f3312124c2
Added in CL 700496, freeSomeSpanSPMCs attempts to bound tail latency by processing at most 64 entries at a time, as well as returning early if it notices a preemption request. Both of those are attempts to reduce tail latency, as we cannot preempt the function while it holds the lock. This scheme is based on a similar scheme in freeSomeWbufs. freeSomeWbufs has a key difference: all workbufs in its list are unconditionally freed. So freeSomeWbufs will always make forward progress in each call (unless it is constantly preempted). In contrast, freeSomeSpanSPMCs only frees "dead" entries. If the list contains >64 live entries, a call may make no progress, and the caller will simply keep calling in a loop forever, until the GC ends at which point it returns success early. The infinite loop likely restarts at the next GC cycle. The queues are used on each P, so it is easy to have 64 permanently live queues if GOMAXPROCS >= 64. If GOMAXPROCS < 64, it is possible to transiently have more queues, but spanQueue.drain increases queue size in an attempt to reach a steady state of one queue per P. We must drop work.spanSPMCs.lock to allow preemption, but dropping the lock allows mutation of the linked list, meaning we cannot simply continue iteration after retaking lock. Since there is no straightforward resolution to this and we expect this to generally only be around 1 entry per P, simply remove the batching and process the entire list without preemption. We may want to revisit this in the future for very high GOMAXPROCS or if application regularly otherwise create very long lists. Fixes #75771. Change-Id: I6a6a636cd3be443aacde5a678c460aa7066b4c4a Reviewed-on: https://go-review.googlesource.com/c/go/+/709575 Reviewed-by: Michael Knyszek <mknyszek@google.com> LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> |
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