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The Go programming language
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The tri-state mutex implementation (unlocked, locked, sleeping) avoids sleep/wake syscalls when contention is low or absent, but its performance degrades when many threads are contending for a mutex to execute a fast critical section. A fast critical section means frequent unlock2 calls. Each of those finds the mutex in the "sleeping" state and so wakes a sleeping thread, even if many other threads are already awake and in the spin loop of lock2 attempting to acquire the mutex for themselves. Many spinning threads means wasting energy and CPU time that could be used by other processes on the machine. Many threads all spinning on the same cache line leads to performance collapse. Merge the futex- and semaphore-based mutex implementations by using a semaphore abstraction for futex platforms. Then, add a bit to the mutex state word that communicates whether one of the waiting threads is awake and spinning. When threads in lock2 see the new "spinning" bit, they can sleep immediately. In unlock2, the "spinning" bit means we can save a syscall and not wake a sleeping thread. This brings up the real possibility of starvation: waiting threads are able to enter a deeper sleep than before, since one of their peers can volunteer to be the sole "spinning" thread and thus cause unlock2 to skip the semawakeup call. Additionally, the waiting threads form a LIFO stack so any wakeups that do occur will target threads that have gone to sleep most recently. Counteract those effects by periodically waking the thread at the bottom of the stack and allowing it to spin. Exempt sched.lock from most of the new behaviors; it's often used by several threads in sequence to do thread-specific work, so low-latency handoff is a priority over improved throughput. Gate use of this implementation behind GOEXPERIMENT=spinbitmutex, so it's easy to disable. Enable it by default on supported platforms (the most efficient implementation requires atomic.Xchg8). Fixes #68578 goos: linux goarch: amd64 pkg: runtime cpu: 13th Gen Intel(R) Core(TM) i7-13700H │ old │ new │ │ sec/op │ sec/op vs base │ MutexContention 17.82n ± 0% 17.74n ± 0% -0.42% (p=0.000 n=10) MutexContention-2 22.17n ± 9% 19.85n ± 12% ~ (p=0.089 n=10) MutexContention-3 26.14n ± 14% 20.81n ± 13% -20.41% (p=0.000 n=10) MutexContention-4 29.28n ± 8% 21.19n ± 10% -27.62% (p=0.000 n=10) MutexContention-5 31.79n ± 2% 21.98n ± 10% -30.83% (p=0.000 n=10) MutexContention-6 34.63n ± 1% 22.58n ± 5% -34.79% (p=0.000 n=10) MutexContention-7 44.16n ± 2% 23.14n ± 7% -47.59% (p=0.000 n=10) MutexContention-8 53.81n ± 3% 23.66n ± 6% -56.04% (p=0.000 n=10) MutexContention-9 65.58n ± 4% 23.91n ± 9% -63.54% (p=0.000 n=10) MutexContention-10 77.35n ± 3% 26.06n ± 9% -66.31% (p=0.000 n=10) MutexContention-11 89.62n ± 1% 25.56n ± 9% -71.47% (p=0.000 n=10) MutexContention-12 102.45n ± 2% 25.57n ± 7% -75.04% (p=0.000 n=10) MutexContention-13 111.95n ± 1% 24.59n ± 8% -78.04% (p=0.000 n=10) MutexContention-14 123.95n ± 3% 24.42n ± 6% -80.30% (p=0.000 n=10) MutexContention-15 120.80n ± 10% 25.54n ± 6% -78.86% (p=0.000 n=10) MutexContention-16 128.10n ± 25% 26.95n ± 4% -78.96% (p=0.000 n=10) MutexContention-17 139.80n ± 18% 24.96n ± 5% -82.14% (p=0.000 n=10) MutexContention-18 141.35n ± 7% 25.05n ± 8% -82.27% (p=0.000 n=10) MutexContention-19 151.35n ± 18% 25.72n ± 6% -83.00% (p=0.000 n=10) MutexContention-20 153.30n ± 20% 24.75n ± 6% -83.85% (p=0.000 n=10) MutexHandoff/Solo-20 13.54n ± 1% 13.61n ± 4% ~ (p=0.206 n=10) MutexHandoff/FastPingPong-20 141.3n ± 209% 164.8n ± 49% ~ (p=0.436 n=10) MutexHandoff/SlowPingPong-20 1.572µ ± 16% 1.804µ ± 19% +14.76% (p=0.015 n=10) geomean 74.34n 30.26n -59.30% goos: darwin goarch: arm64 pkg: runtime cpu: Apple M1 │ old │ new │ │ sec/op │ sec/op vs base │ MutexContention 13.86n ± 3% 12.09n ± 3% -12.73% (p=0.000 n=10) MutexContention-2 15.88n ± 1% 16.50n ± 2% +3.94% (p=0.001 n=10) MutexContention-3 18.45n ± 2% 16.88n ± 2% -8.54% (p=0.000 n=10) MutexContention-4 20.01n ± 2% 18.94n ± 18% ~ (p=0.469 n=10) MutexContention-5 22.60n ± 1% 17.51n ± 9% -22.50% (p=0.000 n=10) MutexContention-6 23.93n ± 2% 17.35n ± 2% -27.48% (p=0.000 n=10) MutexContention-7 24.69n ± 1% 17.15n ± 3% -30.54% (p=0.000 n=10) MutexContention-8 25.01n ± 1% 17.33n ± 2% -30.69% (p=0.000 n=10) MutexHandoff/Solo-8 13.96n ± 4% 12.04n ± 4% -13.78% (p=0.000 n=10) MutexHandoff/FastPingPong-8 68.89n ± 4% 64.62n ± 2% -6.20% (p=0.000 n=10) MutexHandoff/SlowPingPong-8 9.698µ ± 22% 9.646µ ± 35% ~ (p=0.912 n=10) geomean 38.20n 32.53n -14.84% Change-Id: I0058c75eadf282d08eea7fce0d426f0518039f7c Reviewed-on: https://go-review.googlesource.com/c/go/+/620435 Reviewed-by: Michael Knyszek <mknyszek@google.com> LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Junyang Shao <shaojunyang@google.com> Auto-Submit: Rhys Hiltner <rhys.hiltner@gmail.com> |
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The Go Programming Language
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