The architectural registers can be renamed to physical registers. https://en.wikipedia.org/wiki/Register_renaming https://en.wikipedia.org/wiki/Register_renaming
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
ret
Benchmark is simple but I find it worthwhile because of the fact that (1) it is run across 15 different platforms (different CPUs, libc's) and results are pretty much reproducible, and (2) it is run through gbenchmark which has a mechanism to make the measurements statistically significant.
Interesting 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_syscalls.cc 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).
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.
I can't reproduce. When I run The code is here: https://github.com/gsauthof/osjitter/blob/master/bench_syscalls.cc https://github.com/gsauthof/osjitter/blob/master/bench_sysca..., here are the numbers on the computers I have:
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
So, I've tested multiple times in multiple ways, and the results don't seem to match.