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runtime: drive proportional sweep directly off heap_live

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Currently, proportional sweep maintains its own count of how many
bytes have been allocated since the beginning of the sweep cycle so it
can compute how many pages need to be swept for a given allocation.

However, this requires a somewhat complex reimbursement scheme since
proportional sweep must be done before a span is allocated, but we
don't know how many bytes to charge until we've allocated a span. This
means that the allocated byte count used by proportional sweep can go
up and down, which has led to underflow bugs in the past (#18043) and
is going to interfere with adjusting sweep pacing on-the-fly (for #19076).

This approach also means we're maintaining a statistic that is very
closely related to heap_live, but has a different 0 value. This is
particularly confusing because the sweep ratio is computed based on
heap_live, so you have to understand that these two statistics are
very closely related.

Replace all of this and compute the sweep debt directly from the
current value of heap_live. To make this work, we simply save the
value of heap_live when the sweep ratio is computed to use as a
"basis" for later computing the sweep debt.

This eliminates the need for reimbursement as well as the code for
maintaining the sweeper's version of the live heap size.

For #19076.

Coincidentally fixes #18043, since this eliminates sweep reimbursement
entirely.

Change-Id: I1f931ddd6e90c901a3972c7506874c899251dc2a
Reviewed-on: https://go-review.googlesource.com/39832
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rick Hudson <rlh@golang.org>
This commit is contained in:
Austin Clements 2017-04-03 15:22:06 -04:00
parent ee175afac2
commit a5eb3dceaf
4 changed files with 12 additions and 33 deletions
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8
src/runtime/mheap.go
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@ -75,10 +75,10 @@ type mheap struct {
_ uint32 // align uint64 fields on 32-bit for atomics
// Proportional sweep
pagesInUse uint64 // pages of spans in stats _MSpanInUse; R/W with mheap.lock
spanBytesAlloc uint64 // bytes of spans allocated this cycle; updated atomically
pagesSwept uint64 // pages swept this cycle; updated atomically
sweepPagesPerByte float64 // proportional sweep ratio; written with lock, read without
pagesInUse uint64 // pages of spans in stats _MSpanInUse; R/W with mheap.lock
pagesSwept uint64 // pages swept this cycle; updated atomically
sweepHeapLiveBasis uint64 // value of heap_live to use as the origin of sweep ratio; written with lock, read without
sweepPagesPerByte float64 // proportional sweep ratio; written with lock, read without
// TODO(austin): pagesInUse should be a uintptr, but the 386
// compiler can't 8-byte align fields.