Also the step on M1 is slightly above 40ns (41.7ns) , but exactly 40 ns on Ampere Altra.
## M1 on MacBooc Air Testing timing overhead for 3 seconds. Total 24000177 ticks in 1000000056 ns, 24000175.655990 ticks per ns This CPU is running at 24000175 ticks / second, will run test for 72000525 ticks loop_count 1407639953Per loop time including overhead: 2.13 ns, min: 0 ticks (0.0 ns), same: 1335774969 Total ticks in: 72000525, in: 3000002260 nr Log2(x+1) histogram of timing durations: <= ticks ( <= ns) % of total running % count 0 ( 41.7) 94.8946 94.8946 1335774969 2 ( 83.3) 5.1051 99.9997 71861227 6 ( 166.7) 0.0001 99.9998 757 14 ( 333.3) 0.0000 99.9998 0 30 ( 666.7) 0.0002 99.9999 2193 62 ( 1333.3) 0.0000 100.0000 274 126 ( 2666.6) 0.0000 100.0000 446 254 ( 5333.3) 0.0000 100.0000 87 First 64 ticks -- 0 ( 0.0) 94.8946 94.8946 1335774969 1 ( 41.7) 5.1032 99.9997 71834980 2 ( 83.3) 0.0019 99.9998 26247 3 ( 125.0) 0.0001 99.9998 757 15 ( 625.0) 0.0000 100.0000 1 ## Ampere Altra Testing timing overhead for 3 seconds. Total 25000002 ticks in 1000000074 ns, 25000000.150000 ticks per ns This CPU is running at 25000000 ticks / second, will run test for 75000000 ticks loop_count 291630863Per loop time including overhead: 10.29 ns, min: 0 ticks (0.0 ns), same: 217288944 Total ticks in: 75000000, in: 3000000542 nr Log2(x+1) histogram of timing durations: <= ticks ( <= ns) % of total running % count 0 ( 40.0) 74.5082 74.5082 217288944 2 ( 80.0) 25.4886 99.9968 74332703 6 ( 160.0) 0.0000 99.9968 5 14 ( 320.0) 0.0000 99.9968 0 30 ( 640.0) 0.0000 99.9968 31 62 ( 1280.0) 0.0011 99.9979 3123 126 ( 2560.0) 0.0020 99.9999 5848 254 ( 5120.0) 0.0001 100.0000 149 510 ( 10240.0) 0.0000 100.0000 38 1022 ( 20480.0) 0.0000 100.0000 21 2046 ( 40960.0) 0.0000 100.0000 1 First 64 ticks -- 0 ( 0.0) 74.5082 74.5082 217288944 1 ( 40.0) 25.4886 99.9968 74332699 2 ( 80.0) 0.0000 99.9968 4 3 ( 120.0) 0.0000 99.9968 1 4 ( 160.0) 0.0000 99.9968 3 On Tue, Jul 2, 2024 at 7:31 PM Hannu Krosing <han...@google.com> wrote: > > Hi Tom, > > On various Intel CPUs I got either steps close to single nanosecond or > sometimes a little more on older ones > > One specific CPU moved in in 2 tick increments while the ration to ns > was 2,1/1 or 2100 ticks per microsecond. > > On Zen4 AMD the step seems to be 10 ns, even though the tick-to-ns > ratio is 2.6 / 1 , so reading ticks directly gives 26, 54, ... > > Also, reading directly in ticks on M1 gave "loop time including > overhead: 2.13 ns" (attached code works on Clang, not sure about GCC) > > > I'll also take a look at the docs and try to propose something > > Do we also need tests for this one ? > > ---- > Hannu > > > > On Tue, Jul 2, 2024 at 7:20 PM Tom Lane <t...@sss.pgh.pa.us> wrote: > > > > BTW, getting back to the original point of the thread: I duplicated > > Hannu's result showing that on Apple M1 the clock tick seems to be > > about 40ns. But look at what I got with the v2 patch on my main > > workstation (full output attached): > > > > $ ./pg_test_timing > > ... > > Per loop time including overhead: 16.60 ns > > ... > > Timing durations less than 128 ns: > > ns % of total running % count > > 15 3.2738 3.2738 5914914 > > 16 49.0772 52.3510 88668783 > > 17 36.4662 88.8172 65884173 > > 18 9.5639 98.3810 17279249 > > 19 1.5746 99.9556 2844873 > > 20 0.0416 99.9972 75125 > > 21 0.0004 99.9976 757 > > ... > > > > It sure looks like this is exact-to-the-nanosecond results, > > since the modal values match the overall per-loop timing, > > and there are no zero measurements. > > > > This is a Dell tower from 2021, running RHEL8 on an Intel Xeon W-2245. > > Not exactly top-of-the-line stuff. > > > > regards, tom lane > >
