// 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.get() to consume ..
// if gcBlackenPromptly {
// gcw.dispose()
// }
//
// 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
}
// 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
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)
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 returns true if 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.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]
}
// get dequeues a pointer for the garbage collector to trace, blocking
// if necessary to ensure all pointers from all queues and caches have
// been retrieved. get returns 0 if there are no pointers remaining.
//go:nowritebarrierrec
func (w *gcWork) get() 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 = getfull()
if wbuf == nil {
return 0
}
putempty(owbuf)
w.wbuf1 = wbuf
}
}
// TODO: This might be a good place to add prefetch code
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.wbuf1 = nil
wbuf = w.wbuf2
if wbuf.nobj == 0 {
putempty(wbuf)
} else {
putfull(wbuf)
}
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.wbuf2 = getempty()
} else if wbuf := w.wbuf1; wbuf.nobj > 4 {
w.wbuf1 = handoff(wbuf)
} else {
return
}
// We flushed a buffer to the full list, so wake a worker.
if gcphase == _GCmark {
gcController.enlistWorker()
}
}
// empty returns true if 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()
}
}
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, &memstats.gc_sys)
})
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
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
}
// Get a full work buffer off the work.full list.
// If nothing is available wait until all the other gc helpers have
// finished and then return nil.
// getfull acts as a barrier for work.nproc helpers. As long as one
// gchelper is actively marking objects it
// may create a workbuffer that the other helpers can work on.
// The for loop either exits when a work buffer is found
// or when _all_ of the work.nproc GC helpers are in the loop
// looking for work and thus not capable of creating new work.
// This is in fact the termination condition for the STW mark
// phase.
//go:nowritebarrier
func getfull() *workbuf {
b := (*workbuf)(work.full.pop())
if b != nil {
b.checknonempty()
return b
}
incnwait := atomic.Xadd(&work.nwait, +1)
if incnwait > work.nproc {
println("runtime: work.nwait=", incnwait, "work.nproc=", work.nproc)
throw("work.nwait > work.nproc")
}
for i := 0; ; i++ {
if work.full != 0 {
decnwait := atomic.Xadd(&work.nwait, -1)
if decnwait == work.nproc {
println("runtime: work.nwait=", decnwait, "work.nproc=", work.nproc)
throw("work.nwait > work.nproc")
}
b = (*workbuf)(work.full.pop())
if b != nil {
b.checknonempty()
return b
}
incnwait := atomic.Xadd(&work.nwait, +1)
if incnwait > work.nproc {
println("runtime: work.nwait=", incnwait, "work.nproc=", work.nproc)
throw("work.nwait > work.nproc")
}
}
if work.nwait == work.nproc && work.markrootNext >= work.markrootJobs {
return nil
}
if i < 10 {
procyield(20)
} else if i < 20 {
osyield()
} else {
usleep(100)
}
}
}
//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, &memstats.gc_sys)
}
})
more := !work.wbufSpans.free.isEmpty()
unlock(&work.wbufSpans.lock)
return more
}