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80c98fa901
Nearly all the flaky failures we've seen in trace tests have been due to the use of time stamps to determine relative event ordering. This is tricky for many reasons, including: - different cores might not have exactly synchronized clocks - VMs are worse than real hardware - non-x86 chips have different timer resolution than x86 chips - on fast systems two events can end up with the same time stamp Stop trying to make time reliable. It's clearly not going to be for Go 1.5. Instead, record an explicit event sequence number for ordering. Using our own counter solves all of the above problems. The trace still contains time stamps, of course. The sequence number is just used for ordering. Should alleviate #10554 somewhat. Then tickDiv can be chosen to be a useful time unit instead of having to be exact for ordering. Separating ordering and time stamps lets the trace parser diagnose systems where the time stamp order and actual order do not match for one reason or another. This CL adds that check to the end of trace.Parse, after all other sequence order-based checking. If that error is found, we skip the test instead of failing it. Putting the check in trace.Parse means that cmd/trace will pick up the same check, refusing to display a trace where the time stamps do not match actual ordering. Using net/http's BenchmarkClientServerParallel4 on various CPU counts, not tracing vs tracing: name old time/op new time/op delta ClientServerParallel4 50.4µs ± 4% 80.2µs ± 4% +59.06% (p=0.000 n=10+10) ClientServerParallel4-2 33.1µs ± 7% 57.8µs ± 5% +74.53% (p=0.000 n=10+10) ClientServerParallel4-4 18.5µs ± 4% 32.6µs ± 3% +75.77% (p=0.000 n=10+10) ClientServerParallel4-6 12.9µs ± 5% 24.4µs ± 2% +89.33% (p=0.000 n=10+10) ClientServerParallel4-8 11.4µs ± 6% 21.0µs ± 3% +83.40% (p=0.000 n=10+10) ClientServerParallel4-12 14.4µs ± 4% 23.8µs ± 4% +65.67% (p=0.000 n=10+10) Fixes #10512. Change-Id: I173eecf8191e86feefd728a5aad25bf1bc094b12 Reviewed-on: https://go-review.googlesource.com/12579 Reviewed-by: Austin Clements <austin@google.com> |
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