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.
In any case, both the native and the emulated TLS options show the same
results with the GCC and C11 versions.
--Elad
