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Currently the scavenger will reset to the top of the heap every GC. This means if it scavenges a bunch of memory which doesn't get used again, it's going to keep re-scanning that memory on subsequent cycles. This problem is especially bad when it comes to heap spikes: suppose an application's heap spikes to 2x its steady-state size. The scavenger will run over the top half of that heap even if the heap shrinks, for the rest of the application's lifetime. To fix this, we maintain two numbers: a "free" high watermark, which represents the highest address freed to the page allocator in that cycle, and a "scavenged" low watermark, which represents how low of an address the scavenger got to when scavenging. If the "free" watermark exceeds the "scavenged" watermark, then we pick the "free" watermark as the new "top of the heap" for the scavenger when starting the next scavenger cycle. Otherwise, we have the scavenger pick up where it left off. With this mechanism, we only ever re-scan scavenged memory if a random page gets freed very high up in the heap address space while most of the action is happening in the lower parts. This case should be exceedingly unlikely because the page reclaimer walks over the heap from low address to high addresses, and we use a first-fit address-ordered allocation policy. Updates #35788. Change-Id: Id335603b526ce3a0eb79ef286d1a4e876abc9cab Reviewed-on: https://go-review.googlesource.com/c/go/+/218997 Run-TryBot: Michael Knyszek <mknyszek@google.com> TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Austin Clements <austin@google.com> Reviewed-by: David Chase <drchase@google.com> |
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README.md
The Go Programming Language
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