From: Calvin Wan <calvinwan@google.com>
To: Junio C Hamano <gitster@pobox.com>
Cc: git@vger.kernel.org, emilyshaffer@google.com, avarab@gmail.com,
phillip.wood123@gmail.com
Subject: Re: [PATCH v2 0/4] submodule: parallelize diff
Date: Thu, 13 Oct 2022 17:39:21 -0700 [thread overview]
Message-ID: <CAFySSZAi_XtwAQeVDgbjXD7vnGuYgNLNo+6NNJP0QvH64KxPOA@mail.gmail.com> (raw)
In-Reply-To: <xmqqczaxmvv9.fsf@gitster.g>
> Calvin Wan <calvinwan@google.com> writes:
>
> > I also wanted to pose another question to list regarding defaults for
> > parallel processes. For jobs that clearly scale with the number of
> > processes (aka jobs that are mostly processor bound), it is obvious that
> > setting the default number of processes to the number of available cores
> > is the most optimal option. However, this changes when the job is mostly
> > I/O bound or has a combination of I/O and processing. Looking at my use
> > case for `status` on a cold cache (see below), we notice that increasing
> > the number of parallel processes speeds up status, but after a certain
> > number, it actually starts slowing down.
>
> I do not offhand recall how the default parallelism is computed
> there, but if I am correct to suspect that "git grep" has a similar
> scaling pattern, i.e. the threads all need to compete for I/O to
> read from the filesystem to find needles from the haystack, perhaps
> it would give us a precedent to model the behaviour of this part of
> the code, too, hopefully?
Setting grep.threads=0 does default it to the number of available cores
(at least the documentation is clear about this). I tested "git grep" on
my machine and found that it started slowing down after 4 threads --
this is most likely because my NVMe SSD uses 4 PCIe lanes aka it can at
most do 4 reads in parallel. AFAIK, there is no way to tell how many
reads a disk can do in parallel. This coupled with the fact that other
commands have varying levels of IO requirements makes it impossible to
set a "reasonable" amount of threads.
next prev parent reply other threads:[~2022-10-14 0:39 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <https://lore.kernel.org/git/20220922232947.631309-1-calvinwan@google.com/>
2022-10-11 23:26 ` [PATCH v2 0/4] submodule: parallelize diff Calvin Wan
2022-10-12 5:52 ` Junio C Hamano
2022-10-14 0:39 ` Calvin Wan [this message]
2022-10-11 23:26 ` [PATCH v2 1/4] run-command: add pipe_output_fn to run_processes_parallel_opts Calvin Wan
2022-10-12 7:58 ` Ævar Arnfjörð Bjarmason
2022-10-11 23:26 ` [PATCH v2 2/4] submodule: move status parsing into function Calvin Wan
2022-10-12 7:41 ` Ævar Arnfjörð Bjarmason
2022-10-12 8:27 ` Ævar Arnfjörð Bjarmason
2022-10-11 23:26 ` [PATCH v2 3/4] diff-lib: refactor match_stat_with_submodule Calvin Wan
2022-10-11 23:26 ` [PATCH v2 4/4] diff-lib: parallelize run_diff_files for submodules Calvin Wan
2022-10-12 8:31 ` Ævar Arnfjörð Bjarmason
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