src/runtime/mgcwork.go - go - Git at Google

 // 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
 			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
 }

 // 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
 }
	// 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
	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
	}

	// 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
	}