@matheusbsbs / matheus-bagliano / commits / 53b8d931b6

grep: disable threading in non-worktree case

Measurements by various people have shown that grepping in parallel is not beneficial when the object store is involved. For example, with a simple regex: Threads | --cached case | worktree case ---------------------------------------------------------------- 8 (default) | 2.88u 0.21s 0:02.94real | 0.19u 0.32s 0:00.16real 4 | 2.89u 0.29s 0:02.99real | 0.16u 0.34s 0:00.17real 2 | 2.83u 0.36s 0:02.87real | 0.18u 0.32s 0:00.26real NO_PTHREADS | 2.16u 0.08s 0:02.25real | 0.12u 0.17s 0:00.31real This happens because all the threads contend on read_sha1_mutex almost all of the time. A more complex regex allows the threads to do more work in parallel, but as Jeff King found out, the "super boost" (much higher clock when only one core is active) feature of recent CPUs still causes the unthreaded case to win by a large margin. So until the pack machinery allows unthreaded access, we disable grep's threading in all but the worktree case. Helped-by: René Scharfe <rene.scharfe@lsrfire.ath.cx> Helped-by: Jeff King <peff@peff.net> Signed-off-by: Thomas Rast <trast@student.ethz.ch> Signed-off-by: Junio C Hamano <gitster@pobox.com>

Thomas Rast committed Dec 12, 2011 at 22:16 UTC 53b8d931b649a0d82e16358a6bf1dd980dc24a6b
1 file changed +18 -18
builtin/grep.c
+18 -18
@@ -1002,24 +1002,6 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
1002 if (!opt.fixed && opt.ignore_case)
1003 opt.regflags |= REG_ICASE;
1004
1005 -#ifndef NO_PTHREADS
1006 - if (online_cpus() == 1)
1007 - use_threads = 0;
1008 -#else
1009 - use_threads = 0;
1010 -#endif
1011 -
1012 - opt.use_threads = use_threads;
1013 -
1014 -#ifndef NO_PTHREADS
1015 - if (use_threads) {
1016 - if (opt.pre_context || opt.post_context || opt.file_break ||
1017 - opt.funcbody)
1018 - skip_first_line = 1;
1019 - start_threads(&opt);
1020 - }
1021 -#endif
1022 -
1005 compile_grep_patterns(&opt);
1006
1007 /* Check revs and then paths */
@@ -1041,6 +1023,24 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
1023 break;
1024 }
1025
1026 +#ifndef NO_PTHREADS
1027 + if (list.nr || cached || online_cpus() == 1)
1028 + use_threads = 0;
1029 +#else
1030 + use_threads = 0;
1031 +#endif
1032 +
1033 + opt.use_threads = use_threads;
1034 +
1035 +#ifndef NO_PTHREADS
1036 + if (use_threads) {
1037 + if (opt.pre_context || opt.post_context || opt.file_break ||
1038 + opt.funcbody)
1039 + skip_first_line = 1;
1040 + start_threads(&opt);
1041 + }
1042 +#endif
1043 +
1044 /* The rest are paths */
1045 if (!seen_dashdash) {
1046 int j;