[PATCH v3 3/3] grep: disable threading in non-worktree case
- From
Thomas Rast <trast@student.ethz.ch>
- Date
- Dec 12, 2011, 21:16 UTC
- Message-ID
- <faa76319c7863852eb6848614c193b56af5789f9.1323723759.git.trast@student.ethz.ch>
- In-Reply-To
- <cover.1323723759.git.trast@student.ethz.ch>
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> --- builtin/grep.c | 36 ++++++++++++++++++------------------ 1 files changed, 18 insertions(+), 18 deletions(-)
diff --git a/builtin/grep.c b/builtin/grep.c index bc23c3c..7affbda 100644 --- a/builtin/grep.c +++ b/builtin/grep.c @@ -1003,24 +1003,6 @@ int cmd_grep(int argc, const char **argv, const char *prefix) if (!opt.fixed && opt.ignore_case) opt.regflags |= REG_ICASE; -#ifndef NO_PTHREADS - if (online_cpus() == 1) - use_threads = 0; -#else - use_threads = 0; -#endif - - opt.use_threads = use_threads; - -#ifndef NO_PTHREADS - if (use_threads) { - if (opt.pre_context || opt.post_context || opt.file_break || - opt.funcbody) - skip_first_line = 1; - start_threads(&opt); - } -#endif - compile_grep_patterns(&opt); /* Check revs and then paths */ @@ -1042,6 +1024,24 @@ int cmd_grep(int argc, const char **argv, const char *prefix) break; } +#ifndef NO_PTHREADS + if (list.nr || cached || online_cpus() == 1) + use_threads = 0; +#else + use_threads = 0; +#endif + + opt.use_threads = use_threads; + +#ifndef NO_PTHREADS + if (use_threads) { + if (opt.pre_context || opt.post_context || opt.file_break || + opt.funcbody) + skip_first_line = 1; + start_threads(&opt); + } +#endif + /* The rest are paths */ if (!seen_dashdash) { int j;
-- 1.7.8.431.g2abf2