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While their improvements are real and necessary for actual high speed (100 Gb/s and up), 4 Gb/s is not fast. That is only 500 MB/s. Something somewhere, likely not in their code, is terribly slow. I will explain.

As the author cited, kernel context switch is only on the order of 1 us (which seems too high for a system call anyways). You can reach 500 MB/s even if you still call sendmsg() on literally every packet as long as you average ~500 bytes/packet which is ~1/3 of the standard 1500 bytes MTU. So if you average MTU sized packets, you get 2 us of processing in addition to a full system call to reach 4 Gb/s.

The old number of 1 Gb/s could be reached with a average of ~125 bytes/packet, ~1/12 of the MTU or ~11 us of processing.

“But there are also memory copies in the network stack.” A trivial 3 instruction memory copy will go ~10-20 GB/s, 80–160 Gb/s. In 2 us you can drive 20-40 KB of copies. You are arguing the network stack does 40-80(!) copies to put a UDP packet, a thin veneer over a literal packet, into a packet. I have written commercial network drivers. Even without zero-copy, with direct access you can shovel UDP packets into the NIC buffers at basically memory copy speeds.

“But encryption is slow.” Not that slow. Here is some AES-128 GCM performance done what looks like over 5 years ago. [1] The Intel i5-6500, a midline processor from 8 years ago, averages 1729 MB/s. It can do the encryption for a 500 byte packet in 300 ns, 1/6 of the remaining 2 us budget. Modern processors seem to be closer to 3-5 GB/s per core, or about 25-40 Gb/s, 6-10x the stated UDP throughput.

[1] https://calomel.org/aesni_ssl_performance.html

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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

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).

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

---------------------------------------------------------------------------- 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

---------------------------------------------------------------------------- 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