Re: [PATCH] git-completion.bash - avoid excruciatingly slow ref completion on Cygwin

From: Tay Ray Chuan <rctay89@gmail.com>
Date: Oct 18, 2014, 14:47 UTC
Message-ID: <CALUzUxpGXwu=sdh7MHjEcJzDRzZZkTR0NajU0hrBGCZZLGj2aw@mail.gmail.com>
In-Reply-To: <1413042673-24052-1-git-send-email-mlevedahl@gmail.com>

On Sat, Oct 11, 2014 at 11:51 PM, Mark Levedahl <mlevedahl@gmail.com> wrote:

Show 9 quoted lines

>
> $git checkout <tab> was taking about 3.5 seconds to respond on one
> repository having four remotes with about 100 total refs (measured on
> Cygwin).  All of the time was being claimed in "git for-each-ref" to do
> its work.  This working directory was created using git-new-workdir, and
> thus .git/refs and .git/packed-refs are both symlinks.  for-each-ref
> operates in a way that causes the .git/refs symlink to be resolved
> multiple times for each ref in the repository, and Cygwin is especially
> slow in such operations.

Thanks for looking into this.

> Patching refs.c to avoid repeatedly dereferencing the symlink reduced
> execution time from about 3.5 seconds to about 1.1 seconds (but no
> improvement on Linux),

This makes your patch sound bad, but it isn't when one realises it is already fast on Linux!

> [snip]
>
> Relevant timing results using the same repository on both Linux and
> Cygwin:

FWIW, timings on msysgit:

$ time git for-each-ref --format="%(refname:short)" refs

real 0m8.799s user 0m0.109s sys 0m0.250s

$ time (cd "$GIT_DIR" ; cat packed-refs ; find refs/ -type f) \
        2>/dev/null | sed -ne 's@^.*refs/@refs/@p' | sort | uniq

real 0m3.406s user 0m1.073s sys 0m2.398s

so while your symlink-analysis might not accurately describe msysgit (I believe copies are made in place of a symlink), msysgit benefits from this too.

-- 
Cheers,
Ray Chuan

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