[weekly-r317] PostingsForLabelMatching: check for waiting writers #760
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There's still some contention on MemPostings.mtx, I checked goroutines from a profile at a spiking mutex wait time moment, I filtered by `MemPostings` and all I see is: - 4 goroutines waiting on `p.mtx.RLock()` in `LabelValues` - 203 goroutines waiting on `p.mtx.RLock()` in `Get` - 6 goroutines waiting on `p.mtx.Lock()` in `Add` - 4 goroutines waiting on first `p.mtx.RLock` in `PostingsForLabelMatching` - 2 goroutines in `for _, v := range vals` loop in `PostingsForLabelMatching` So, what I undersand is going on here: - The last two goroutines were able to enter the `PostingsForLabelMatching` second mutex, so they're creating ListPostings as there were no tomorrow (I'll send a separate PR to Prometheus to avoid creating one pointer for each one there) - The six goroutines waiting on `Add` are blocking the reset 200+ read goroutines. So, here we have 2 read goroutines blocking 200 goroutines (plus six read ones, if someone cares). What this change proposes is to check every few iterations (1024, because I expect this loop to go quite fast) whether there's a Lock() waiting. If there is one, we'll stop, unlock, and let it happen, because if we're being selfish, no other reader can get in here. Signed-off-by: Oleg Zaytsev <[email protected]>
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Cherry-pick of #757