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As a result of changes earlier in Go 1.12, the scavenger became much more aggressive. In particular, when scavenged and unscavenged spans coalesced, they would always become scavenged. This resulted in most spans becoming scavenged over time. While this is good for keeping the RSS of the program low, it also causes many more undue page faults and many more calls to madvise. For most applications, the impact of this was negligible. But for applications that repeatedly grow and shrink the heap by large amounts, the overhead can be significant. The overhead was especially obvious on older versions of Linux where MADV_FREE isn't available and MADV_DONTNEED must be used. This change makes it so that scavenged spans will never coalesce with unscavenged spans. This results in fewer page faults overall. Aside from this, the expected impact of this change is more heap growths on average, as span allocations will be less likely to be fulfilled. To mitigate this slightly, this change also coalesces spans eagerly after scavenging, to at least ensure that all scavenged spans and all unscavenged spans are coalesced with each other. Also, this change adds additional logic in the case where two adjacent spans cannot coalesce. In this case, on platforms where the physical page size is larger than the runtime's page size, we realign the boundary between the two adjacent spans to a physical page boundary. The advantage of this approach is that "unscavengable" spans, that is, spans which cannot be scavenged because they don't cover at least a single physical page are grown to a size where they have a higher likelihood of being discovered by the runtime's scavenging mechanisms when they border a scavenged span. This helps prevent the runtime from accruing pockets of "unscavengable" memory in between scavenged spans, preventing them from coalescing. We specifically choose to apply this logic to all spans because it simplifies the code, even though it isn't strictly necessary. The expectation is that this change will result in a slight loss in performance on platforms where the physical page size is larger than the runtime page size. Update #14045. Change-Id: I64fd43eac1d6de6f51d7a2ecb72670f10bb12589 Reviewed-on: https://go-review.googlesource.com/c/158078 Run-TryBot: Michael Knyszek <mknyszek@google.com> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Austin Clements <austin@google.com> |
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