osjitter

ock_gettime 15.9 ns 15.9 ns 44010857 bench_clock_gettime_tai 15.9 ns 15.9 ns 43997256 bench_clock_gettime_monotonic 15.9 ns 15.9 ns 44012908 bench_clock_gettime_monotonic_raw 15.9 ns 15.9 ns 43982277 bench_nanosleep0 49961 ns 370 ns 100000 bench_nanosleep0_slack1 10839 ns 351 ns 1000000 bench_nanosleep1_slack1 10878 ns 358 ns 1000000 bench_pthread_cond_signal 1.37 ns 1.37 ns 503715097 bench_assign 0.563 ns 0.562 ns 1000000000 bench_sqrt 1.63 ns 1.63 ns 430096636 bench_sqrtrec 5.33 ns 5.33 ns 132574542 bench_nothing 0.394 ns 0.394 ns 1000000000 12th Gen Intel(R) Core(TM) i5-12600H ---------------------------------------------------------------------------- Benchmark Time CPU Iterations ---------------------------------------------------------------------------- bench_getuid 70.0 ns 70.0 ns 9985369 bench_getpid 71.6 ns 71.6 ns 9763016 bench_close 76.7 ns 76.7 ns 9131090 bench_syscall 66.8 ns 66.8 ns 10533946 bench_sched_yield 160 ns 160 ns 4377987 bench_clock_gettime 12.2 ns 12.2 ns 57432496 bench_clock_gettime_tai 12.1 ns 12.1 ns 57826299 bench_clock_gettime_monotonic 12.2 ns 12.2 ns 57736141 bench_clock_gettime_monotonic_raw 12.3 ns 12.3 ns 57070425 bench_nanosleep0 63154 ns 11834 ns 55756 bench_nanosleep0_slack1 2933 ns 1700 ns 348675 bench_nanosleep1_slack1 2654 ns 1479 ns 467420 bench_pthread_cond_signal 1.39 ns 1.39 ns 483995101 bench_assign 0.868 ns 0.868 ns 821103909 bench_sqrt 1.69 ns 1.69 ns 422094139 bench_sqrtrec 4.06 ns 4.06 ns 174511095 bench_nothing 0.750 ns 0.750 ns 941204159 AMD Ryzen 5 PRO 7545U Laptop: ---------------------------------------------------------------------------- Benchmark Time CPU Iterations ---------------------------------------------------------------------------- bench_getuid 106 ns 106 ns 6581746 bench_getpid 111 ns 111 ns 6271878 bench_close 116 ns 116 ns 5944154 bench_syscall 85.9 ns 85.9 ns 7317584 bench_sched_yield 315 ns 315 ns 2249333 bench_clock_gettime 17.6 ns 17.6 ns 39935693 bench_clock_gettime_tai 17.6 ns 17.6 ns 39920957 bench_clock_gettime_monotonic 17.5 ns 17.5 ns 39962966 bench_clock_gettime_monotonic_raw 17.5 ns 17.5 ns 39561163 bench_nanosleep0 52720 ns 3058 ns 100000 bench_nanosleep0_slack1 13815 ns 2969 ns 244790 bench_nanosleep1_slack1 13710 ns 2722 ns 254666 bench_pthread_cond_signal 2.66 ns 2.66 ns 264735233 bench_assign 0.930 ns 0.930 ns 813279743 bench_sqrt 2.43 ns 2.43 ns 286953468 bench_sqrtrec 5.67 ns 5.67 ns 123889652 bench_nothing 0.812 ns 0.812 ns 860562208 So, I've tested multiple times in multiple ways, and the results don't seem to match.

. syscall() row invokes a simple syscall(423) and it seems to be expensive. Other calls such as close(999), getpid(), getuid(), clock_gettime(CLOCK_MONOTONIC_RAW, &ts), and sched_yield() are also producing the similar results. All of them basically an order of magnitude larger than 50ns. As for the register renaming, I know what this is, but I still don't get it what register renaming has to do with making the state (registers) storage a cheaper operation. This is from Intel manual: Instructions following a SYSCALL may be fetched from memory before earlier instructions complete execution, but they will not execute (even speculatively) until all instructions prior to the SYSCALL have completed execution (the later instructions may execute before data stored by the earlier instructions have become globally visible). So, I wrongly assumed that the core has to wait before the data is completely written but it seems it acts more like a memory barrier but with relaxed properties - instructions are serialized but the data written doesn't have to become globally visible. I think the most important aspect of it is "until all instructions prior to the SYSCALL have completed". This means that the whole pipeline has to be drained. With 20+ deep instruction pipeline, and whatnot instructions in it, I can imagine that this can likely become the most expensive part of the syscall.