1 files changed, 243 insertions, 0 deletions
diff --git a/src/runtime/mcentral.go b/src/runtime/mcentral.go
new file mode 100644
index 0000000..cd20dec
--- /dev/null
+++ b/src/runtime/mcentral.go
@@ -0,0 +1,243 @@
+// 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.
+
+// Central free lists.
+//
+// See malloc.go for an overview.
+//
+// The mcentral doesn't actually contain the list of free objects; the mspan does.
+// Each mcentral is two lists of mspans: those with free objects (c->nonempty)
+// and those that are completely allocated (c->empty).
+
+package runtime
+
+import "runtime/internal/atomic"
+
+// Central list of free objects of a given size.
+//
+//go:notinheap
+type mcentral struct {
+	spanclass spanClass
+
+	// partial and full contain two mspan sets: one of swept in-use
+	// spans, and one of unswept in-use spans. These two trade
+	// roles on each GC cycle. The unswept set is drained either by
+	// allocation or by the background sweeper in every GC cycle,
+	// so only two roles are necessary.
+	//
+	// sweepgen is increased by 2 on each GC cycle, so the swept
+	// spans are in partial[sweepgen/2%2] and the unswept spans are in
+	// partial[1-sweepgen/2%2]. Sweeping pops spans from the
+	// unswept set and pushes spans that are still in-use on the
+	// swept set. Likewise, allocating an in-use span pushes it
+	// on the swept set.
+	//
+	// Some parts of the sweeper can sweep arbitrary spans, and hence
+	// can't remove them from the unswept set, but will add the span
+	// to the appropriate swept list. As a result, the parts of the
+	// sweeper and mcentral that do consume from the unswept list may
+	// encounter swept spans, and these should be ignored.
+	partial [2]spanSet // list of spans with a free object
+	full    [2]spanSet // list of spans with no free objects
+}
+
+// Initialize a single central free list.
+func (c *mcentral) init(spc spanClass) {
+	c.spanclass = spc
+	lockInit(&c.partial[0].spineLock, lockRankSpanSetSpine)
+	lockInit(&c.partial[1].spineLock, lockRankSpanSetSpine)
+	lockInit(&c.full[0].spineLock, lockRankSpanSetSpine)
+	lockInit(&c.full[1].spineLock, lockRankSpanSetSpine)
+}
+
+// partialUnswept returns the spanSet which holds partially-filled
+// unswept spans for this sweepgen.
+func (c *mcentral) partialUnswept(sweepgen uint32) *spanSet {
+	return &c.partial[1-sweepgen/2%2]
+}
+
+// partialSwept returns the spanSet which holds partially-filled
+// swept spans for this sweepgen.
+func (c *mcentral) partialSwept(sweepgen uint32) *spanSet {
+	return &c.partial[sweepgen/2%2]
+}
+
+// fullUnswept returns the spanSet which holds unswept spans without any
+// free slots for this sweepgen.
+func (c *mcentral) fullUnswept(sweepgen uint32) *spanSet {
+	return &c.full[1-sweepgen/2%2]
+}
+
+// fullSwept returns the spanSet which holds swept spans without any
+// free slots for this sweepgen.
+func (c *mcentral) fullSwept(sweepgen uint32) *spanSet {
+	return &c.full[sweepgen/2%2]
+}
+
+// Allocate a span to use in an mcache.
+func (c *mcentral) cacheSpan() *mspan {
+	// Deduct credit for this span allocation and sweep if necessary.
+	spanBytes := uintptr(class_to_allocnpages[c.spanclass.sizeclass()]) * _PageSize
+	deductSweepCredit(spanBytes, 0)
+
+	sg := mheap_.sweepgen
+
+	traceDone := false
+	if trace.enabled {
+		traceGCSweepStart()
+	}
+
+	// If we sweep spanBudget spans without finding any free
+	// space, just allocate a fresh span. This limits the amount
+	// of time we can spend trying to find free space and
+	// amortizes the cost of small object sweeping over the
+	// benefit of having a full free span to allocate from. By
+	// setting this to 100, we limit the space overhead to 1%.
+	//
+	// TODO(austin,mknyszek): This still has bad worst-case
+	// throughput. For example, this could find just one free slot
+	// on the 100th swept span. That limits allocation latency, but
+	// still has very poor throughput. We could instead keep a
+	// running free-to-used budget and switch to fresh span
+	// allocation if the budget runs low.
+	spanBudget := 100
+
+	var s *mspan
+
+	// Try partial swept spans first.
+	if s = c.partialSwept(sg).pop(); s != nil {
+		goto havespan
+	}
+
+	// Now try partial unswept spans.
+	for ; spanBudget >= 0; spanBudget-- {
+		s = c.partialUnswept(sg).pop()
+		if s == nil {
+			break
+		}
+		if atomic.Load(&s.sweepgen) == sg-2 && atomic.Cas(&s.sweepgen, sg-2, sg-1) {
+			// We got ownership of the span, so let's sweep it and use it.
+			s.sweep(true)
+			goto havespan
+		}
+		// We failed to get ownership of the span, which means it's being or
+		// has been swept by an asynchronous sweeper that just couldn't remove it
+		// from the unswept list. That sweeper took ownership of the span and
+		// responsibility for either freeing it to the heap or putting it on the
+		// right swept list. Either way, we should just ignore it (and it's unsafe
+		// for us to do anything else).
+	}
+	// Now try full unswept spans, sweeping them and putting them into the
+	// right list if we fail to get a span.
+	for ; spanBudget >= 0; spanBudget-- {
+		s = c.fullUnswept(sg).pop()
+		if s == nil {
+			break
+		}
+		if atomic.Load(&s.sweepgen) == sg-2 && atomic.Cas(&s.sweepgen, sg-2, sg-1) {
+			// We got ownership of the span, so let's sweep it.
+			s.sweep(true)
+			// Check if there's any free space.
+			freeIndex := s.nextFreeIndex()
+			if freeIndex != s.nelems {
+				s.freeindex = freeIndex
+				goto havespan
+			}
+			// Add it to the swept list, because sweeping didn't give us any free space.
+			c.fullSwept(sg).push(s)
+		}
+		// See comment for partial unswept spans.
+	}
+	if trace.enabled {
+		traceGCSweepDone()
+		traceDone = true
+	}
+
+	// We failed to get a span from the mcentral so get one from mheap.
+	s = c.grow()
+	if s == nil {
+		return nil
+	}
+
+	// At this point s is a span that should have free slots.
+havespan:
+	if trace.enabled && !traceDone {
+		traceGCSweepDone()
+	}
+	n := int(s.nelems) - int(s.allocCount)
+	if n == 0 || s.freeindex == s.nelems || uintptr(s.allocCount) == s.nelems {
+		throw("span has no free objects")
+	}
+	freeByteBase := s.freeindex &^ (64 - 1)
+	whichByte := freeByteBase / 8
+	// Init alloc bits cache.
+	s.refillAllocCache(whichByte)
+
+	// Adjust the allocCache so that s.freeindex corresponds to the low bit in
+	// s.allocCache.
+	s.allocCache >>= s.freeindex % 64
+
+	return s
+}
+
+// Return span from an mcache.
+//
+// s must have a span class corresponding to this
+// mcentral and it must not be empty.
+func (c *mcentral) uncacheSpan(s *mspan) {
+	if s.allocCount == 0 {
+		throw("uncaching span but s.allocCount == 0")
+	}
+
+	sg := mheap_.sweepgen
+	stale := s.sweepgen == sg+1
+
+	// Fix up sweepgen.
+	if stale {
+		// Span was cached before sweep began. It's our
+		// responsibility to sweep it.
+		//
+		// Set sweepgen to indicate it's not cached but needs
+		// sweeping and can't be allocated from. sweep will
+		// set s.sweepgen to indicate s is swept.
+		atomic.Store(&s.sweepgen, sg-1)
+	} else {
+		// Indicate that s is no longer cached.
+		atomic.Store(&s.sweepgen, sg)
+	}
+
+	// Put the span in the appropriate place.
+	if stale {
+		// It's stale, so just sweep it. Sweeping will put it on
+		// the right list.
+		s.sweep(false)
+	} else {
+		if int(s.nelems)-int(s.allocCount) > 0 {
+			// Put it back on the partial swept list.
+			c.partialSwept(sg).push(s)
+		} else {
+			// There's no free space and it's not stale, so put it on the
+			// full swept list.
+			c.fullSwept(sg).push(s)
+		}
+	}
+}
+
+// grow allocates a new empty span from the heap and initializes it for c's size class.
+func (c *mcentral) grow() *mspan {
+	npages := uintptr(class_to_allocnpages[c.spanclass.sizeclass()])
+	size := uintptr(class_to_size[c.spanclass.sizeclass()])
+
+	s := mheap_.alloc(npages, c.spanclass, true)
+	if s == nil {
+		return nil
+	}
+
+	// Use division by multiplication and shifts to quickly compute:
+	// n := (npages << _PageShift) / size
+	n := (npages << _PageShift) >> s.divShift * uintptr(s.divMul) >> s.divShift2
+	s.limit = s.base() + size*n
+	heapBitsForAddr(s.base()).initSpan(s)
+	return s
+}