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runtime: scavenge memory upon allocating from scavenged memory
Because scavenged and unscavenged spans no longer coalesce, memory that is freed no longer has a high likelihood of being re-scavenged. As a result, if an application is allocating at a fast rate, it may work fast enough to undo all the scavenging work performed by the runtime's current scavenging mechanisms. This behavior is exacerbated by the global best-fit allocation policy the runtime uses, since scavenged spans are just as likely to be chosen as unscavenged spans on average. To remedy that, we treat each allocation of scavenged space as a heap growth, and scavenge other memory to make up for the allocation. This change makes performance of the runtime slightly worse, as now we're scavenging more often during allocation. The regression is particularly obvious with the garbage benchmark (3%) but most of the Go1 benchmarks are within the margin of noise. A follow-up change should help. Garbage: https://perf.golang.org/search?q=upload:20190131.3 Go1: https://perf.golang.org/search?q=upload:20190131.2 Updates #14045. Change-Id: I44a7e6586eca33b5f97b6d40418db53a8a7ae715 Reviewed-on: https://go-review.googlesource.com/c/159500 Reviewed-by: Austin Clements <austin@google.com>
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@ -1190,6 +1190,16 @@ HaveSpan:
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// heap_released since we already did so earlier.
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sysUsed(unsafe.Pointer(s.base()), s.npages<<_PageShift)
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s.scavenged = false
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// Since we allocated out of a scavenged span, we just
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// grew the RSS. Mitigate this by scavenging enough free
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// space to make up for it.
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//
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// Also, scavengeLargest may cause coalescing, so prevent
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// coalescing with s by temporarily changing its state.
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s.state = mSpanManual
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h.scavengeLargest(s.npages * pageSize)
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s.state = mSpanFree
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}
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s.unusedsince = 0
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