Re: [PATCH] subtree: performance improvement for finding unexpected parent commits

From: Junio C Hamano <gitster@pobox.com>
Date: Oct 12, 2018, 14:22 UTC
Message-ID: <xmqqmurj9re9.fsf@gitster-ct.c.googlers.com>
In-Reply-To: <20181012135218.14439-1-rstrain@swri.org>

Roger Strain <rstrain@swri.org> writes:

Show 10 quoted lines

> After testing a previous patch at larger scale, a performance issue was
> detected when using git show to locate parent revisions, with a single
> run of the git show command taking 2 seconds or longer in a complex repo.
> When the command is required tens or hundreds of times in a run of the
> script, the additional wait time is unaccepatable. Replacing the command
> with git rev-parse resulted in significantly increased performance, with
> the command in question returning instantly.
>
> Signed-off-by: Roger Strain <rstrain@swri.org>
> Thanks-to: Junio C Hamano <gitster@pobox.com>

That usually is spelled as "Helped-by:".

Will queue.  Thanks.

I still find it disturbing not to know why "show -s --format=..." takes measurable time, though. "-s" means "we do not need any diff output", so it ought to be comparable to "git cat-file commit $rev" with some formatting, but apparently your repository is making Git spend a lot more than that. Puzzled...

Show 16 quoted lines

> ---
>  contrib/subtree/git-subtree.sh | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/contrib/subtree/git-subtree.sh b/contrib/subtree/git-subtree.sh
> index 1c157dbd9..147201dc6 100755
> --- a/contrib/subtree/git-subtree.sh
> +++ b/contrib/subtree/git-subtree.sh
> @@ -633,7 +633,7 @@ process_split_commit () {
>  	else
>  		# processing commit without normal parent information;
>  		# fetch from repo
> -		parents=$(git show -s --pretty=%P "$rev")
> +		parents=$(git rev-parse "$rev^@")
>  		extracount=$(($extracount + 1))
>  	fi