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This change modifies mheap's span allocation API to have each caller
declare a purpose, defined as a new enum called spanAllocType.

The purpose behind this change is two-fold:
1. Tight control over who gets to allocate heap memory is, generally
   speaking, a good thing. Every codepath that allocates heap memory
   places additional implicit restrictions on the allocator. A notable
   example of a restriction is work bufs coming from heap memory: write
   barriers are not allowed in allocation paths because then we could
   have a situation where the allocator calls into the allocator.
2. Memory statistic updating is explicit. Instead of passing an opaque
   pointer for statistic updating, which places restrictions on how that
   statistic may be updated, we use the spanAllocType to determine which
   statistic to update and how.

We also take this opportunity to group all the statistic updating code
together, which should make the accounting code a little easier to
follow.

Change-Id: Ic0b0898959ba2a776f67122f0e36c9d7d60e3085
Reviewed-on: https://go-review.googlesource.com/c/go/+/246970
Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>
8 contributors

Users who have contributed to this file

@aclements @RLH @matloob @bradfitz @prattmic @mknyszek @martisch @ainar-g
482 lines (439 sloc) 12.8 KB
// 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.
package runtime
import (
"runtime/internal/atomic"
"runtime/internal/sys"
"unsafe"
)
const (
_WorkbufSize = 2048 // in bytes; larger values result in less contention
// workbufAlloc is the number of bytes to allocate at a time
// for new workbufs. This must be a multiple of pageSize and
// should be a multiple of _WorkbufSize.
//
// Larger values reduce workbuf allocation overhead. Smaller
// values reduce heap fragmentation.
workbufAlloc = 32 << 10
)
func init() {
if workbufAlloc%pageSize != 0 || workbufAlloc%_WorkbufSize != 0 {
throw("bad workbufAlloc")
}
}
// Garbage collector work pool abstraction.
//
// This implements a producer/consumer model for pointers to grey
// objects. A grey object is one that is marked and on a work
// queue. A black object is marked and not on a work queue.
//
// Write barriers, root discovery, stack scanning, and object scanning
// produce pointers to grey objects. Scanning consumes pointers to
// grey objects, thus blackening them, and then scans them,
// potentially producing new pointers to grey objects.
// A gcWork provides the interface to produce and consume work for the
// garbage collector.
//
// A gcWork can be used on the stack as follows:
//
// (preemption must be disabled)
// gcw := &getg().m.p.ptr().gcw
// .. call gcw.put() to produce and gcw.tryGet() to consume ..
//
// It's important that any use of gcWork during the mark phase prevent
// the garbage collector from transitioning to mark termination since
// gcWork may locally hold GC work buffers. This can be done by
// disabling preemption (systemstack or acquirem).
type gcWork struct {
// wbuf1 and wbuf2 are the primary and secondary work buffers.
//
// This can be thought of as a stack of both work buffers'
// pointers concatenated. When we pop the last pointer, we
// shift the stack up by one work buffer by bringing in a new
// full buffer and discarding an empty one. When we fill both
// buffers, we shift the stack down by one work buffer by
// bringing in a new empty buffer and discarding a full one.
// This way we have one buffer's worth of hysteresis, which
// amortizes the cost of getting or putting a work buffer over
// at least one buffer of work and reduces contention on the
// global work lists.
//
// wbuf1 is always the buffer we're currently pushing to and
// popping from and wbuf2 is the buffer that will be discarded
// next.
//
// Invariant: Both wbuf1 and wbuf2 are nil or neither are.
wbuf1, wbuf2 *workbuf
// Bytes marked (blackened) on this gcWork. This is aggregated
// into work.bytesMarked by dispose.
bytesMarked uint64
// Scan work performed on this gcWork. This is aggregated into
// gcController by dispose and may also be flushed by callers.
scanWork int64
// flushedWork indicates that a non-empty work buffer was
// flushed to the global work list since the last gcMarkDone
// termination check. Specifically, this indicates that this
// gcWork may have communicated work to another gcWork.
flushedWork bool
}
// Most of the methods of gcWork are go:nowritebarrierrec because the
// write barrier itself can invoke gcWork methods but the methods are
// not generally re-entrant. Hence, if a gcWork method invoked the
// write barrier while the gcWork was in an inconsistent state, and
// the write barrier in turn invoked a gcWork method, it could
// permanently corrupt the gcWork.
func (w *gcWork) init() {
w.wbuf1 = getempty()
wbuf2 := trygetfull()
if wbuf2 == nil {
wbuf2 = getempty()
}
w.wbuf2 = wbuf2
}
// put enqueues a pointer for the garbage collector to trace.
// obj must point to the beginning of a heap object or an oblet.
//go:nowritebarrierrec
func (w *gcWork) put(obj uintptr) {
flushed := false
wbuf := w.wbuf1
// Record that this may acquire the wbufSpans or heap lock to
// allocate a workbuf.
lockWithRankMayAcquire(&work.wbufSpans.lock, lockRankWbufSpans)
lockWithRankMayAcquire(&mheap_.lock, lockRankMheap)
if wbuf == nil {
w.init()
wbuf = w.wbuf1
// wbuf is empty at this point.
} else if wbuf.nobj == len(wbuf.obj) {
w.wbuf1, w.wbuf2 = w.wbuf2, w.wbuf1
wbuf = w.wbuf1
if wbuf.nobj == len(wbuf.obj) {
putfull(wbuf)
w.flushedWork = true
wbuf = getempty()
w.wbuf1 = wbuf
flushed = true
}
}
wbuf.obj[wbuf.nobj] = obj
wbuf.nobj++
// If we put a buffer on full, let the GC controller know so
// it can encourage more workers to run. We delay this until
// the end of put so that w is in a consistent state, since
// enlistWorker may itself manipulate w.
if flushed && gcphase == _GCmark {
gcController.enlistWorker()
}
}
// putFast does a put and reports whether it can be done quickly
// otherwise it returns false and the caller needs to call put.
//go:nowritebarrierrec
func (w *gcWork) putFast(obj uintptr) bool {
wbuf := w.wbuf1
if wbuf == nil {
return false
} else if wbuf.nobj == len(wbuf.obj) {
return false
}
wbuf.obj[wbuf.nobj] = obj
wbuf.nobj++
return true
}
// putBatch performs a put on every pointer in obj. See put for
// constraints on these pointers.
//
//go:nowritebarrierrec
func (w *gcWork) putBatch(obj []uintptr) {
if len(obj) == 0 {
return
}
flushed := false
wbuf := w.wbuf1
if wbuf == nil {
w.init()
wbuf = w.wbuf1
}
for len(obj) > 0 {
for wbuf.nobj == len(wbuf.obj) {
putfull(wbuf)
w.flushedWork = true
w.wbuf1, w.wbuf2 = w.wbuf2, getempty()
wbuf = w.wbuf1
flushed = true
}
n := copy(wbuf.obj[wbuf.nobj:], obj)
wbuf.nobj += n
obj = obj[n:]
}
if flushed && gcphase == _GCmark {
gcController.enlistWorker()
}
}
// tryGet dequeues a pointer for the garbage collector to trace.
//
// If there are no pointers remaining in this gcWork or in the global
// queue, tryGet returns 0. Note that there may still be pointers in
// other gcWork instances or other caches.
//go:nowritebarrierrec
func (w *gcWork) tryGet() uintptr {
wbuf := w.wbuf1
if wbuf == nil {
w.init()
wbuf = w.wbuf1
// wbuf is empty at this point.
}
if wbuf.nobj == 0 {
w.wbuf1, w.wbuf2 = w.wbuf2, w.wbuf1
wbuf = w.wbuf1
if wbuf.nobj == 0 {
owbuf := wbuf
wbuf = trygetfull()
if wbuf == nil {
return 0
}
putempty(owbuf)
w.wbuf1 = wbuf
}
}
wbuf.nobj--
return wbuf.obj[wbuf.nobj]
}
// tryGetFast dequeues a pointer for the garbage collector to trace
// if one is readily available. Otherwise it returns 0 and
// the caller is expected to call tryGet().
//go:nowritebarrierrec
func (w *gcWork) tryGetFast() uintptr {
wbuf := w.wbuf1
if wbuf == nil {
return 0
}
if wbuf.nobj == 0 {
return 0
}
wbuf.nobj--
return wbuf.obj[wbuf.nobj]
}
// dispose returns any cached pointers to the global queue.
// The buffers are being put on the full queue so that the
// write barriers will not simply reacquire them before the
// GC can inspect them. This helps reduce the mutator's
// ability to hide pointers during the concurrent mark phase.
//
//go:nowritebarrierrec
func (w *gcWork) dispose() {
if wbuf := w.wbuf1; wbuf != nil {
if wbuf.nobj == 0 {
putempty(wbuf)
} else {
putfull(wbuf)
w.flushedWork = true
}
w.wbuf1 = nil
wbuf = w.wbuf2
if wbuf.nobj == 0 {
putempty(wbuf)
} else {
putfull(wbuf)
w.flushedWork = true
}
w.wbuf2 = nil
}
if w.bytesMarked != 0 {
// dispose happens relatively infrequently. If this
// atomic becomes a problem, we should first try to
// dispose less and if necessary aggregate in a per-P
// counter.
atomic.Xadd64(&work.bytesMarked, int64(w.bytesMarked))
w.bytesMarked = 0
}
if w.scanWork != 0 {
atomic.Xaddint64(&gcController.scanWork, w.scanWork)
w.scanWork = 0
}
}
// balance moves some work that's cached in this gcWork back on the
// global queue.
//go:nowritebarrierrec
func (w *gcWork) balance() {
if w.wbuf1 == nil {
return
}
if wbuf := w.wbuf2; wbuf.nobj != 0 {
putfull(wbuf)
w.flushedWork = true
w.wbuf2 = getempty()
} else if wbuf := w.wbuf1; wbuf.nobj > 4 {
w.wbuf1 = handoff(wbuf)
w.flushedWork = true // handoff did putfull
} else {
return
}
// We flushed a buffer to the full list, so wake a worker.
if gcphase == _GCmark {
gcController.enlistWorker()
}
}
// empty reports whether w has no mark work available.
//go:nowritebarrierrec
func (w *gcWork) empty() bool {
return w.wbuf1 == nil || (w.wbuf1.nobj == 0 && w.wbuf2.nobj == 0)
}
// Internally, the GC work pool is kept in arrays in work buffers.
// The gcWork interface caches a work buffer until full (or empty) to
// avoid contending on the global work buffer lists.
type workbufhdr struct {
node lfnode // must be first
nobj int
}
//go:notinheap
type workbuf struct {
workbufhdr
// account for the above fields
obj [(_WorkbufSize - unsafe.Sizeof(workbufhdr{})) / sys.PtrSize]uintptr
}
// workbuf factory routines. These funcs are used to manage the
// workbufs.
// If the GC asks for some work these are the only routines that
// make wbufs available to the GC.
func (b *workbuf) checknonempty() {
if b.nobj == 0 {
throw("workbuf is empty")
}
}
func (b *workbuf) checkempty() {
if b.nobj != 0 {
throw("workbuf is not empty")
}
}
// getempty pops an empty work buffer off the work.empty list,
// allocating new buffers if none are available.
//go:nowritebarrier
func getempty() *workbuf {
var b *workbuf
if work.empty != 0 {
b = (*workbuf)(work.empty.pop())
if b != nil {
b.checkempty()
}
}
// Record that this may acquire the wbufSpans or heap lock to
// allocate a workbuf.
lockWithRankMayAcquire(&work.wbufSpans.lock, lockRankWbufSpans)
lockWithRankMayAcquire(&mheap_.lock, lockRankMheap)
if b == nil {
// Allocate more workbufs.
var s *mspan
if work.wbufSpans.free.first != nil {
lock(&work.wbufSpans.lock)
s = work.wbufSpans.free.first
if s != nil {
work.wbufSpans.free.remove(s)
work.wbufSpans.busy.insert(s)
}
unlock(&work.wbufSpans.lock)
}
if s == nil {
systemstack(func() {
s = mheap_.allocManual(workbufAlloc/pageSize, spanAllocWorkBuf)
})
if s == nil {
throw("out of memory")
}
// Record the new span in the busy list.
lock(&work.wbufSpans.lock)
work.wbufSpans.busy.insert(s)
unlock(&work.wbufSpans.lock)
}
// Slice up the span into new workbufs. Return one and
// put the rest on the empty list.
for i := uintptr(0); i+_WorkbufSize <= workbufAlloc; i += _WorkbufSize {
newb := (*workbuf)(unsafe.Pointer(s.base() + i))
newb.nobj = 0
lfnodeValidate(&newb.node)
if i == 0 {
b = newb
} else {
putempty(newb)
}
}
}
return b
}
// putempty puts a workbuf onto the work.empty list.
// Upon entry this go routine owns b. The lfstack.push relinquishes ownership.
//go:nowritebarrier
func putempty(b *workbuf) {
b.checkempty()
work.empty.push(&b.node)
}
// putfull puts the workbuf on the work.full list for the GC.
// putfull accepts partially full buffers so the GC can avoid competing
// with the mutators for ownership of partially full buffers.
//go:nowritebarrier
func putfull(b *workbuf) {
b.checknonempty()
work.full.push(&b.node)
}
// trygetfull tries to get a full or partially empty workbuffer.
// If one is not immediately available return nil
//go:nowritebarrier
func trygetfull() *workbuf {
b := (*workbuf)(work.full.pop())
if b != nil {
b.checknonempty()
return b
}
return b
}
//go:nowritebarrier
func handoff(b *workbuf) *workbuf {
// Make new buffer with half of b's pointers.
b1 := getempty()
n := b.nobj / 2
b.nobj -= n
b1.nobj = n
memmove(unsafe.Pointer(&b1.obj[0]), unsafe.Pointer(&b.obj[b.nobj]), uintptr(n)*unsafe.Sizeof(b1.obj[0]))
// Put b on full list - let first half of b get stolen.
putfull(b)
return b1
}
// prepareFreeWorkbufs moves busy workbuf spans to free list so they
// can be freed to the heap. This must only be called when all
// workbufs are on the empty list.
func prepareFreeWorkbufs() {
lock(&work.wbufSpans.lock)
if work.full != 0 {
throw("cannot free workbufs when work.full != 0")
}
// Since all workbufs are on the empty list, we don't care
// which ones are in which spans. We can wipe the entire empty
// list and move all workbuf spans to the free list.
work.empty = 0
work.wbufSpans.free.takeAll(&work.wbufSpans.busy)
unlock(&work.wbufSpans.lock)
}
// freeSomeWbufs frees some workbufs back to the heap and returns
// true if it should be called again to free more.
func freeSomeWbufs(preemptible bool) bool {
const batchSize = 64 // ~1–2 µs per span.
lock(&work.wbufSpans.lock)
if gcphase != _GCoff || work.wbufSpans.free.isEmpty() {
unlock(&work.wbufSpans.lock)
return false
}
systemstack(func() {
gp := getg().m.curg
for i := 0; i < batchSize && !(preemptible && gp.preempt); i++ {
span := work.wbufSpans.free.first
if span == nil {
break
}
work.wbufSpans.free.remove(span)
mheap_.freeManual(span, spanAllocWorkBuf)
}
})
more := !work.wbufSpans.free.isEmpty()
unlock(&work.wbufSpans.lock)
return more
}