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runtime: make markrootSpans time proportional to in-use spans

Currently markrootSpans iterates over all spans ever allocated to find
the in-use spans. Since we now have a list of in-use spans, change it
to iterate over that instead.

This, combined with the previous change, fixes #9265. Before these two
changes, blowing up the heap to 8GB and then shrinking it to a 0MB
live set caused the small-heap portion of the test to run 60x slower
than without the initial blowup. With these two changes, the time is
indistinguishable.

No significant effect on other benchmarks.

Change-Id: I4a27e533efecfb5d18cba3a87c0181a81d0ddc1e
Reviewed-on: https://go-review.googlesource.com/30536
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>
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aclements committed Oct 5, 2016
1 parent f9497a6 commit c95a8e458fdf9f3cb0c176ac92a513e5dc9b32c1
Showing with 55 additions and 8 deletions.
  1. +9 −7 src/runtime/mgcmark.go
  2. +46 −1 src/runtime/mgcsweepbuf.go
View
@@ -77,7 +77,13 @@ func gcMarkRootPrepare() {
// above invariants for objects that get finalizers
// after concurrent mark. In STW GC, this will happen
// during mark termination.
- work.nSpanRoots = (len(work.spans) + rootBlockSpans - 1) / rootBlockSpans
+ //
+ // We're only interested in scanning the in-use spans,
+ // which will all be swept at this point. More spans
+ // may be added to this list during concurrent GC, but
+ // we only care about spans that were allocated before
+ // this mark phase.
+ work.nSpanRoots = mheap_.sweepSpans[mheap_.sweepgen/2%2].numBlocks()
// On the first markroot, we need to scan all Gs. Gs
// may be created after this point, but it's okay that
@@ -332,18 +338,14 @@ func markrootSpans(gcw *gcWork, shard int) {
}
sg := mheap_.sweepgen
- startSpan := shard * rootBlockSpans
- endSpan := (shard + 1) * rootBlockSpans
- if endSpan > len(work.spans) {
- endSpan = len(work.spans)
- }
+ spans := mheap_.sweepSpans[mheap_.sweepgen/2%2].block(shard)
// Note that work.spans may not include spans that were
// allocated between entering the scan phase and now. This is
// okay because any objects with finalizers in those spans
// must have been allocated and given finalizers after we
// entered the scan phase, so addfinalizer will have ensured
// the above invariants for them.
- for _, s := range work.spans[startSpan:endSpan] {
+ for _, s := range spans {
if s.state != mSpanInUse {
continue
}
View
@@ -129,5 +129,50 @@ func (b *gcSweepBuf) pop() *mspan {
top, bottom := cursor/gcSweepBlockEntries, cursor%gcSweepBlockEntries
blockp := (**gcSweepBlock)(add(b.spine, sys.PtrSize*uintptr(top)))
block := *blockp
- return block.spans[bottom]
+ s := block.spans[bottom]
+ // Clear the pointer for block(i).
+ block.spans[bottom] = nil
+ return s
+}
+
+// numBlocks returns the number of blocks in buffer b. numBlocks is
+// safe to call concurrently with any other operation. Spans that have
+// been pushed prior to the call to numBlocks are guaranteed to appear
+// in some block in the range [0, numBlocks()), assuming there are no
+// intervening pops. Spans that are pushed after the call may also
+// appear in these blocks.
+func (b *gcSweepBuf) numBlocks() int {
+ return int((atomic.Load(&b.index) + gcSweepBlockEntries - 1) / gcSweepBlockEntries)
+}
+
+// block returns the spans in the i'th block of buffer b. block is
+// safe to call concurrently with push.
+func (b *gcSweepBuf) block(i int) []*mspan {
+ // Perform bounds check before loading spine address since
+ // push ensures the allocated length is at least spineLen.
+ if i < 0 || uintptr(i) >= atomic.Loaduintptr(&b.spineLen) {
+ throw("block index out of range")
+ }
+
+ // Get block i.
+ spine := atomic.Loadp(unsafe.Pointer(&b.spine))
+ blockp := add(spine, sys.PtrSize*uintptr(i))
+ block := (*gcSweepBlock)(atomic.Loadp(blockp))
+
+ // Slice the block if necessary.
+ cursor := uintptr(atomic.Load(&b.index))
+ top, bottom := cursor/gcSweepBlockEntries, cursor%gcSweepBlockEntries
+ var spans []*mspan
+ if uintptr(i) < top {
+ spans = block.spans[:]
+ } else {
+ spans = block.spans[:bottom]
+ }
+
+ // push may have reserved a slot but not filled it yet, so
+ // trim away unused entries.
+ for len(spans) > 0 && spans[len(spans)-1] == nil {
+ spans = spans[:len(spans)-1]
+ }
+ return spans
}

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