspan) base() uintptr {
return s.startAddr
}
func (s *mspan) layout() (size, n, total uintptr) {
total = s.npages << _PageShift
size = s.elemsize
if size > 0 {
n = total / size
}
return
}
// recordspan adds a newly allocated span to h.allspans.
//
// This only happens the first time a span is allocated from
// mheap.spanalloc (it is not called when a span is reused).
//
// Write barriers are disallowed here because it can be called from
// gcWork when allocating new workbufs. However, because it's an
// indirect call from the fixalloc initializer, the compiler can't see
// this.
//
// The heap lock must be held.
//
//go:nowritebarrierrec
func recordspan(vh unsafe.Pointer, p unsafe.Pointer) {
h := (*mheap)(vh)
s := (*mspan)(p)
assertLockHeld(&h.lock)
if len(h.allspans) >= cap(h.allspans) {
n := 64 * 1024 / sys.PtrSize
if n < cap(h.allspans)*3/2 {
n = cap(h.allspans) * 3 / 2
}
var new []*mspan
sp := (*slice)(unsafe.Pointer(&new))
sp.array = sysAlloc(uintptr(n)*sys.PtrSize, &memstats.other_sys)
if sp.array == nil {
throw("runtime: cannot allocate memory")
}
sp.len = len(h.allspans)
sp.cap = n
if len(h.allspans) > 0 {
copy(new, h.allspans)
}
oldAllspans := h.allspans
*(*notInHeapSlice)(unsafe.Pointer(&h.allspans)) = *(*notInHeapSlice)(unsafe.Pointer(&new))
if len(oldAllspans) != 0 {
sysFree(unsafe.Pointer(&oldAllspans[0]), uintptr(cap(oldAllspans))*unsafe.Sizeof(oldAllspans[0]), &memstats.other_sys)
}
}
h.allspans = h.allspans[:len(h.allspans)+1]
h.allspans[len(h.allspans)-1] = s
}
// A spanClass represents the size class and noscan-ness of a span.
//
// Each size class has a noscan spanClass and a scan spanClass. The
// noscan spanClass contains only noscan objects, which do not contain
// pointers and thus do not need to be scanned by the garbage
// collector.
type spanClass uint8
const (
numSpanClasses = _NumSizeClasses << 1
tinySpanClass = spanClass(tinySizeClass<<1 | 1)
)
func makeSpanClass(sizeclass uint8, noscan bool) spanClass {
return spanClass(sizeclass<<1) | spanClass(bool2int(noscan))
}
func (sc spanClass) sizeclass() int8 {
return int8(sc >> 1)
}
func (sc spanClass) noscan() bool {
return sc&1 != 0
}
// arenaIndex returns the index into mheap_.arenas of the arena
// containing metadata for p. This index combines of an index into the
// L1 map and an index into the L2 map and should be used as
// mheap_.arenas[ai.l1()][ai.l2()].
//
// If p is outside the range of valid heap addresses, either l1() or
// l2() will be out of bounds.
//
// It is nosplit because it's called by spanOf and several other
// nosplit functions.
//
//go:nosplit
func arenaIndex(p uintptr) arenaIdx {
return arenaIdx((p - arenaBaseOffset) / heapArenaBytes)
}
// arenaBase returns the low address of the region covered by heap
// arena i.
func arenaBase(i arenaIdx) uintptr {
return uintptr(i)*heapArenaBytes + arenaBaseOffset
}
type arenaIdx uint
func (i arenaIdx) l1() uint {
if arenaL1Bits == 0 {
// Let the compiler optimize this away if there's no
// L1 map.
return 0
} else {
return uint(i) >> arenaL1Shift
}
}
func (i arenaIdx) l2() uint {
if arenaL1Bits == 0 {
return uint(i)
} else {
return uint(i) & (1<<arenaL2Bits - 1)
}
}
// inheap reports whether b is a pointer into a (potentially dead) heap object.
// It returns false for pointers into mSpanManual spans.
// Non-preemptible because it is used by write barriers.
//go:nowritebarrier
//go:nosplit
func inheap(b uintptr) bool {
return spanOfHeap(b) != nil
}
// inHeapOrStack is a variant of inheap that returns true for pointers
// into any allocated heap span.
//
//go:nowritebarrier
//go:nosplit
func inHeapOrStack(b uintptr) bool {
s := spanOf(b)
if s == nil || b < s.base() {
return false
}
switch s.state.get() {
case mSpanInUse, mSpanManual:
return b < s.limit
default:
return false
}
}
// spanOf returns the span of p. If p does not point into the heap
// arena or no span has ever contained p, spanOf returns nil.
//
// If p does not point to allocated memory, this may return a non-nil
// span that does *not* contain p. If this is a possibility, the
// caller should either call spanOfHeap or check the span bounds
// explicitly.
//
// Must be nosplit because it has callers that are nosplit.
//
//go:nosplit
func spanOf(p uintptr) *mspan {
// This function looks big, but we use a lot of constant
// folding around arenaL1Bits to get it under the inlining
// budget. Also, many of the checks here are safety checks
// that Go needs to do anyway, so the generated code is quite
// short.
ri := arenaIndex(p)
if arenaL1Bits == 0 {
// If there's no L1, then ri.l1() can't be out of bounds but ri.l2() can.
if ri.l2() >= uint(len(mheap_.arenas[0])) {
return nil
}
} else {
// If there's an L1, then ri.l1() can be out of bounds but ri.l2() can't.
if ri.l1() >= uint(len(mheap_.arenas)) {
return nil
}
}
l2 := mheap_.arenas[ri.l1()]
if arenaL1Bits != 0 && l2 == nil { // Should never happen if there's no L1.
return nil
}
ha := l2[ri.l2()]
if ha == nil {
return nil
}
return ha.spans[(p/pageSize)%pagesPerArena]
}
// spanOfUnchecked is equivalent to spanOf, but the caller must ensure
// that p points into an allocated heap arena.
//
// Must be nosplit because it has callers that are nosplit.
//
//go:nosplit
func spanOfUnchecked(p uintptr) *mspan {
ai := arenaIndex(p)
return mheap_.arenas[ai.l1()][ai.l2()].spans[(p/pageSize)%pagesPerArena]
}
// spanOfHeap is like spanOf, but returns nil if p does not point to a
// heap object.
//
// Must be nosplit because it has callers that are nosplit.
//
//go:nosplit
func spanOfHeap(p uintptr) *mspan {
s := spanOf(p)
// s is nil if it's never been allocated. Otherwise, we check
// its state first because we don't trust this pointer, so we
// have to synchronize with span initialization. Then, it's
// still possible we picked up a stale span pointer, so we
// have to check the span's bounds.
if s == nil || s.state.get() != mSpanInUse || p < s.base() || p >= s.limit {
return nil
}
return s
}
// pageIndexOf returns the arena, page index, and page mask for pointer p.
// The caller must ensure p is in the heap.
func pageIndexOf(p uintptr) (arena *heapArena, pageIdx uintptr, pageMask uint8) {
ai := arenaIndex(p)
arena = mheap_.arenas[ai.l1()][ai.l2()]
pageIdx = ((p / pageSize) / 8) % uintptr(len(arena.pageInUse))
pageMask = byte(1 << ((p / pageSize) % 8))
return
}
// Initialize the heap.
func (h *mheap) init() {
lockInit(&h.lock, lockRankMheap)
lockInit(&h.speciallock, lockRankMheapSpecial)
h.spanalloc.init(unsafe.Sizeof(mspan{}), recordspan, unsafe.Pointer(h), &memstats.mspan_sys)
h.cachealloc.init(unsafe.Sizeof(mcache{}), nil, nil, &memstats.mcache_sys)
h.specialfinalizeralloc.init(unsafe.Sizeof(specialfinalizer{}), nil, nil, &memstats.other_sys)
h.specialprofilealloc.init(unsafe.Sizeof(specialprofile{}), nil, nil, &memstats.other_sys)
h.arenaHintAlloc.init(unsafe.Sizeof(arenaHint{}), nil, nil, &memstats.other_sys)
// Don't zero mspan allocations. Background sweeping can
// inspect a span concurrently with allocating it, so it's
// important that the span's sweepgen survive across freeing
// and re-allocating a span to prevent background sweeping
// from improperly cas'ing it from 0.
//
// This is safe because mspan contains no heap pointers.
h.spanalloc.zero = false
// h->mapcache needs no init
for i := range h.central {
h.central[i].mcentral.init(spanClass(i))
}
h.pages.init(&h.lock, &memstats.gcMiscSys)
}
// reclaim sweeps and reclaims at least npage pages into the heap.
// It is called before allocating npage pages to keep growth in check.
//
// reclaim implements the page-reclaimer half of the sweeper.
//
// h.lock must NOT be held.
func (h *mheap) reclaim(npage uintptr) {
// TODO(austin): Half of the time spent freeing spans is in
// locking/unlocking the heap (even with low contention). We
// could make the slow path here several times faster by
// batching heap frees.
// Bail early if there's no more reclaim work.
if atomic.Load64(&h.reclaimIndex) >= 1<<63 {
return
}
// Disable preemption so the GC can't start while we're
// sweeping, so we can read h.sweepArenas, and so
// traceGCSweepStart/Done pair on the P.
mp := acquirem()
if trace.enabled {
traceGCSweepStart()
}
arenas := h.sweepArenas
locked := false
for npage > 0 {
// Pull from accumulated credit first.
if credit := atomic.Loaduintptr(&h.reclaimCredit); credit > 0 {
take := credit
if take > npage {
// Take only what we need.
take = npage
}
if atomic.Casuintptr(&h.reclaimCredit, credit, credit-take) {
npage -= take
}
continue
}
// Claim a chunk of work.
idx := uintptr(atomic.Xadd64(&h.reclaimIndex, pagesPerReclaimerChunk) - pagesPerReclaimerChunk)
if idx/pagesPerArena >= uintptr(len(arenas)) {
// Page reclaiming is done.
atomic.Store64(&h.reclaimIndex, 1<<63)
break
}
if !locked {
// Lock the heap for reclaimChunk.
lock(&h.lock)
locked = true
}
// Scan this chunk.
nfound := h.reclaimChunk(arenas, idx, pagesPerReclaimerChunk)
if nfound <= npage {
npage -= nfound
} else {
// Put spare pages toward global credit.
atomic.Xadduintptr(&h.reclaimCredit, nfound-npage)
npage = 0
}
}
if locked {
unlock(&h.lock)
}
if trace.enabled {
traceGCSweepDone()
}
releasem(mp)
}
// reclaimChunk sweeps unmarked spans that start at page indexes [pageIdx, pageIdx+n).
// It returns the number of pages returned to the heap.
//
// h.lock must be held and the caller must be non-preemptible. Note: h.lock may be
// temporarily unlocked and re-locked in order to do sweeping or if tracing is
// enabled.
func (h *mheap) reclaimChunk(arenas []arenaIdx, pageIdx, n uintptr) uintptr {
// The heap lock must be held because this accesses the
// heapArena.spans arrays using potentially non-live pointers.
// In particular, if a span were freed and merged concurrently
// with this probing heapArena.spans, it would be possible to
// observe arbitrary, stale span pointers.
assertLockHeld(&h.lock)
n0 := n
var nFreed uintptr
sg := h.sweepgen
for n > 0 {
ai := arenas[pageIdx/pagesPerArena]
ha := h.arenas[ai.l1()][ai.l2()]
// Get a chunk of the bitmap to work on.
arenaPage := uint(pageIdx % pagesPerArena)
inUse := ha.pageInUse[arenaPage/8:]
marked := ha.pageMarks[arenaPage/8:]
if uintptr(len(inUse)) > n/8 {
inUse = inUse[:n/8]
marked = marked[:n/8]
}
// Scan this bitmap chunk for spans that are in-use
// but have no marked objects on them.
for i := range inUse {
inUseUnmarked := atomic.Load8(&inUse[i]) &^ marked[i]
if inUseUnmarked == 0 {
continue
}
for j := uint(0); j < 8; j++ {
if inUseUnmarked&(1<<j) != 0 {
s := ha.spans[arenaPage+uint(i)*8+j]
if atomic.Load(&s.sweepgen) == sg-2 && atomic.Cas(&s.sweepgen, sg-2, sg-1) {
npages := s.npages
unlock(&h.lock)
if s.sweep(false) {
nFreed += npages
}
lock(&h.lock)
// Reload inUse. It's possible nearby
// spans were freed when we dropped the
// lock and we don't want to get stale
// pointers from the spans array.
inUseUnmarked = atomic.Load8(&inUse[i]) &^ marked[i]
}
}
}
}
// Advance.
pageIdx += uintptr(len(inUse) * 8)
n -= uintptr(len(inUse) * 8)
}
if trace.enabled {
unlock(&h.lock)
// Account for pages scanned but not reclaimed.
traceGCSweepSpan((n0 - nFreed) * pageSize)
lock(&h.lock)
}
assertLockHeld(&h.lock) // Must be locked on return.
return nFreed
}
// spanAllocType represents the type of allocation to make, or
// the type of allocation to be freed.
type spanAllocType uint8
const (
spanAllocHeap spanAllocType = iota // heap span
spanAllocStack // stack span
spanAllocPtrScalarBits // unrolled GC prog bitmap span
spanAllocWorkBuf // work buf span
)
// manual returns true if the span allocation is manually managed.
func (s spanAllocType) manual() bool {
return s != spanAllocHeap
}
// alloc allocates a new span of npage pages from the GC'd heap.
//
// spanclass indicates the span's size class and scannability.
//
// If needzero is true, the memory for the returned span will be zeroed.
func (h *mheap) alloc(npages uintptr, spanclass spanClass, needzero bool) *mspan {
// Don't do any operations that lock the heap on the G stack.
// It might trigger stack growth, and the stack growth code needs
// to be able to allocate heap.
var s *mspan
systemstack(func() {
// To prevent excessive heap growth, before allocating n pages
// we need to sweep and reclaim at least n pages.
if h.sweepdone == 0 {
h.reclaim(npages)
}
s = h.allocSpan(npages, spanAllocHeap, spanclass)
})
if s != nil {
if needzero && s.needzero != 0 {
memclrNoHeapPointers(unsafe.Pointer(s.base()), s.npages<<_PageShift)
}
s.needzero = 0
}
return s
}
// allocManual allocates a manually-managed span of npage pages.
// allocManual returns nil if allocation fails.
//
// allocManual adds the bytes used to *stat, which should be a
// memstats in-use field. Unlike allocations in the GC'd heap, the
// allocation does *not* count toward heap_inuse or heap_sys.
//
// The memory backing the returned span may not be zeroed if
// span.needzero is set.
//
// allocManual must be called on the system stack because it may
// acquire the heap lock via allocSpan. See mheap for details.
//
// If new code is written to call allocManual, do NOT use an
// existing spanAllocType value and instead declare a new one.
//
//go:systemstack
func (h *mheap) allocManual(npages uintptr, typ spanAllocType) *mspan {
if !typ.manual() {
throw("manual span allocation called with non-manually-managed type")
}
return h.allocSpan(npages, typ, 0)
}
// setSpans modifies the span map so [spanOf(base), spanOf(base+npage*pageSize))
// is s.
func (h *mheap) setSpans(base, npage uintptr, s *mspan) {
p := base / pageSize
ai := arenaIndex(base)
ha := h.arenas[ai.l1()][ai.l2()]
for n := uintptr(0); n < npage; n++ {
i := (p + n) % pagesPerArena
if i == 0 {
ai = arenaIndex(base + n*pageSize)
ha = h.arenas[ai.l1()][ai.l2()]
}
ha.spans[i] = s
}
}
// allocNeedsZero checks if the region of address space [base, base+npage*pageSize),
// assumed to be allocated, needs to be zeroed, updating heap arena metadata for
// future allocations.
//
// This must be called each time pages are allocated from the heap, even if the page
// allocator can otherwise prove the memory it's allocating is already zero because
// they're fresh from the operating system. It updates heapArena metadata that is
// critical for future page allocations.
//
// There are no locking constraints on this method.
func (h *mheap) allocNeedsZero(base, npage uintptr) (needZero bool) {
for npage > 0 {
ai := arenaIndex(base)
ha := h.arenas[ai.l1()][ai.l2()]
zeroedBase := atomic.Loaduintptr(&ha.zeroedBase)
arenaBase := base % heapArenaBytes
if arenaBase < zeroedBase {
// We extended into the non-zeroed part of the
// arena, so this region needs to be zeroed before use.
//
// zeroedBase is monotonically increasing, so if we see this now then
// we can be sure we need to zero this memory region.
//
// We still need to update zeroedBase for this arena, and
// potentially more arenas.
needZero = true
}
// We may observe arenaBase > zeroedBase if we're racing with one or more
// allocations which are acquiring memory directly before us in the address
// space. But, because we know no one else is acquiring *this* memory, it's
// still safe to not zero.
// Compute how far into the arena we extend into, capped
// at heapArenaBytes.
arenaLimit := arenaBase + npage*pageSize
if arenaLimit > heapArenaBytes {
arenaLimit = heapArenaBytes
}
// Increase ha.zeroedBase so it's >= arenaLimit.
// We may be racing with other updates.
for arenaLimit > zeroedBase {
if atomic.Casuintptr(&ha.zeroedBase, zeroedBase, arenaLimit) {
break
}
zeroedBase = atomic.Loaduintptr(&ha.zeroedBase)
// Sanity check zeroedBase.
if zeroedBase <= arenaLimit && zeroedBase > arenaBase {
// The zeroedBase moved into the space we were trying to
// claim. That's very bad, and indicates someone allocated
// the same region we did.
throw("potentially overlapping in-use allocations detected")
}
}
// Move base forward and subtract from npage to move into
// the next arena, or finish.
base += arenaLimit - arenaBase
npage -= (arenaLimit - arenaBase) / pageSize
}
return
}
// tryAllocMSpan attempts to allocate an mspan object from
// the P-local cache, but may fail.
//
// h.lock need not be held.
//
// This caller must ensure that its P won't change underneath
// it during this function. Currently to ensure that we enforce
// that the function is run on the system stack, because that's
// the only place it is used now. In the future, this requirement
// may be relaxed if its use is necessary elsewhere.
//
//go:systemstack
func (h *mheap) tryAllocMSpan() *mspan {
pp := getg().m.p.ptr()
// If we don't have a p or the cache is empty, we can't do
// anything here.
if pp == nil || pp.mspancache.len == 0 {
return nil
}
// Pull off the last entry in the cache.
s := pp.mspancache.buf[pp.mspancache.len-1]
pp.mspancache.len--
return s
}
// allocMSpanLocked allocates an mspan object.
//
// h.lock must be held.
//
// allocMSpanLocked must be called on the system stack because
// its caller holds the heap lock. See mheap for details.
// Running on the system stack also ensures that we won't
// switch Ps during this function. See tryAllocMSpan for details.
//
//go:systemstack
func (h *mheap) allocMSpanLocked() *mspan {
assertLockHeld(&h.lock)
pp := getg().m.p.ptr()
if pp == nil {
// We don't have a p so just do the normal thing.
return (*mspan)(h.spanalloc.alloc())
}
// Refill the cache if necessary.
if pp.mspancache.len == 0 {
const refillCount = len(pp.mspancache.buf) / 2
for i := 0; i < refillCount; i++ {
pp.mspancache.buf[i] = (*mspan)(h.spanalloc.alloc())
}
pp.mspancache.len = refillCount
}
// Pull off the last entry in the cache.
s := pp.mspancache.buf[pp.mspancache.len-1]
pp.mspancache.len--
return s
}
// freeMSpanLocked free an mspan object.
//
// h.lock must be held.
//
// freeMSpanLocked must be called on the system stack because
// its caller holds the heap lock. See mheap for details.
// Running on the system stack also ensures that we won't
// switch Ps during this function. See tryAllocMSpan for details.
//
//go:systemstack
func (h *mheap) freeMSpanLocked(s *mspan) {
assertLockHeld(&h.lock)
pp := getg().m.p.ptr()
// First try to free the mspan directly to the cache.
if pp != nil && pp.mspancache.len < len(pp.mspancache.buf) {
pp.mspancache.buf[pp.mspancache.len] = s
pp.mspancache.len++
return
}
// Failing that (or if we don't have a p), just free it to
// the heap.
h.spanalloc.free(unsafe.Pointer(s))
}
// allocSpan allocates an mspan which owns npages worth of memory.
//
// If typ.manual() == false, allocSpan allocates a heap span of class spanclass
// and updates heap accounting. If manual == true, allocSpan allocates a
// manually-managed span (spanclass is ignored), and the caller is
// responsible for any accounting related to its use of the span. Either
// way, allocSpan will atomically add the bytes in the newly allocated
// span to *sysStat.
//
// The returned span is fully initialized.
//
// h.lock must not be held.
//
// allocSpan must be called on the system stack both because it acquires
// the heap lock and because it must block GC transitions.
//
//go:systemstack
func (h *mheap) allocSpan(npages uintptr, typ spanAllocType, spanclass spanClass) (s *mspan) {
// Function-global state.
gp := getg()
base, scav := uintptr(0), uintptr(0)
// On some platforms we need to provide physical page aligned stack
// allocations. Where the page size is less than the physical page
// size, we already manage to do this by default.
needPhysPageAlign := physPageAlignedStacks && typ == spanAllocStack && pageSize < physPageSize
// If the allocation is small enough, try the page cache!
// The page cache does not support aligned allocations, so we cannot use
// it if we need to provide a physical page aligned stack allocation.
pp := gp.m.p.ptr()
if !needPhysPageAlign && pp != nil && npages < pageCachePages/4 {
c := &pp.pcache
// If the cache is empty, refill it.
if c.empty() {
lock(&h.lock)
*c = h.pages.allocToCache()
unlock(&h.lock)
}
// Try to allocate from the cache.
base, scav = c.alloc(npages)
if base != 0 {
s = h.tryAllocMSpan()
if s != nil {
goto HaveSpan
}
// We have a base but no mspan, so we need
// to lock the heap.
}
}
// For one reason or another, we couldn't get the
// whole job done without the heap lock.
lock(&h.lock)
if needPhysPageAlign {
// Overallocate by a physical page to allow for later alignment.
npages += physPageSize / pageSize
}
if base == 0 {
// Try to acquire a base address.
base, scav = h.pages.alloc(npages)
if base == 0 {
if !h.grow(npages) {
unlock(&h.lock)
return nil
}
base, scav = h.pages.alloc(npages)
if base == 0 {
throw("grew heap, but no adequate free space found")
}
}
}
if s == nil {
// We failed to get an mspan earlier, so grab
// one now that we have the heap lock.
s = h.allocMSpanLocked()
}
if needPhysPageAlign {
allocBase, allocPages := base, npages
base = alignUp(allocBase, physPageSize)
npages -= physPageSize / pageSize
// Return memory around the aligned allocation.
spaceBefore := base - allocBase
if spaceBefore > 0 {
h.pages.free(allocBase, spaceBefore/pageSize)
}
spaceAfter := (allocPages-npages)*pageSize - spaceBefore
if spaceAfter > 0 {
h.pages.free(base+npages*pageSize, spaceAfter/pageSize)
}
}
unlock(&h.lock)
HaveSpan:
// At this point, both s != nil and base != 0, and the heap
// lock is no longer held. Initialize the span.
s.init(base, npages)
if h.allocNeedsZero(base, npages) {
s.needzero = 1
}
nbytes := npages * pageSize
if typ.manual() {
s.manualFreeList = 0
s.nelems = 0
s.limit = s.base() + s.npages*pageSize
s.state.set(mSpanManual)
} else {
// We must set span properties before the span is published anywhere
// since we're not holding the heap lock.
s.spanclass = spanclass
if sizeclass := spanclass.sizeclass(); sizeclass == 0 {
s.elemsize = nbytes
s.nelems = 1
s.divShift = 0
s.divMul = 0
s.divShift2 = 0
s.baseMask = 0
} else {
s.elemsize = uintptr(class_to_size[sizeclass])
s.nelems = nbytes / s.elemsize
m := &class_to_divmagic[sizeclass]
s.divShift = m.shift
s.divMul = m.mul
s.divShift2 = m.shift2
s.baseMask = m.baseMask
}
// Initialize mark and allocation structures.
s.freeindex = 0
s.allocCache = ^uint64(0) // all 1s indicating all free.
s.gcmarkBits = newMarkBits(s.nelems)
s.allocBits = newAllocBits(s.nelems)
// It's safe to access h.sweepgen without the heap lock because it's
// only ever updated with the world stopped and we run on the
// systemstack which blocks a STW transition.
atomic.Store(&s.sweepgen, h.sweepgen)
// Now that the span is filled in, set its state. This
// is a publication barrier for the other fields in
// the span. While valid pointers into this span
// should never be visible until the span is returned,
// if the garbage collector finds an invalid pointer,
// access to the span may race with initialization of
// the span. We resolve this race by atomically
// setting the state after the span is fully
// initialized, and atomically checking the state in
// any situation where a pointer is suspect.
s.state.set(mSpanInUse)
}
// Commit and account for any scavenged memory that the span now owns.
if scav != 0 {
// sysUsed all the pages that are actually available
// in the span since some of them might be scavenged.
sysUsed(unsafe.Pointer(base), nbytes)
atomic.Xadd64(&memstats.heap_released, -int64(scav))
}
// Update stats.
if typ == spanAllocHeap {
atomic.Xadd64(&memstats.heap_inuse, int64(nbytes))
}
if typ.manual() {
// Manually managed memory doesn't count toward heap_sys.
memstats.heap_sys.add(-int64(nbytes))
}
// Update consistent stats.
stats := memstats.heapStats.acquire()
atomic.Xaddint64(&stats.committed, int64(scav))
atomic.Xaddint64(&stats.released, -int64(scav))
switch typ {
case spanAllocHeap:
atomic.Xaddint64(&stats.inHeap, int64(nbytes))
case spanAllocStack:
atomic.Xaddint64(&stats.inStacks, int64(nbytes))
case spanAllocPtrScalarBits:
atomic.Xaddint64(&stats.inPtrScalarBits, int64(nbytes))
case spanAllocWorkBuf:
atomic.Xaddint64(&stats.inWorkBufs, int64(nbytes))
}
memstats.heapStats.release()
// Publish the span in various locations.
// This is safe to call without the lock held because the slots
// related to this span will only ever be read or modified by
// this thread until pointers into the span are published (and
// we execute a publication barrier at the end of this function
// before that happens) or pageInUse is updated.
h.setSpans(s.base(), npages, s)
if !typ.manual() {
// Mark in-use span in arena page bitmap.
//
// This publishes the span to the page sweeper, so
// it's imperative that the span be completely initialized
// prior to this line.
arena, pageIdx, pageMask := pageIndexOf(s.base())
atomic.Or8(&arena.pageInUse[pageIdx], pageMask)
// Update related page sweeper stats.
atomic.Xadd64(&h.pagesInUse, int64(npages))
}
// Make sure the newly allocated span will be observed
// by the GC before pointers into the span are published.
publicationBarrier()
return s
}
// Try to add at least npage pages of memory to the heap,
// returning whether it worked.
//
// h.lock must be held.
func (h *mheap) grow(npage uintptr) bool {
assertLockHeld(&h.lock)
// We must grow the heap in whole palloc chunks.
ask := alignUp(npage, pallocChunkPages) * pageSize
totalGrowth := uintptr(0)
// This may overflow because ask could be very large
// and is otherwise unrelated to h.curArena.base.
end := h.curArena.base + ask
nBase := alignUp(end, physPageSize)
if nBase > h.curArena.end || /* overflow */ end < h.curArena.base {
// Not enough room in the current arena. Allocate more
// arena space. This may not be contiguous with the
// current arena, so we have to request the full ask.
av, asize := h.sysAlloc(ask)
if av == nil {
print("runtime: out of memory: cannot allocate ", ask, "-byte block (", memstats.heap_sys, " in use)\n")
return false
}
if uintptr(av) == h.curArena.end {
// The new space is contiguous with the old
// space, so just extend the current space.
h.curArena.end = uintptr(av) + asize
} else {
// The new space is discontiguous. Track what
// remains of the current space and switch to
// the new space. This should be rare.
if size := h.curArena.end - h.curArena.base; size != 0 {
h.pages.grow(h.curArena.base, size)
totalGrowth += size
}
// Switch to the new space.
h.curArena.base = uintptr(av)
h.curArena.end = uintptr(av) + asize
}
// The memory just allocated counts as both released
// and idle, even though it's not yet backed by spans.
//
// The allocation is always aligned to the heap arena
// size which is always > physPageSize, so its safe to
// just add directly to heap_released.
atomic.Xadd64(&memstats.heap_released, int64(asize))
stats := memstats.heapStats.acquire()
atomic.Xaddint64(&stats.released, int64(asize))
memstats.heapStats.release()
// Recalculate nBase.
// We know this won't overflow, because sysAlloc returned
// a valid region starting at h.curArena.base which is at
// least ask bytes in size.
nBase = alignUp(h.curArena.base+ask, physPageSize)
}
// Grow into the current arena.
v := h.curArena.base
h.curArena.base = nBase
h.pages.grow(v, nBase-v)
totalGrowth += nBase - v
// We just caused a heap growth, so scavenge down what will soon be used.
// By scavenging inline we deal with the failure to allocate out of
// memory fragments by scavenging the memory fragments that are least
// likely to be re-used.
if retained := heapRetained(); retained+uint64(totalGrowth) > h.scavengeGoal {
todo := totalGrowth
if overage := uintptr(retained + uint64(totalGrowth) - h.scavengeGoal); todo > overage {
todo = overage
}
h.pages.scavenge(todo, false)
}
return true
}
// Free the span back into the heap.
func (h *mheap) freeSpan(s *mspan) {
systemstack(func() {
lock(&h.lock)
if msanenabled {
// Tell msan that this entire span is no longer in use.
base := unsafe.Pointer(s.base())
bytes := s.npages << _PageShift
msanfree(base, bytes)
}
h.freeSpanLocked(s, spanAllocHeap)
unlock(&h.lock)
})
}
// freeManual frees a manually-managed span returned by allocManual.
// typ must be the same as the spanAllocType passed to the allocManual that
// allocated s.
//
// This must only be called when gcphase == _GCoff. See mSpanState for
// an explanation.
//
// freeManual must be called on the system stack because it acquires
// the heap lock. See mheap for details.
//
//go:systemstack
func (h *mheap) freeManual(s *mspan, typ spanAllocType) {
s.needzero = 1
lock(&h.lock)
h.freeSpanLocked(s, typ)
unlock(&h.lock)
}
func (h *mheap) freeSpanLocked(s *mspan, typ spanAllocType) {
assertLockHeld(&h.lock)
switch s.state.get() {
case mSpanManual:
if s.allocCount != 0 {
throw("mheap.freeSpanLocked - invalid stack free")
}
case mSpanInUse:
if s.allocCount != 0 || s.sweepgen != h.sweepgen {
print("mheap.freeSpanLocked - span ", s, " ptr ", hex(s.base()), " allocCount ", s.allocCount, " sweepgen ", s.sweepgen, "/", h.sweepgen, "\n")
throw("mheap.freeSpanLocked - invalid free")
}
atomic.Xadd64(&h.pagesInUse, -int64(s.npages))
// Clear in-use bit in arena page bitmap.
arena, pageIdx, pageMask := pageIndexOf(s.base())
atomic.And8(&arena.pageInUse[pageIdx], ^pageMask)
default:
throw("mheap.freeSpanLocked - invalid span state")
}
// Update stats.
//
// Mirrors the code in allocSpan.
nbytes := s.npages * pageSize
if typ == spanAllocHeap {
atomic.Xadd64(&memstats.heap_inuse, -int64(nbytes))
}
if typ.manual() {
// Manually managed memory doesn't count toward heap_sys, so add it back.
memstats.heap_sys.add(int64(nbytes))
}
// Update consistent stats.
stats := memstats.heapStats.acquire()
switch typ {
case spanAllocHeap:
atomic.Xaddint64(&stats.inHeap, -int64(nbytes))
case spanAllocStack:
atomic.Xaddint64(&stats.inStacks, -int64(nbytes))
case spanAllocPtrScalarBits:
atomic.Xaddint64(&stats.inPtrScalarBits, -int64(nbytes))
case spanAllocWorkBuf:
atomic.Xaddint64(&stats.inWorkBufs, -int64(nbytes))
}
memstats.heapStats.release()
// Mark the space as free.
h.pages.free(s.base(), s.npages)
// Free the span structure. We no longer have a use for it.
s.state.set(mSpanDead)
h.freeMSpanLocked(s)
}
// scavengeAll acquires the heap lock (blocking any additional
// manipulation of the page allocator) and iterates over the whole
// heap, scavenging every free page available.
func (h *mheap) scavengeAll() {
// Disallow malloc or panic while holding the heap lock. We do
// this here because this is a non-mallocgc entry-point to
// the mheap API.
gp := getg()
gp.m.mallocing++
lock(&h.lock)
// Start a new scavenge generation so we have a chance to walk
// over the whole heap.
h.pages.scavengeStartGen()
released := h.pages.scavenge(^uintptr(0), false)
gen := h.pages.scav.gen
unlock(&h.lock)
gp.m.mallocing--
if debug.scavtrace > 0 {
printScavTrace(gen, released, true)
}
}
//go:linkname runtime_debug_freeOSMemory runtime/debug.freeOSMemory
func runtime_debug_freeOSMemory() {
GC()
systemstack(func() { mheap_.scavengeAll() })
}
// Initialize a new span with the given start and npages.
func (span *mspan) init(base uintptr, npages uintptr) {
// span is *not* zeroed.
span.next = nil
span.prev = nil
span.list = nil
span.startAddr = base
span.npages = npages
span.allocCount = 0
span.spanclass = 0
span.elemsize = 0
span.speciallock.key = 0
span.specials = nil
span.needzero = 0
span.freeindex = 0
span.allocBits = nil
span.gcmarkBits = nil
span.state.set(mSpanDead)
lockInit(&span.speciallock, lockRankMspanSpecial)
}
func (span *mspan) inList() bool {
return span.list != nil
}
// Initialize an empty doubly-linked list.
func (list *mSpanList) init() {
list.first = nil
list.last = nil
}
func (list *mSpanList) remove(span *mspan) {
if span.list != list {
print("runtime: failed mSpanList.remove span.npages=", span.npages,
" span=", span, " prev=", span.prev, " span.list=", span.list, " list=", list, "\n")
throw("mSpanList.remove")
}
if list.first == span {
list.first = span.next
} else {
span.prev.next = span.next
}
if list.last == span {
list.last = span.prev
} else {
span.next.prev = span.prev
}
span.next = nil
span.prev = nil
span.list = nil
}
func (list *mSpanList) isEmpty() bool {
return list.first == nil
}
func (list *mSpanList) insert(span *mspan) {
if span.next != nil || span.prev != nil || span.list != nil {
println("runtime: failed mSpanList.insert", span, span.next, span.prev, span.list)
throw("mSpanList.insert")
}
span.next = list.first
if list.first != nil {
// The list contains at least one span; link it in.
// The last span in the list doesn't change.
list.first.prev = span
} else {
// The list contains no spans, so this is also the last span.
list.last = span
}
list.first = span
span.list = list
}
func (list *mSpanList) insertBack(span *mspan) {
if span.next != nil || span.prev != nil || span.list != nil {
println("runtime: failed mSpanList.insertBack", span, span.next, span.prev, span.list)
throw("mSpanList.insertBack")
}
span.prev = list.last
if list.last != nil {
// The list contains at least one span.
list.last.next = span
} else {
// The list contains no spans, so this is also the first span.
list.first = span
}
list.last = span
span.list = list
}
// takeAll removes all spans from other and inserts them at the front
// of list.
func (list *mSpanList) takeAll(other *mSpanList) {
if other.isEmpty() {
return
}
// Reparent everything in other to list.
for s := other.first; s != nil; s = s.next {
s.list = list
}
// Concatenate the lists.
if list.isEmpty() {
*list = *other
} else {
// Neither list is empty. Put other before list.
other.last.next = list.first
list.first.prev = other.last
list.first = other.first
}
other.first, other.last = nil, nil
}
const (
_KindSpecialFinalizer = 1
_KindSpecialProfile = 2
// Note: The finalizer special must be first because if we're freeing
// an object, a finalizer special will cause the freeing operation
// to abort, and we want to keep the other special records around
// if that happens.
)
//go:notinheap
type special struct {
next *special // linked list in span
offset uint16 // span offset of object
kind byte // kind of special
}
// spanHasSpecials marks a span as having specials in the arena bitmap.
func spanHasSpecials(s *mspan) {
arenaPage := (s.base() / pageSize) % pagesPerArena
ai := arenaIndex(s.base())
ha := mheap_.arenas[ai.l1()][ai.l2()]
atomic.Or8(&ha.pageSpecials[arenaPage/8], uint8(1)<<(arenaPage%8))
}
// spanHasNoSpecials marks a span as having no specials in the arena bitmap.
func spanHasNoSpecials(s *mspan) {
arenaPage := (s.base() / pageSize) % pagesPerArena
ai := arenaIndex(s.base())
ha := mheap_.arenas[ai.l1()][ai.l2()]
atomic.And8(&ha.pageSpecials[arenaPage/8], ^(uint8(1) << (arenaPage % 8)))
}
// Adds the special record s to the list of special records for
// the object p. All fields of s should be filled in except for
// offset & next, which this routine will fill in.
// Returns true if the special was successfully added, false otherwise.
// (The add will fail only if a record with the same p and s->kind
// already exists.)
func addspecial(p unsafe.Pointer, s *special) bool {
span := spanOfHeap(uintptr(p))
if span == nil {
throw("addspecial on invalid pointer")
}
// Ensure that the span is swept.
// Sweeping accesses the specials list w/o locks, so we have
// to synchronize with it. And it's just much safer.
mp := acquirem()
span.ensureSwept()
offset := uintptr(p) - span.base()
kind := s.kind
lock(&span.speciallock)
// Find splice point, check for existing record.
t := &span.specials
for {
x := *t
if x == nil {
break
}
if offset == uintptr(x.offset) && kind == x.kind {
unlock(&span.speciallock)
releasem(mp)
return false // already exists
}
if offset < uintptr(x.offset) || (offset == uintptr(x.offset) && kind < x.kind) {
break
}
t = &x.next
}
// Splice in record, fill in offset.
s.offset = uint16(offset)
s.next = *t
*t = s
spanHasSpecials(span)
unlock(&span.speciallock)
releasem(mp)
return true
}
// Removes the Special record of the given kind for the object p.
// Returns the record if the record existed, nil otherwise.
// The caller must FixAlloc_Free the result.
func removespecial(p unsafe.Pointer, kind uint8) *special {
span := spanOfHeap(uintptr(p))
if span == nil {
throw("removespecial on invalid pointer")
}
// Ensure that the span is swept.
// Sweeping accesses the specials list w/o locks, so we have
// to synchronize with it. And it's just much safer.
mp := acquirem()
span.ensureSwept()
offset := uintptr(p) - span.base()
var result *special
lock(&span.speciallock)
t := &span.specials
for {
s := *t
if s == nil {
break
}
// This function is used for finalizers only, so we don't check for
// "interior" specials (p must be exactly equal to s->offset).
if offset == uintptr(s.offset) && kind == s.kind {
*t = s.next
result = s
break
}
t = &s.next
}
if span.specials == nil {
spanHasNoSpecials(span)
}
unlock(&span.speciallock)
releasem(mp)
return result
}
// The described object has a finalizer set for it.
//
// specialfinalizer is allocated from non-GC'd memory, so any heap
// pointers must be specially handled.
//
//go:notinheap
type specialfinalizer struct {
special special
fn *funcval // May be a heap pointer.
nret uintptr
fint *_type // May be a heap pointer, but always live.
ot *ptrtype // May be a heap pointer, but always live.
}
// Adds a finalizer to the object p. Returns true if it succeeded.
func addfinalizer(p unsafe.Pointer, f *funcval, nret uintptr, fint *_type, ot *ptrtype) bool {
lock(&mheap_.speciallock)
s := (*specialfinalizer)(mheap_.specialfinalizeralloc.alloc())
unlock(&mheap_.speciallock)
s.special.kind = _KindSpecialFinalizer
s.fn = f
s.nret = nret
s.fint = fint
s.ot = ot
if addspecial(p, &s.special) {
// This is responsible for maintaining the same
// GC-related invariants as markrootSpans in any
// situation where it's possible that markrootSpans
// has already run but mark termination hasn't yet.
if gcphase != _GCoff {
base, _, _ := findObject(uintptr(p), 0, 0)
mp := acquirem()
gcw := &mp.p.ptr().gcw
// Mark everything reachable from the object
// so it's retained for the finalizer.
scanobject(base, gcw)
// Mark the finalizer itself, since the
// special isn't part of the GC'd heap.
scanblock(uintptr(unsafe.Pointer(&s.fn)), sys.PtrSize, &oneptrmask[0], gcw, nil)
releasem(mp)
}
return true
}
// There was an old finalizer
lock(&mheap_.speciallock)
mheap_.specialfinalizeralloc.free(unsafe.Pointer(s))
unlock(&mheap_.speciallock)
return false
}
// Removes the finalizer (if any) from the object p.
func removefinalizer(p unsafe.Pointer) {
s := (*specialfinalizer)(unsafe.Pointer(removespecial(p, _KindSpecialFinalizer)))
if s == nil {
return // there wasn't a finalizer to remove
}
lock(&mheap_.speciallock)
mheap_.specialfinalizeralloc.free(unsafe.Pointer(s))
unlock(&mheap_.speciallock)
}
// The described object is being heap profiled.
//
//go:notinheap
type specialprofile struct {
special special
b *bucket
}
// Set the heap profile bucket associated with addr to b.
func setprofilebucket(p unsafe.Pointer, b *bucket) {
lock(&mheap_.speciallock)
s := (*specialprofile)(mheap_.specialprofilealloc.alloc())
unlock(&mheap_.speciallock)
s.special.kind = _KindSpecialProfile
s.b = b
if !addspecial(p, &s.special) {
throw("setprofilebucket: profile already set")
}
}
// Do whatever cleanup needs to be done to deallocate s. It has
// already been unlinked from the mspan specials list.
func freespecial(s *special, p unsafe.Pointer, size uintptr) {
switch s.kind {
case _KindSpecialFinalizer:
sf := (*specialfinalizer)(unsafe.Pointer(s))
queuefinalizer(p, sf.fn, sf.nret, sf.fint, sf.ot)
lock(&mheap_.speciallock)
mheap_.specialfinalizeralloc.free(unsafe.Pointer(sf))
unlock(&mheap_.speciallock)
case _KindSpecialProfile:
sp := (*specialprofile)(unsafe.Pointer(s))
mProf_Free(sp.b, size)
lock(&mheap_.speciallock)
mheap_.specialprofilealloc.free(unsafe.Pointer(sp))
unlock(&mheap_.speciallock)
default:
throw("bad special kind")
panic("not reached")
}
}
// gcBits is an alloc/mark bitmap. This is always used as *gcBits.
//
//go:notinheap
type gcBits uint8
// bytep returns a pointer to the n'th byte of b.
func (b *gcBits) bytep(n uintptr) *uint8 {
return addb((*uint8)(b), n)
}
// bitp returns a pointer to the byte containing bit n and a mask for
// selecting that bit from *bytep.
func (b *gcBits) bitp(n uintptr) (bytep *uint8, mask uint8) {
return b.bytep(n / 8), 1 << (n % 8)
}
const gcBitsChunkBytes = uintptr(64 << 10)
const gcBitsHeaderBytes = unsafe.Sizeof(gcBitsHeader{})
type gcBitsHeader struct {
free uintptr // free is the index into bits of the next free byte.
next uintptr // *gcBits triggers recursive type bug. (issue 14620)
}
//go:notinheap
type gcBitsArena struct {
// gcBitsHeader // side step recursive type bug (issue 14620) by including fields by hand.
free uintptr // free is the index into bits of the next free byte; read/write atomically
next *gcBitsArena
bits [gcBitsChunkBytes - gcBitsHeaderBytes]gcBits
}
var gcBitsArenas struct {
lock mutex
free *gcBitsArena
next *gcBitsArena // Read atomically. Write atomically under lock.
current *gcBitsArena
previous *gcBitsArena
}
// tryAlloc allocates from b or returns nil if b does not have enough room.
// This is safe to call concurrently.
func (b *gcBitsArena) tryAlloc(bytes uintptr) *gcBits {
if b == nil || atomic.Loaduintptr(&b.free)+bytes > uintptr(len(b.bits)) {
return nil
}
// Try to allocate from this block.
end := atomic.Xadduintptr(&b.free, bytes)
if end > uintptr(len(b.bits)) {
return nil
}
// There was enough room.
start := end - bytes
return &b.bits[start]
}
// newMarkBits returns a pointer to 8 byte aligned bytes
// to be used for a span's mark bits.
func newMarkBits(nelems uintptr) *gcBits {
blocksNeeded := uintptr((nelems + 63) / 64)
bytesNeeded := blocksNeeded * 8
// Try directly allocating from the current head arena.
head := (*gcBitsArena)(atomic.Loadp(unsafe.Pointer(&gcBitsArenas.next)))
if p := head.tryAlloc(bytesNeeded); p != nil {
return p
}
// There's not enough room in the head arena. We may need to
// allocate a new arena.
lock(&gcBitsArenas.lock)
// Try the head arena again, since it may have changed. Now
// that we hold the lock, the list head can't change, but its
// free position still can.
if p := gcBitsArenas.next.tryAlloc(bytesNeeded); p != nil {
unlock(&gcBitsArenas.lock)
return p
}
// Allocate a new arena. This may temporarily drop the lock.
fresh := newArenaMayUnlock()
// If newArenaMayUnlock dropped the lock, another thread may
// have put a fresh arena on the "next" list. Try allocating
// from next again.
if p := gcBitsArenas.next.tryAlloc(bytesNeeded); p != nil {
// Put fresh back on the free list.
// TODO: Mark it "already zeroed"
fresh.next = gcBitsArenas.free
gcBitsArenas.free = fresh
unlock(&gcBitsArenas.lock)
return p
}
// Allocate from the fresh arena. We haven't linked it in yet, so
// this cannot race and is guaranteed to succeed.
p := fresh.tryAlloc(bytesNeeded)
if p == nil {
throw("markBits overflow")
}
// Add the fresh arena to the "next" list.
fresh.next = gcBitsArenas.next
atomic.StorepNoWB(unsafe.Pointer(&gcBitsArenas.next), unsafe.Pointer(fresh))
unlock(&gcBitsArenas.lock)
return p
}
// newAllocBits returns a pointer to 8 byte aligned bytes
// to be used for this span's alloc bits.
// newAllocBits is used to provide newly initialized spans
// allocation bits. For spans not being initialized the
// mark bits are repurposed as allocation bits when
// the span is swept.
func newAllocBits(nelems uintptr) *gcBits {
return newMarkBits(nelems)
}
// nextMarkBitArenaEpoch establishes a new epoch for the arenas
// holding the mark bits. The arenas are named relative to the
// current GC cycle which is demarcated by the call to finishweep_m.
//
// All current spans have been swept.
// During that sweep each span allocated room for its gcmarkBits in
// gcBitsArenas.next block. gcBitsArenas.next becomes the gcBitsArenas.current
// where the GC will mark objects and after each span is swept these bits
// will be used to allocate objects.
// gcBitsArenas.current becomes gcBitsArenas.previous where the span's
// gcAllocBits live until all the spans have been swept during this GC cycle.
// The span's sweep extinguishes all the references to gcBitsArenas.previous
// by pointing gcAllocBits into the gcBitsArenas.current.
// The gcBitsArenas.previous is released to the gcBitsArenas.free list.
func nextMarkBitArenaEpoch() {
lock(&gcBitsArenas.lock)
if gcBitsArenas.previous != nil {
if gcBitsArenas.free == nil {
gcBitsArenas.free = gcBitsArenas.previous
} else {
// Find end of previous arenas.
last := gcBitsArenas.previous
for last = gcBitsArenas.previous; last.next != nil; last = last.next {
}
last.next = gcBitsArenas.free
gcBitsArenas.free = gcBitsArenas.previous
}
}
gcBitsArenas.previous = gcBitsArenas.current
gcBitsArenas.current = gcBitsArenas.next
atomic.StorepNoWB(unsafe.Pointer(&gcBitsArenas.next), nil) // newMarkBits calls newArena when needed
unlock(&gcBitsArenas.lock)
}
// newArenaMayUnlock allocates and zeroes a gcBits arena.
// The caller must hold gcBitsArena.lock. This may temporarily release it.
func newArenaMayUnlock() *gcBitsArena {
var result *gcBitsArena
if gcBitsArenas.free == nil {
unlock(&gcBitsArenas.lock)
result = (*gcBitsArena)(sysAlloc(gcBitsChunkBytes, &memstats.gcMiscSys))
if result == nil {
throw("runtime: cannot allocate memory")
}
lock(&gcBitsArenas.lock)
} else {
result = gcBitsArenas.free
gcBitsArenas.free = gcBitsArenas.free.next
memclrNoHeapPointers(unsafe.Pointer(result), gcBitsChunkBytes)
}
result.next = nil
// If result.bits is not 8 byte aligned adjust index so
// that &result.bits[result.free] is 8 byte aligned.
if uintptr(unsafe.Offsetof(gcBitsArena{}.bits))&7 == 0 {
result.free = 0
} else {
result.free = 8 - (uintptr(unsafe.Pointer(&result.bits[0])) & 7)
}
return result
}
 |
The pages are generated with Golds v0.3.2. (GOOS=linux GOARCH=amd64)
Golds is a Go 101 project developed by Tapir Liu.
PR and bug reports are welcome and can be submitted to the issue list.
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