timeutil: stack-allocate Timer, remove pooling #119901
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mgartner
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Reviewed 1 of 1 files at r1, 47 of 47 files at r2, 2 of 2 files at r3, all commit messages.
Reviewable status:complete! 1 of 0 LGTMs obtained (waiting on @herkolategan, @msbutler, @renatolabs, @xinhaoz, and @yuzefovich)
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Nice, thanks!
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This commit adds a new benchmark for timeutil.Timer creation and use. ``` name time/op Timer-10 153µs ± 1% name alloc/op Timer-10 200B ± 0% name allocs/op Timer-10 3.00 ± 0% ``` Epic: None Release note: None
Informs cockroachdb#119593. Closes cockroachdb#38055. This commit removes the pooling of timeutil.Timer structs and, in doing so, permits the structs to be stack allocated so that no pooling is necessary. This superfluous (and in hindsight, harmful) memory pooling was introduced in f11ec1c, which also added very necessary pooling for the internal time.Timer structs. The pooling was harmful because it mandated a contract where Timer structs could not be used after their Stop method was called. This was surprising (time.Timer has no such limitation) and led to subtle use-after-free bugs over time (cockroachdb#61373 and cockroachdb#119595). It was also unnecessary because the outer Timer structs can be stack allocated. Ironically, the only thing that causes them to escape to the heap was the pooling mechanism itself. Removing pooling solves the issue. ``` name old time/op new time/op delta Timer-10 153µs ± 1% 152µs ± 1% ~ (p=0.589 n=10+9) name old alloc/op new alloc/op delta Timer-10 200B ± 0% 200B ± 0% ~ (all equal) name old allocs/op new allocs/op delta Timer-10 3.00 ± 0% 3.00 ± 0% ~ (all equal) ``` Epic: None Release note: None
This commit eliminates a case where a Timer's inner timer is not recycled. This was originally identified by @andreimatei in cockroachdb#13466 (review). ``` name old time/op new time/op delta Timer-10 152µs ± 1% 153µs ± 1% ~ (p=0.133 n=9+10) name old alloc/op new alloc/op delta Timer-10 200B ± 0% 0B -100.00% (p=0.000 n=10+10) name old allocs/op new allocs/op delta Timer-10 3.00 ± 0% 0.00 -100.00% (p=0.000 n=10+10) ``` Epic: None Release note: None
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TFTRs! bors r+ |
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Informs #119593.
Closes #38055.
This PR removes the pooling of
timeutil.Timerstructs and, in doing so, permits the structs to be stack allocated so that no pooling is necessary. This superfluous (and in hindsight, harmful) memory pooling was introduced in f11ec1c, which also added very necessary pooling for the internal time.Timer structs.The pooling was harmful because it mandated a contract where Timer structs could not be used after their Stop method was called. This was surprising (time.Timer has no such limitation) and led to subtle use-after-free bugs over time (#61373 and #119595). It was also unnecessary because the outer Timer structs can be stack allocated. Ironically, the only thing that causes them to escape to the heap was the pooling mechanism itself. Removing pooling solves the issue.
The PR then improves the memory pooling of the inner
time.Timerso that it is always recycled. This was originally identified by @andreimatei in #13466 (review).Doing so has a positive impact on the microbenchmark introduced in the first commit, demonstrating that timers can be stack-allocated and require zero heap allocations:
cc. @andreimatei @ajwerner
Epic: None
Release note: None