Sebastian Pop c064b6560b [Arm64] use isb instruction instead of yield in spin loops ... On arm64 we have seen on several databases that ISB (instruction synchronization barrier) is better to use than yield in a spin loop. The yield instruction is a nop. The isb instruction puts the processor to sleep for some short time. isb is a good equivalent to the pause instruction on x86. Below is an experiment that shows the effects of yield and isb on Arm64 and the time of a pause instruction on x86 Intel processors. The micro-benchmarks use https://github.com/google/benchmark.git $ cat a.cc static void BM_scalar_increment(benchmark::State& state) { int i = 0; for (auto _ : state) benchmark::DoNotOptimize(i++); } BENCHMARK(BM_scalar_increment); static void BM_yield(benchmark::State& state) { for (auto _ : state) asm volatile("yield"::); } BENCHMARK(BM_yield); static void BM_isb(benchmark::State& state) { for (auto _ : state) asm volatile("isb"::); } BENCHMARK(BM_isb); BENCHMARK_MAIN(); $ g++ -o run a.cc -O2 -lbenchmark -lpthread $ ./run -------------------------------------------------------------- Benchmark Time CPU Iterations -------------------------------------------------------------- AWS Graviton2 (Neoverse-N1) processor: BM_scalar_increment 0.485 ns 0.485 ns 1000000000 BM_yield 0.400 ns 0.400 ns 1000000000 BM_isb 13.2 ns 13.2 ns 52993304 AWS Graviton (A-72) processor: BM_scalar_increment 0.897 ns 0.874 ns 801558633 BM_yield 0.877 ns 0.875 ns 800002377 BM_isb 13.0 ns 12.7 ns 55169412 Apple Arm64 M1 processor: BM_scalar_increment 0.315 ns 0.315 ns 1000000000 BM_yield 0.313 ns 0.313 ns 1000000000 BM_isb 9.06 ns 9.06 ns 77259282 static void BM_pause(benchmark::State& state) { for (auto _ : state) asm volatile("pause"::); } BENCHMARK(BM_pause); Intel Skylake processor: BM_scalar_increment 0.295 ns 0.295 ns 1000000000 BM_pause 41.7 ns 41.7 ns 16780553 Tested on Graviton2 aarch64-linux with `./x.py test`.		2021-04-29 23:05:40 +00:00
..
alloc	Auto merge of #83530 - Mark-Simulacrum:bootstrap-bump, r=Mark-Simulacrum	2021-04-04 22:45:56 +00:00
backtrace@710fc18ddc	Update backtrace to 0.3.56	2021-03-25 22:25:34 -06:00
core	[Arm64] use isb instruction instead of yield in spin loops	2021-04-29 23:05:40 +00:00
panic_abort
panic_unwind
proc_macro
profiler_builtins
rtstartup
rustc-std-workspace-alloc
rustc-std-workspace-core
rustc-std-workspace-std
std	Rollup merge of #83831 - AngelicosPhosphoros:issue-77583-inline-for-ip, r=m-ou-se	2021-04-05 13:03:42 +02:00
stdarch@9c732a56f6
term
test	lazily calls some fns	2021-03-27 10:20:32 +03:00
unwind