Grepping bottleneck is not cpu, but IO. Maybe it is more reasonable to
use not online_cpus() but online_cpus()*2?
--
Victor
On Thu, Jan 16, 2020 at 5:41 AM Matheus Tavares
<matheus.bernardino@xxxxxx> wrote:
>
> When --threads is not specified, git-grep will use 8 threads by default.
> This fixed number may be too many for machines with fewer cores and too
> little for machines with more cores. So, instead, use the number of
> logical cores available in the machine, which seems to result in the
> best overall performance: The following measurements correspond to the
> mean elapsed times for 30 git-grep executions in chromium's
> repository[1] with a 95% confidence interval (each set of 30 were
> performed after 2 warmup runs). Regex 1 is 'abcd[02]' and Regex 2 is
> '(static|extern) (int|double) \*'.
>
> | Working tree | Object Store
> ------|-------------------------------|--------------------------------
> #ths | Regex 1 | Regex 2 | Regex 1 | Regex 2
> ------|---------------|---------------|----------------|---------------
> 32 | 2.92s ± 0.01 | 3.72s ± 0.21 | 5.36s ± 0.01 | 6.07s ± 0.01
> 16 | 2.84s ± 0.01 | 3.57s ± 0.21 | 5.05s ± 0.01 | 5.71s ± 0.01
> > 8 | 2.53s ± 0.00 | 3.24s ± 0.21 | 4.86s ± 0.01 | 5.48s ± 0.01
> 4 | 2.43s ± 0.02 | 3.22s ± 0.20 | 5.22s ± 0.02 | 6.03s ± 0.02
> 2 | 3.06s ± 0.20 | 4.52s ± 0.01 | 7.52s ± 0.01 | 9.06s ± 0.01
> 1 | 6.16s ± 0.01 | 9.25s ± 0.02 | 14.10s ± 0.01 | 17.22s ± 0.01
>
> The above tests were performed in a desktop running Debian 10.0 with
> Intel(R) Xeon(R) CPU E3-1230 V2 (4 cores w/ hyper-threading), 32GB of
> RAM and a 7200 rpm, SATA 3.1 HDD.
>
> Bellow, the tests were repeated for a machine with SSD: a Manjaro laptop
> with Intel(R) i7-7700HQ (4 cores w/ hyper-threading) and 16GB of RAM:
>
> | Working tree | Object Store
> ------|--------------------------------|--------------------------------
> #ths | Regex 1 | Regex 2 | Regex 1 | Regex 2
> ------|---------------|----------------|----------------|---------------
> 32 | 3.29s ± 0.21 | 4.30s ± 0.01 | 6.30s ± 0.01 | 7.30s ± 0.02
> 16 | 3.19s ± 0.20 | 4.14s ± 0.02 | 5.91s ± 0.01 | 6.83s ± 0.01
> > 8 | 2.90s ± 0.04 | 3.82s ± 0.20 | 5.70s ± 0.02 | 6.53s ± 0.01
> 4 | 2.84s ± 0.02 | 3.77s ± 0.20 | 6.19s ± 0.02 | 7.18s ± 0.02
> 2 | 3.73s ± 0.21 | 5.57s ± 0.02 | 9.28s ± 0.01 | 11.22s ± 0.01
> 1 | 7.48s ± 0.02 | 11.36s ± 0.03 | 17.75s ± 0.01 | 21.87s ± 0.08
>
> [1]: chromium’s repo at commit 03ae96f (“Add filters testing at DSF=2”,
> 04-06-2019), after a 'git gc' execution.
>
> Signed-off-by: Matheus Tavares <matheus.bernardino@xxxxxx>
> ---
> Documentation/git-grep.txt | 4 ++--
> builtin/grep.c | 3 +--
> 2 files changed, 3 insertions(+), 4 deletions(-)
>
> diff --git a/Documentation/git-grep.txt b/Documentation/git-grep.txt
> index de628741fa..eb5412724f 100644
> --- a/Documentation/git-grep.txt
> +++ b/Documentation/git-grep.txt
> @@ -59,8 +59,8 @@ grep.extendedRegexp::
> other than 'default'.
>
> grep.threads::
> - Number of grep worker threads to use. If unset (or set to 0),
> - 8 threads are used by default (for now).
> + Number of grep worker threads to use. If unset (or set to 0), Git will
> + use as many threads as the number of logical cores available.
>
> grep.fullName::
> If set to true, enable `--full-name` option by default.
> diff --git a/builtin/grep.c b/builtin/grep.c
> index a85b710b48..629eaf5dbc 100644
> --- a/builtin/grep.c
> +++ b/builtin/grep.c
> @@ -33,7 +33,6 @@ static char const * const grep_usage[] = {
>
> static int recurse_submodules;
>
> -#define GREP_NUM_THREADS_DEFAULT 8
> static int num_threads;
>
> static pthread_t *threads;
> @@ -1064,7 +1063,7 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
> } else if (num_threads < 0)
> die(_("invalid number of threads specified (%d)"), num_threads);
> else if (num_threads == 0)
> - num_threads = HAVE_THREADS ? GREP_NUM_THREADS_DEFAULT : 1;
> + num_threads = HAVE_THREADS ? online_cpus() : 1;
>
> if (num_threads > 1) {
> if (!HAVE_THREADS)
> --
> 2.24.1
>
