I also ran the count_end test on Linux/aarch64 on the same board, and got the same results as on QNX. No surprise there, but the discrepancy with the x86_64 results on Linux made me want to double-check that the QNX build is not doing something stupid. I guess that what it shows is that, Apple's M1 chip notwithstanding, ARM still hasn't closed the gap when it comes to raw performance. You can always claim that we are comparing apples and oranges here, but I noticed the same trend on many different boards. --Elad On Sun, 14 Aug 2022 at 23:59, Paul E. McKenney <paulmck@xxxxxxxxxx> wrote: > > On Sun, Aug 14, 2022 at 06:11:44AM -0400, Elad Lahav wrote: > > On 2022-08-13 19:29, Paul E. McKenney wrote: > > > On Sat, Aug 13, 2022 at 07:23:40PM -0400, Elad Lahav wrote: > > > > I believe that performance is statistically the same, but I will > > > > double check. I assume both GCC and C11 end up using the same > > > > underlying mechanism for thread-local storage: > > > > > > > > https://uclibc.org/docs/tls.pdf > > > > > > > > If not implemented, TLS falls back on pthread_[gs]et_specific(), but > > > > again it would be the same for __thread and _Thread_local. > > > > > > Sounds likely to me, but I have been surprised before. > > > > Results: > > > > Linux/x86-64, i7-8550U, GCC TLS > > > > elahav@lamia:~/src/other/perfbook/CodeSamples/count$ ./count_end > > n_reads: 408000 n_updates: 689366000 nreaders: 1 nupdaters: 1 duration: > > 240 > > ns/read: 588.235 ns/update: 0.348146 > > elahav@lamia:~/src/other/perfbook/CodeSamples/count$ ./count_end > > n_reads: 443000 n_updates: 762876000 nreaders: 1 nupdaters: 1 duration: > > 240 > > ns/read: 541.761 ns/update: 0.314599 > > elahav@lamia:~/src/other/perfbook/CodeSamples/count$ ./count_end > > n_reads: 395000 n_updates: 666718000 nreaders: 1 nupdaters: 1 duration: > > 240 > > ns/read: 607.595 ns/update: 0.359972 > > > > Linux/x86-64, i7-8550U, C11 TLS > > > > elahav@lamia:~/src/other/perfbook/CodeSamples/count$ ./count_end > > n_reads: 410000 n_updates: 684880000 nreaders: 1 nupdaters: 1 duration: > > 240 > > ns/read: 585.366 ns/update: 0.350426 > > elahav@lamia:~/src/other/perfbook/CodeSamples/count$ ./count_end > > n_reads: 411000 n_updates: 698148000 nreaders: 1 nupdaters: 1 duration: > > 240 > > ns/read: 583.942 ns/update: 0.343767 > > elahav@lamia:~/src/other/perfbook/CodeSamples/count$ ./count_end > > n_reads: 409000 n_updates: 704072000 nreaders: 1 nupdaters: 1 duration: > > 240 > > ns/read: 586.797 ns/update: 0.340874 > > > > QNX/aarch64, NXP LX2160A, GCC TLS (emulated) > > > > elahav@honeycomb:~/src/CodeSamples/count$ ./count_end > > n_reads: 221000 n_updates: 67876000 nreaders: 1 nupdaters: 1 duration: > > 240 > > ns/read: 1085.97 ns/update: 3.53586 > > elahav@honeycomb:~/src/CodeSamples/count$ ./count_end > > n_reads: 227000 n_updates: 67901000 nreaders: 1 nupdaters: 1 duration: > > 240 > > ns/read: 1057.27 ns/update: 3.53456 > > elahav@honeycomb:~/src/CodeSamples/count$ ./count_end > > n_reads: 211000 n_updates: 65043000 nreaders: 1 nupdaters: 1 duration: > > 240 > > ns/read: 1137.44 ns/update: 3.68987 > > > > QNX/aarch64, NXP LX2160A, C11 TLS (emulated) > > > > elahav@honeycomb:~/src/CodeSamples/count$ ./count_end > > n_reads: 217000 n_updates: 67814000 nreaders: 1 nupdaters: 1 duration: > > 240 > > ns/read: 1105.99 ns/update: 3.53909 > > elahav@honeycomb:~/src/CodeSamples/count$ ./count_end > > n_reads: 223000 n_updates: 67860000 nreaders: 1 nupdaters: 1 duration: > > 240 > > ns/read: 1076.23 ns/update: 3.53669 > > elahav@honeycomb:~/src/CodeSamples/count$ ./count_end > > n_reads: 218000 n_updates: 68116000 nreaders: 1 nupdaters: 1 duration: > > 240 > > ns/read: 1100.92 ns/update: 3.5234 > > > > Looking at the disassembly for the QNX binary I realized that it is still > > using the emulated TLS option (i.e., the compiler generates a shim layer > > that uses pthread_[gs]et_specific()). I will need to rebuild the compiler > > with native TLS support and retest, though I doubt it will have a > > significant impact. > > That would explain the lack of statistical significance. If there is > a change, I would hope that the new style is faster. > > > In any case, both the native and the emulated TLS options show the same > > results with the GCC and C11 versions. > > Sounds good, and thank you for checking! > > Thanx, Paul
