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// 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.
// Memory statistics
package runtime
import (
// Statistics.
// For detailed descriptions see the documentation for MemStats.
// Fields that differ from MemStats are further documented here.
// Many of these fields are updated on the fly, while others are only
// updated when updatememstats is called.
type mstats struct {
// General statistics.
alloc uint64 // bytes allocated and not yet freed
total_alloc uint64 // bytes allocated (even if freed)
sys uint64 // bytes obtained from system (should be sum of xxx_sys below, no locking, approximate)
nlookup uint64 // number of pointer lookups (unused)
nmalloc uint64 // number of mallocs
nfree uint64 // number of frees
// Statistics about malloc heap.
// Updated atomically, or with the world stopped.
// Like MemStats, heap_sys and heap_inuse do not count memory
// in manually-managed spans.
heap_sys sysMemStat // virtual address space obtained from system for GC'd heap
heap_inuse uint64 // bytes in mSpanInUse spans
heap_released uint64 // bytes released to the os
// heap_objects is not used by the runtime directly and instead
// computed on the fly by updatememstats.
heap_objects uint64 // total number of allocated objects
// Statistics about stacks.
stacks_inuse uint64 // bytes in manually-managed stack spans; computed by updatememstats
stacks_sys sysMemStat // only counts newosproc0 stack in mstats; differs from MemStats.StackSys
// Statistics about allocation of low-level fixed-size structures.
// Protected by FixAlloc locks.
mspan_inuse uint64 // mspan structures
mspan_sys sysMemStat
mcache_inuse uint64 // mcache structures
mcache_sys sysMemStat
buckhash_sys sysMemStat // profiling bucket hash table
// Statistics about GC overhead.
gcWorkBufInUse uint64 // computed by updatememstats
gcProgPtrScalarBitsInUse uint64 // computed by updatememstats
gcMiscSys sysMemStat // updated atomically or during STW
// Miscellaneous statistics.
other_sys sysMemStat // updated atomically or during STW
// Statistics about the garbage collector.
// Protected by mheap or stopping the world during GC.
last_gc_unix uint64 // last gc (in unix time)
pause_total_ns uint64
pause_ns [256]uint64 // circular buffer of recent gc pause lengths
pause_end [256]uint64 // circular buffer of recent gc end times (nanoseconds since 1970)
numgc uint32
numforcedgc uint32 // number of user-forced GCs
gc_cpu_fraction float64 // fraction of CPU time used by GC
enablegc bool
debuggc bool
// Statistics about allocation size classes.
by_size [_NumSizeClasses]struct {
size uint32
nmalloc uint64
nfree uint64
// Add an uint32 for even number of size classes to align below fields
// to 64 bits for atomic operations on 32 bit platforms.
_ [1 - _NumSizeClasses%2]uint32
last_gc_nanotime uint64 // last gc (monotonic time)
last_heap_inuse uint64 // heap_inuse at mark termination of the previous GC
// heapStats is a set of statistics
heapStats consistentHeapStats
// _ uint32 // ensure gcPauseDist is aligned
// gcPauseDist represents the distribution of all GC-related
// application pauses in the runtime.
// Each individual pause is counted separately, unlike pause_ns.
gcPauseDist timeHistogram
var memstats mstats
// A MemStats records statistics about the memory allocator.
type MemStats struct {
// General statistics.
// Alloc is bytes of allocated heap objects.
// This is the same as HeapAlloc (see below).
Alloc uint64
// TotalAlloc is cumulative bytes allocated for heap objects.
// TotalAlloc increases as heap objects are allocated, but
// unlike Alloc and HeapAlloc, it does not decrease when
// objects are freed.
TotalAlloc uint64
// Sys is the total bytes of memory obtained from the OS.
// Sys is the sum of the XSys fields below. Sys measures the
// virtual address space reserved by the Go runtime for the
// heap, stacks, and other internal data structures. It's
// likely that not all of the virtual address space is backed
// by physical memory at any given moment, though in general
// it all was at some point.
Sys uint64
// Lookups is the number of pointer lookups performed by the
// runtime.
// This is primarily useful for debugging runtime internals.
Lookups uint64
// Mallocs is the cumulative count of heap objects allocated.
// The number of live objects is Mallocs - Frees.
Mallocs uint64
// Frees is the cumulative count of heap objects freed.
Frees uint64
// Heap memory statistics.
// Interpreting the heap statistics requires some knowledge of
// how Go organizes memory. Go divides the virtual address
// space of the heap into "spans", which are contiguous
// regions of memory 8K or larger. A span may be in one of
// three states:
// An "idle" span contains no objects or other data. The
// physical memory backing an idle span can be released back
// to the OS (but the virtual address space never is), or it
// can be converted into an "in use" or "stack" span.
// An "in use" span contains at least one heap object and may
// have free space available to allocate more heap objects.
// A "stack" span is used for goroutine stacks. Stack spans
// are not considered part of the heap. A span can change
// between heap and stack memory; it is never used for both
// simultaneously.
// HeapAlloc is bytes of allocated heap objects.
// "Allocated" heap objects include all reachable objects, as
// well as unreachable objects that the garbage collector has
// not yet freed. Specifically, HeapAlloc increases as heap
// objects are allocated and decreases as the heap is swept
// and unreachable objects are freed. Sweeping occurs
// incrementally between GC cycles, so these two processes
// occur simultaneously, and as a result HeapAlloc tends to
// change smoothly (in contrast with the sawtooth that is
// typical of stop-the-world garbage collectors).
HeapAlloc uint64
// HeapSys is bytes of heap memory obtained from the OS.
// HeapSys measures the amount of virtual address space
// reserved for the heap. This includes virtual address space
// that has been reserved but not yet used, which consumes no
// physical memory, but tends to be small, as well as virtual
// address space for which the physical memory has been
// returned to the OS after it became unused (see HeapReleased
// for a measure of the latter).
// HeapSys estimates the largest size the heap has had.
HeapSys uint64
// HeapIdle is bytes in idle (unused) spans.
// Idle spans have no objects in them. These spans could be
// (and may already have been) returned to the OS, or they can
// be reused for heap allocations, or they can be reused as
// stack memory.
// HeapIdle minus HeapReleased estimates the amount of memory
// that could be returned to the OS, but is being retained by
// the runtime so it can grow the heap without requesting more
// memory from the OS. If this difference is significantly
// larger than the heap size, it indicates there was a recent
// transient spike in live heap size.
HeapIdle uint64
// HeapInuse is bytes in in-use spans.
// In-use spans have at least one object in them. These spans
// can only be used for other objects of roughly the same
// size.
// HeapInuse minus HeapAlloc estimates the amount of memory
// that has been dedicated to particular size classes, but is
// not currently being used. This is an upper bound on
// fragmentation, but in general this memory can be reused
// efficiently.
HeapInuse uint64
// HeapReleased is bytes of physical memory returned to the OS.
// This counts heap memory from idle spans that was returned
// to the OS and has not yet been reacquired for the heap.
HeapReleased uint64
// HeapObjects is the number of allocated heap objects.
// Like HeapAlloc, this increases as objects are allocated and
// decreases as the heap is swept and unreachable objects are
// freed.
HeapObjects uint64
// Stack memory statistics.
// Stacks are not considered part of the heap, but the runtime
// can reuse a span of heap memory for stack memory, and
// vice-versa.
// StackInuse is bytes in stack spans.
// In-use stack spans have at least one stack in them. These
// spans can only be used for other stacks of the same size.
// There is no StackIdle because unused stack spans are
// returned to the heap (and hence counted toward HeapIdle).
StackInuse uint64
// StackSys is bytes of stack memory obtained from the OS.
// StackSys is StackInuse, plus any memory obtained directly
// from the OS for OS thread stacks (which should be minimal).
StackSys uint64
// Off-heap memory statistics.
// The following statistics measure runtime-internal
// structures that are not allocated from heap memory (usually
// because they are part of implementing the heap). Unlike
// heap or stack memory, any memory allocated to these
// structures is dedicated to these structures.
// These are primarily useful for debugging runtime memory
// overheads.
// MSpanInuse is bytes of allocated mspan structures.
MSpanInuse uint64
// MSpanSys is bytes of memory obtained from the OS for mspan
// structures.
MSpanSys uint64
// MCacheInuse is bytes of allocated mcache structures.
MCacheInuse uint64
// MCacheSys is bytes of memory obtained from the OS for
// mcache structures.
MCacheSys uint64
// BuckHashSys is bytes of memory in profiling bucket hash tables.
BuckHashSys uint64
// GCSys is bytes of memory in garbage collection metadata.
GCSys uint64
// OtherSys is bytes of memory in miscellaneous off-heap
// runtime allocations.
OtherSys uint64
// Garbage collector statistics.
// NextGC is the target heap size of the next GC cycle.
// The garbage collector's goal is to keep HeapAlloc ≤ NextGC.
// At the end of each GC cycle, the target for the next cycle
// is computed based on the amount of reachable data and the
// value of GOGC.
NextGC uint64
// LastGC is the time the last garbage collection finished, as
// nanoseconds since 1970 (the UNIX epoch).
LastGC uint64
// PauseTotalNs is the cumulative nanoseconds in GC
// stop-the-world pauses since the program started.
// During a stop-the-world pause, all goroutines are paused
// and only the garbage collector can run.
PauseTotalNs uint64
// PauseNs is a circular buffer of recent GC stop-the-world
// pause times in nanoseconds.
// The most recent pause is at PauseNs[(NumGC+255)%256]. In
// general, PauseNs[N%256] records the time paused in the most
// recent N%256th GC cycle. There may be multiple pauses per
// GC cycle; this is the sum of all pauses during a cycle.
PauseNs [256]uint64
// PauseEnd is a circular buffer of recent GC pause end times,
// as nanoseconds since 1970 (the UNIX epoch).
// This buffer is filled the same way as PauseNs. There may be
// multiple pauses per GC cycle; this records the end of the
// last pause in a cycle.
PauseEnd [256]uint64
// NumGC is the number of completed GC cycles.
NumGC uint32
// NumForcedGC is the number of GC cycles that were forced by
// the application calling the GC function.
NumForcedGC uint32
// GCCPUFraction is the fraction of this program's available
// CPU time used by the GC since the program started.
// GCCPUFraction is expressed as a number between 0 and 1,
// where 0 means GC has consumed none of this program's CPU. A
// program's available CPU time is defined as the integral of
// GOMAXPROCS since the program started. That is, if
// GOMAXPROCS is 2 and a program has been running for 10
// seconds, its "available CPU" is 20 seconds. GCCPUFraction
// does not include CPU time used for write barrier activity.
// This is the same as the fraction of CPU reported by
// GODEBUG=gctrace=1.
GCCPUFraction float64
// EnableGC indicates that GC is enabled. It is always true,
// even if GOGC=off.
EnableGC bool
// DebugGC is currently unused.
DebugGC bool
// BySize reports per-size class allocation statistics.
// BySize[N] gives statistics for allocations of size S where
// BySize[N-1].Size < S ≤ BySize[N].Size.
// This does not report allocations larger than BySize[60].Size.
BySize [61]struct {
// Size is the maximum byte size of an object in this
// size class.
Size uint32
// Mallocs is the cumulative count of heap objects
// allocated in this size class. The cumulative bytes
// of allocation is Size*Mallocs. The number of live
// objects in this size class is Mallocs - Frees.
Mallocs uint64
// Frees is the cumulative count of heap objects freed
// in this size class.
Frees uint64
func init() {
if offset := unsafe.Offsetof(memstats.heapStats); offset%8 != 0 {
throw("memstats.heapStats not aligned to 8 bytes")
if offset := unsafe.Offsetof(memstats.gcPauseDist); offset%8 != 0 {
throw("memstats.gcPauseDist not aligned to 8 bytes")
// Ensure the size of heapStatsDelta causes adjacent fields/slots (e.g.
// [3]heapStatsDelta) to be 8-byte aligned.
if size := unsafe.Sizeof(heapStatsDelta{}); size%8 != 0 {
throw("heapStatsDelta not a multiple of 8 bytes in size")
// ReadMemStats populates m with memory allocator statistics.
// The returned memory allocator statistics are up to date as of the
// call to ReadMemStats. This is in contrast with a heap profile,
// which is a snapshot as of the most recently completed garbage
// collection cycle.
func ReadMemStats(m *MemStats) {
stopTheWorld("read mem stats")
systemstack(func() {
func readmemstats_m(stats *MemStats) {
stats.Alloc = memstats.alloc
stats.TotalAlloc = memstats.total_alloc
stats.Sys = memstats.sys
stats.Mallocs = memstats.nmalloc
stats.Frees = memstats.nfree
stats.HeapAlloc = memstats.alloc
stats.HeapSys = memstats.heap_sys.load()
// By definition, HeapIdle is memory that was mapped
// for the heap but is not currently used to hold heap
// objects. It also specifically is memory that can be
// used for other purposes, like stacks, but this memory
// is subtracted out of HeapSys before it makes that
// transition. Put another way:
// heap_sys = bytes allocated from the OS for the heap - bytes ultimately used for non-heap purposes
// heap_idle = bytes allocated from the OS for the heap - bytes ultimately used for any purpose
// or
// heap_sys = sys - stacks_inuse - gcWorkBufInUse - gcProgPtrScalarBitsInUse
// heap_idle = sys - stacks_inuse - gcWorkBufInUse - gcProgPtrScalarBitsInUse - heap_inuse
// => heap_idle = heap_sys - heap_inuse
stats.HeapIdle = memstats.heap_sys.load() - memstats.heap_inuse
stats.HeapInuse = memstats.heap_inuse
stats.HeapReleased = memstats.heap_released
stats.HeapObjects = memstats.heap_objects
stats.StackInuse = memstats.stacks_inuse
// memstats.stacks_sys is only memory mapped directly for OS stacks.
// Add in heap-allocated stack memory for user consumption.
stats.StackSys = memstats.stacks_inuse + memstats.stacks_sys.load()
stats.MSpanInuse = memstats.mspan_inuse
stats.MSpanSys = memstats.mspan_sys.load()
stats.MCacheInuse = memstats.mcache_inuse
stats.MCacheSys = memstats.mcache_sys.load()
stats.BuckHashSys = memstats.buckhash_sys.load()
// MemStats defines GCSys as an aggregate of all memory related
// to the memory management system, but we track this memory
// at a more granular level in the runtime.
stats.GCSys = memstats.gcMiscSys.load() + memstats.gcWorkBufInUse + memstats.gcProgPtrScalarBitsInUse
stats.OtherSys = memstats.other_sys.load()
stats.NextGC = gcController.heapGoal
stats.LastGC = memstats.last_gc_unix
stats.PauseTotalNs = memstats.pause_total_ns
stats.PauseNs = memstats.pause_ns
stats.PauseEnd = memstats.pause_end
stats.NumGC = memstats.numgc
stats.NumForcedGC = memstats.numforcedgc
stats.GCCPUFraction = memstats.gc_cpu_fraction
stats.EnableGC = true
// Handle BySize. Copy N values, where N is
// the minimum of the lengths of the two arrays.
// Unfortunately copy() won't work here because
// the arrays have different structs.
// TODO(mknyszek): Consider renaming the fields
// of by_size's elements to align so we can use
// the copy built-in.
bySizeLen := len(stats.BySize)
if l := len(memstats.by_size); l < bySizeLen {
bySizeLen = l
for i := 0; i < bySizeLen; i++ {
stats.BySize[i].Size = memstats.by_size[i].size
stats.BySize[i].Mallocs = memstats.by_size[i].nmalloc
stats.BySize[i].Frees = memstats.by_size[i].nfree
//go:linkname readGCStats runtime_1debug.readGCStats
func readGCStats(pauses *[]uint64) {
systemstack(func() {
// readGCStats_m must be called on the system stack because it acquires the heap
// lock. See mheap for details.
func readGCStats_m(pauses *[]uint64) {
p := *pauses
// Calling code in runtime/debug should make the slice large enough.
if cap(p) < len(memstats.pause_ns)+3 {
throw("short slice passed to readGCStats")
// Pass back: pauses, pause ends, last gc (absolute time), number of gc, total pause ns.
n := memstats.numgc
if n > uint32(len(memstats.pause_ns)) {
n = uint32(len(memstats.pause_ns))
// The pause buffer is circular. The most recent pause is at
// pause_ns[(numgc-1)%len(pause_ns)], and then backward
// from there to go back farther in time. We deliver the times
// most recent first (in p[0]).
p = p[:cap(p)]
for i := uint32(0); i < n; i++ {
j := (memstats.numgc - 1 - i) % uint32(len(memstats.pause_ns))
p[i] = memstats.pause_ns[j]
p[n+i] = memstats.pause_end[j]
p[n+n] = memstats.last_gc_unix
p[n+n+1] = uint64(memstats.numgc)
p[n+n+2] = memstats.pause_total_ns
*pauses = p[:n+n+3]
// Updates the memstats structure.
// The world must be stopped.
func updatememstats() {
// Flush mcaches to mcentral before doing anything else.
// Flushing to the mcentral may in general cause stats to
// change as mcentral data structures are manipulated.
memstats.mcache_inuse = uint64(mheap_.cachealloc.inuse)
memstats.mspan_inuse = uint64(mheap_.spanalloc.inuse)
memstats.sys = memstats.heap_sys.load() + memstats.stacks_sys.load() + memstats.mspan_sys.load() +
memstats.mcache_sys.load() + memstats.buckhash_sys.load() + memstats.gcMiscSys.load() +
// Calculate memory allocator stats.
// During program execution we only count number of frees and amount of freed memory.
// Current number of alive objects in the heap and amount of alive heap memory
// are calculated by scanning all spans.
// Total number of mallocs is calculated as number of frees plus number of alive objects.
// Similarly, total amount of allocated memory is calculated as amount of freed memory
// plus amount of alive heap memory.
memstats.alloc = 0
memstats.total_alloc = 0
memstats.nmalloc = 0
memstats.nfree = 0
for i := 0; i < len(memstats.by_size); i++ {
memstats.by_size[i].nmalloc = 0
memstats.by_size[i].nfree = 0
// Collect consistent stats, which are the source-of-truth in the some cases.
var consStats heapStatsDelta
// Collect large allocation stats.
totalAlloc := uint64(consStats.largeAlloc)
memstats.nmalloc += uint64(consStats.largeAllocCount)
totalFree := uint64(consStats.largeFree)
memstats.nfree += uint64(consStats.largeFreeCount)
// Collect per-sizeclass stats.
for i := 0; i < _NumSizeClasses; i++ {
// Malloc stats.
a := uint64(consStats.smallAllocCount[i])
totalAlloc += a * uint64(class_to_size[i])
memstats.nmalloc += a
memstats.by_size[i].nmalloc = a
// Free stats.
f := uint64(consStats.smallFreeCount[i])
totalFree += f * uint64(class_to_size[i])
memstats.nfree += f
memstats.by_size[i].nfree = f
// Account for tiny allocations.
memstats.nfree += uint64(consStats.tinyAllocCount)
memstats.nmalloc += uint64(consStats.tinyAllocCount)
// Calculate derived stats.
memstats.total_alloc = totalAlloc
memstats.alloc = totalAlloc - totalFree
memstats.heap_objects = memstats.nmalloc - memstats.nfree
memstats.stacks_inuse = uint64(consStats.inStacks)
memstats.gcWorkBufInUse = uint64(consStats.inWorkBufs)
memstats.gcProgPtrScalarBitsInUse = uint64(consStats.inPtrScalarBits)
// We also count stacks_inuse, gcWorkBufInUse, and gcProgPtrScalarBitsInUse as sys memory.
memstats.sys += memstats.stacks_inuse + memstats.gcWorkBufInUse + memstats.gcProgPtrScalarBitsInUse
// The world is stopped, so the consistent stats (after aggregation)
// should be identical to some combination of memstats. In particular:
// * heap_inuse == inHeap
// * heap_released == released
// * heap_sys - heap_released == committed - inStacks - inWorkBufs - inPtrScalarBits
// Check if that's actually true.
// TODO(mknyszek): Maybe don't throw here. It would be bad if a
// bug in otherwise benign accounting caused the whole application
// to crash.
if memstats.heap_inuse != uint64(consStats.inHeap) {
print("runtime: heap_inuse=", memstats.heap_inuse, "\n")
print("runtime: consistent value=", consStats.inHeap, "\n")
throw("heap_inuse and consistent stats are not equal")
if memstats.heap_released != uint64(consStats.released) {
print("runtime: heap_released=", memstats.heap_released, "\n")
print("runtime: consistent value=", consStats.released, "\n")
throw("heap_released and consistent stats are not equal")
globalRetained := memstats.heap_sys.load() - memstats.heap_released
consRetained := uint64(consStats.committed - consStats.inStacks - consStats.inWorkBufs - consStats.inPtrScalarBits)
if globalRetained != consRetained {
print("runtime: global value=", globalRetained, "\n")
print("runtime: consistent value=", consRetained, "\n")
throw("measures of the retained heap are not equal")
// flushmcache flushes the mcache of allp[i].
// The world must be stopped.
func flushmcache(i int) {
p := allp[i]
c := p.mcache
if c == nil {
// flushallmcaches flushes the mcaches of all Ps.
// The world must be stopped.
func flushallmcaches() {
for i := 0; i < int(gomaxprocs); i++ {
// sysMemStat represents a global system statistic that is managed atomically.
// This type must structurally be a uint64 so that mstats aligns with MemStats.
type sysMemStat uint64
// load atomically reads the value of the stat.
// Must be nosplit as it is called in runtime initialization, e.g. newosproc0.
func (s *sysMemStat) load() uint64 {
return atomic.Load64((*uint64)(s))
// add atomically adds the sysMemStat by n.
// Must be nosplit as it is called in runtime initialization, e.g. newosproc0.
func (s *sysMemStat) add(n int64) {
if s == nil {
val := atomic.Xadd64((*uint64)(s), n)
if (n > 0 && int64(val) < n) || (n < 0 && int64(val)+n < n) {
print("runtime: val=", val, " n=", n, "\n")
throw("sysMemStat overflow")
// heapStatsDelta contains deltas of various runtime memory statistics
// that need to be updated together in order for them to be kept
// consistent with one another.
type heapStatsDelta struct {
// Memory stats.
committed int64 // byte delta of memory committed
released int64 // byte delta of released memory generated
inHeap int64 // byte delta of memory placed in the heap
inStacks int64 // byte delta of memory reserved for stacks
inWorkBufs int64 // byte delta of memory reserved for work bufs
inPtrScalarBits int64 // byte delta of memory reserved for unrolled GC prog bits
// Allocator stats.
tinyAllocCount uintptr // number of tiny allocations
largeAlloc uintptr // bytes allocated for large objects
largeAllocCount uintptr // number of large object allocations
smallAllocCount [_NumSizeClasses]uintptr // number of allocs for small objects
largeFree uintptr // bytes freed for large objects (>maxSmallSize)
largeFreeCount uintptr // number of frees for large objects (>maxSmallSize)
smallFreeCount [_NumSizeClasses]uintptr // number of frees for small objects (<=maxSmallSize)
// Add a uint32 to ensure this struct is a multiple of 8 bytes in size.
// Only necessary on 32-bit platforms.
_ [(sys.PtrSize / 4) % 2]uint32
// merge adds in the deltas from b into a.
func (a *heapStatsDelta) merge(b *heapStatsDelta) {
a.committed += b.committed
a.released += b.released
a.inHeap += b.inHeap
a.inStacks += b.inStacks
a.inWorkBufs += b.inWorkBufs
a.inPtrScalarBits += b.inPtrScalarBits
a.tinyAllocCount += b.tinyAllocCount
a.largeAlloc += b.largeAlloc
a.largeAllocCount += b.largeAllocCount
for i := range b.smallAllocCount {
a.smallAllocCount[i] += b.smallAllocCount[i]
a.largeFree += b.largeFree
a.largeFreeCount += b.largeFreeCount
for i := range b.smallFreeCount {
a.smallFreeCount[i] += b.smallFreeCount[i]
// consistentHeapStats represents a set of various memory statistics
// whose updates must be viewed completely to get a consistent
// state of the world.
// To write updates to memory stats use the acquire and release
// methods. To obtain a consistent global snapshot of these statistics,
// use read.
type consistentHeapStats struct {
// stats is a ring buffer of heapStatsDelta values.
// Writers always atomically update the delta at index gen.
// Readers operate by rotating gen (0 -> 1 -> 2 -> 0 -> ...)
// and synchronizing with writers by observing each P's
// statsSeq field. If the reader observes a P not writing,
// it can be sure that it will pick up the new gen value the
// next time it writes.
// The reader then takes responsibility by clearing space
// in the ring buffer for the next reader to rotate gen to
// that space (i.e. it merges in values from index (gen-2) mod 3
// to index (gen-1) mod 3, then clears the former).
// Note that this means only one reader can be reading at a time.
// There is no way for readers to synchronize.
// This process is why we need a ring buffer of size 3 instead
// of 2: one is for the writers, one contains the most recent
// data, and the last one is clear so writers can begin writing
// to it the moment gen is updated.
stats [3]heapStatsDelta
// gen represents the current index into which writers
// are writing, and can take on the value of 0, 1, or 2.
// This value is updated atomically.
gen uint32
// noPLock is intended to provide mutual exclusion for updating
// stats when no P is available. It does not block other writers
// with a P, only other writers without a P and the reader. Because
// stats are usually updated when a P is available, contention on
// this lock should be minimal.
noPLock mutex
// acquire returns a heapStatsDelta to be updated. In effect,
// it acquires the shard for writing. release must be called
// as soon as the relevant deltas are updated.
// The returned heapStatsDelta must be updated atomically.
// The caller's P must not change between acquire and
// release. This also means that the caller should not
// acquire a P or release its P in between.
func (m *consistentHeapStats) acquire() *heapStatsDelta {
if pp := getg().m.p.ptr(); pp != nil {
seq := atomic.Xadd(&pp.statsSeq, 1)
if seq%2 == 0 {
// Should have been incremented to odd.
print("runtime: seq=", seq, "\n")
throw("bad sequence number")
} else {
gen := atomic.Load(&m.gen) % 3
return &m.stats[gen]
// release indicates that the writer is done modifying
// the delta. The value returned by the corresponding
// acquire must no longer be accessed or modified after
// release is called.
// The caller's P must not change between acquire and
// release. This also means that the caller should not
// acquire a P or release its P in between.
func (m *consistentHeapStats) release() {
if pp := getg().m.p.ptr(); pp != nil {
seq := atomic.Xadd(&pp.statsSeq, 1)
if seq%2 != 0 {
// Should have been incremented to even.
print("runtime: seq=", seq, "\n")
throw("bad sequence number")
} else {
// unsafeRead aggregates the delta for this shard into out.
// Unsafe because it does so without any synchronization. The
// world must be stopped.
func (m *consistentHeapStats) unsafeRead(out *heapStatsDelta) {
for i := range m.stats {
// unsafeClear clears the shard.
// Unsafe because the world must be stopped and values should
// be donated elsewhere before clearing.
func (m *consistentHeapStats) unsafeClear() {
for i := range m.stats {
m.stats[i] = heapStatsDelta{}
// read takes a globally consistent snapshot of m
// and puts the aggregated value in out. Even though out is a
// heapStatsDelta, the resulting values should be complete and
// valid statistic values.
// Not safe to call concurrently. The world must be stopped
// or metricsSema must be held.
func (m *consistentHeapStats) read(out *heapStatsDelta) {
// Getting preempted after this point is not safe because
// we read allp. We need to make sure a STW can't happen
// so it doesn't change out from under us.
mp := acquirem()
// Get the current generation. We can be confident that this
// will not change since read is serialized and is the only
// one that modifies currGen.
currGen := atomic.Load(&m.gen)
prevGen := currGen - 1
if currGen == 0 {
prevGen = 2
// Prevent writers without a P from writing while we update gen.
// Rotate gen, effectively taking a snapshot of the state of
// these statistics at the point of the exchange by moving
// writers to the next set of deltas.
// This exchange is safe to do because we won't race
// with anyone else trying to update this value.
atomic.Xchg(&m.gen, (currGen+1)%3)
// Allow P-less writers to continue. They'll be writing to the
// next generation now.
for _, p := range allp {
// Spin until there are no more writers.
for atomic.Load(&p.statsSeq)%2 != 0 {
// At this point we've observed that each sequence
// number is even, so any future writers will observe
// the new gen value. That means it's safe to read from
// the other deltas in the stats buffer.
// Perform our responsibilities and free up
// stats[prevGen] for the next time we want to take
// a snapshot.
m.stats[prevGen] = heapStatsDelta{}
// Finally, copy out the complete delta.
*out = m.stats[currGen]