> spectre & meltdown.
I just measured. On my Ryzen 7 9700X, with Linux 6.12, it's about 50ns to call syscall(__NR_gettimeofday). Even post-spectre, entering the kernel isn't so expensive.
I just measured. On my Ryzen 7 9700X, with Linux 6.12, it's about 50ns to call syscall(__NR_gettimeofday). Even post-spectre, entering the kernel isn't so expensive.
Using the libc wrapper will use the vdso. Using syscall() will enter the kernel.
I haven't measured, but calling the vdso should be closer to 5ns.
Someone else did more detailed measurements here:
https://arkanis.de/weblog/2017-01-05-measurements-of-system-...
Also: register renaming is a thing, as is write combining and pipelining. You're not flushing to L1 synchronously for every register, or ordinary userspace function calls would regularly take hundreds of cycles for handling saved registers. They don't.
It's easy to measure how long it takes to push and pop all registers, as well as writing a moderate number of entries to the stack. It's very cheap.
As far as switching into the kernel -- the syscall instruction is more or less just setting a few permission bits and acting as a speculation barrier; there's no reason for that to be expensive. I don't have information on the cost in isolation, but it's entirely unsurprising to me that the majority of the cost is in shuffling around registers. (The post-spectre TLB flush has a cost, but ASIDs mitigate the cost, and measuring the time spent entering and exiting the kernel wouldn't show it even if ASIDs weren't in use)
What do you say about the measurements from https://gms.tf/on-the-costs-of-syscalls.html? Table suggests that the cost is by a magnitude larger, depending on the CPU host, from 250 to 620ns.
As far as that article, it's interesting that the numbers vary between 76 and 560 ns; the benchmark itself has an order of magnitude variation. It also doesn't say what syscall is being done -- __NR_clock_gettime is very cheap, but, for example, __NR_sched_yield will be relatively expensive.
That makes me suspect something else is up in that benchmark.
For what it's worth, here's some more evidence that touching the stack with easily pipelined/parallelized MOV is very cheap. 100 million calls to this assembly costs 200ms, or about 2ns/call:
f:
.LFB6:
.cfi_startproc
pushq %rbp
.cfi_def_cfa_offset 16
.cfi_offset 6, -16
movq %rsp, %rbp
.cfi_def_cfa_register 6
subq $8, %rsp
movq $42, -128(%rbp)
movq $42, -120(%rbp)
movq $42, -112(%rbp)
movq $42, -104(%rbp)
movq $42, -96(%rbp)
movq $42, -88(%rbp)
movq $42, -80(%rbp)
movq $42, -72(%rbp)
movq $42, -64(%rbp)
movq $42, -56(%rbp)
movq $42, -48(%rbp)
movq $42, -40(%rbp)
movq $42, -32(%rbp)
movq $42, -24(%rbp)
movq $42, -16(%rbp)
movq $42, -8(%rbp)
nop
leave
.cfi_def_cfa 7, 8
retInteresting thing that enforces their hypothesis, and measurements, is the fact that, for example, getpid and clock_gettime_mono_raw on some platforms run much faster (vDSO) than on the rest.
Also, the variance between different CPUs is what IMO is enforcing their results and not the other way around - I don't expect the same call to have the same cost on different CPU models. Different CPUs, different cores, different clock frequencies, different tradeoffs in design, etc.
The code is here: https://github.com/gsauthof/osjitter/blob/master/bench_sysca...
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).
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.
AMD Ryzen 7 9700X Desktop:
----------------------------------------------------------------------------
Benchmark Time CPU Iterations
----------------------------------------------------------------------------
bench_getuid 38.6 ns 38.5 ns 18160546
bench_getpid 39.9 ns 39.9 ns 17703749
bench_close 45.2 ns 45.1 ns 15711379
bench_syscall 42.2 ns 42.1 ns 16638675
bench_sched_yield 81.7 ns 81.6 ns 8623522
bench_clock_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
----------------------------------------------------------------------------
Benchmark Time CPU Iterations
----------------------------------------------------------------------------
bench_getuid 384 ns 384 ns 1822307
bench_getpid 382 ns 382 ns 1835289
bench_close 390 ns 390 ns 1796493
bench_syscall 374 ns 374 ns 1874165
bench_sched_yield 611 ns 611 ns 1143456
bench_clock_gettime 44.1 ns 44.1 ns 15872740
bench_clock_gettime_tai 44.1 ns 44.1 ns 15879915
bench_clock_gettime_monotonic 44.1 ns 44.1 ns 15887383
bench_clock_gettime_monotonic_raw 44.4 ns 44.4 ns 15755225
bench_nanosleep0 55617 ns 4647 ns 100000
bench_nanosleep0_slack1 7144 ns 4362 ns 160448
bench_nanosleep1_slack1 7159 ns 4369 ns 160645
bench_pthread_cond_signal 7.38 ns 7.38 ns 94670062
bench_assign 0.523 ns 0.523 ns 1000000000
bench_sqrt 8.04 ns 8.04 ns 86998912
bench_sqrtrec 11.4 ns 11.4 ns 61428535
bench_nothing 0.000 ns 0.000 ns 1000000000
With turbo-boost @3.7Ghz enabled:
----------------------------------------------------------------------------
Benchmark Time CPU Iterations
----------------------------------------------------------------------------
bench_getuid 619 ns 616 ns 1153007
bench_getpid 632 ns 627 ns 1150829
bench_close 629 ns 626 ns 1110226
bench_syscall 617 ns 613 ns 1160239
bench_sched_yield 974 ns 969 ns 702773
bench_clock_gettime 17.9 ns 17.8 ns 39368735
bench_clock_gettime_tai 17.8 ns 17.7 ns 39109544
bench_clock_gettime_monotonic 17.9 ns 17.8 ns 39591364
bench_clock_gettime_monotonic_raw 19.0 ns 18.8 ns 38902038
bench_nanosleep0 63993 ns 4381 ns 100000
bench_nanosleep0_slack1 7445 ns 2115 ns 328474
bench_nanosleep1_slack1 7346 ns 2111 ns 334833
bench_pthread_cond_signal 2.13 ns 2.12 ns 327903411
bench_assign 0.167 ns 0.166 ns 1000000000
bench_sqrt 1.87 ns 1.85 ns 374885774
bench_sqrtrec 0.000 ns 0.000 ns 1000000000
bench_nothing 0.000 ns 0.000 ns 1000000000
----------------------------------------------------------------------------
Benchmark Time CPU Iterations
----------------------------------------------------------------------------
bench_getuid 1019 ns 1012 ns 688965
bench_getpid 1057 ns 1048 ns 688020
bench_close 1039 ns 1029 ns 684537
bench_syscall 1010 ns 1003 ns 696919
bench_sched_yield 1653 ns 1642 ns 434212
bench_clock_gettime 30.7 ns 30.4 ns 22999055
bench_clock_gettime_tai 30.5 ns 30.2 ns 23716873
bench_clock_gettime_monotonic 29.8 ns 29.6 ns 23643198
bench_clock_gettime_monotonic_raw 30.5 ns 30.3 ns 23277717
bench_nanosleep0 65256 ns 5114 ns 100000
bench_nanosleep0_slack1 11649 ns 3402 ns 197983
bench_nanosleep1_slack1 11572 ns 3528 ns 209371
bench_pthread_cond_signal 3.62 ns 3.60 ns 195696177
bench_assign 0.255 ns 0.253 ns 1000000000
bench_sqrt 3.13 ns 3.10 ns 225561559
bench_sqrtrec 0.000 ns 0.000 ns 1000000000
bench_nothing 0.000 ns 0.000 ns 1000000000
Why are your calls to sqrt so slow on your newest machine? Why is sqrtrec free on the others?
As for the sqrt, I don't think it is unusually slow if we compare it against the results from the table above - it's definitely not an outlier since the recorded range is from 1ns to 15ns and I recorded the value of 8ns. Why is that so is not a question here.
Better question is why are your results such a big outlier?
https://arkanis.de/weblog/2017-01-05-measurements-of-system-...
Google also reported similar numbers in 2011, when publicizing their fiber work.
I can also get similar numbers (~68ns) on 9front, though a little higher.
Since you have been laser-focused on sqrt "bad" performance, and obvious optimization with sqrtrec, but also decided to ignore the rest of the results, maybe you can explain why there is such a large difference in your measurements between seemingly very similar platforms in terms of compute. After all this is pure compute problem.
For example, why does 4.9GHz CPU (AMD Ryzen™ 5 7545U) yield 2x to 4x worse results than 5.5GHz CPU (AMD Ryzen™ 7 9700X)?
AMD Ryzen 7 9700X Desktop:
----------------------------------------------------------------------------
Benchmark Time CPU Iterations
----------------------------------------------------------------------------
bench_getuid 38.6 ns 38.5 ns 18160546
bench_getpid 39.9 ns 39.9 ns 17703749
bench_close 45.2 ns 45.1 ns 15711379
bench_syscall 42.2 ns 42.1 ns 16638675
bench_sched_yield 81.7 ns 81.6 ns 8623522
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 2249333Edit: And, apparently, because regardless of what I do with `cpupower`, and twiddling the governors, cpu frequency on this machine is getting scaled. I've run out of time to debug that, I'll update later.
https://www.cpubenchmark.net/compare/6205vs6367vs4835/AMD-Ry...
I'm not sure what's up with sched_yield.
I can also replicate these numbers with `perf bench syscall basic`.
I reran the experiment in a VM, on a company's Xeon server clocked @2.2GHz, and results are again pretty much the same as before:
----------------------------------------------------------------------------
Benchmark Time CPU Iterations
----------------------------------------------------------------------------
bench_getuid 778 ns 778 ns 901999
bench_getpid 774 ns 774 ns 902699
bench_close 779 ns 779 ns 896939
bench_syscall 761 ns 761 ns 916941
bench_sched_yield 1121 ns 1121 ns 566012
bench_clock_gettime 22.1 ns 22.1 ns 31579512
bench_clock_gettime_tai 22.0 ns 22.0 ns 31502402
bench_clock_gettime_monotonic 22.1 ns 22.1 ns 31848177
bench_clock_gettime_monotonic_raw 22.4 ns 22.4 ns 30953415
bench_nanosleep0 57424 ns 6967 ns 98218
bench_nanosleep0_slack1 6342 ns 6340 ns 110862
bench_nanosleep1_slack1 6310 ns 6308 ns 111064
bench_pthread_cond_signal 3.23 ns 3.23 ns 216726274
bench_assign 0.323 ns 0.323 ns 1000000000
bench_sqrt 2.64 ns 2.64 ns 265275643
bench_sqrtrec 4.40 ns 4.40 ns 160328959
bench_nothing 0.000 ns 0.000 ns 1000000000