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Recent page allocator changes (#35112) forced changes to scavenger as well. AFAICT these changes came with two problems.
Firstly, the scavenger maintains an address which it uses to mark which part of the address space has already been searched. Unfortunately the updates of this value are racy, and it's difficult to determine what kind of check makes sense to prevent these races. Today these races mean that the scavenger can miss updates and end up not working for a whole GC cycle (since it only runs down the heap once), or it could end up iterating over address space that a partially concurrent scavenge (e.g. from heap-growth) had already looked at. The affect on application performance is a higher average RSS than in Go 1.13.
Secondly, the scavenger is awoken on each "pacing" update, but today that only means an update to the goal. Because the scavenger is now self-paced, this wake-up is mostly errant, and is generally not a good indicator that there's new work to be done. What this means for application performance is that it might be scavenging memory further down the heap than it should (violating some principles of the original scavenge design) and thus causing undue page faults, resulting in worse CPU performance than in Go 1.13.
I suggest that we fix these for Go 1.14 (fixes are already implemented), but they need not block the beta.