Re: [PATCH v3 12/12] grep: use no. of cores as the default no. of threads

>
> 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@usp.br>
> ---
>  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
>