Re: [PATCH 1/1] diffcore-rename: speed up register_rename_src

> On Sat, Jun 08, 2019 at 07:42:42AM -0700, Jeff Hostetler via GitGitGadget wrote:
>> From: Jeff Hostetler <jeffhost@microsoft.com>
>>
>> Teach register_rename_src() to see if new file pair
>> can simply be appended to the rename_src[] array before
>> performing the binary search to find the proper insertion
>> point.
>>
>> This is a performance optimization.  This routine is called
>> during run_diff_files in status and the caller is iterating
>> over the sorted index, so we should expect to be able to
>> append in the normal case.  The existing insert logic is
>> preserved so we don't have to assume that, but simply take
>> advantage of it if possible.
> 
> Could you add a command and performance figures to the commit message
> to show off the benefits of this patch?
> 
>  From the description it's clear that it's a performance optimization,
> but it's unclear whether it has a noticeable, or at least measurable
> effect, or it's "only" a micro-optimization.
> 
> I tried something more substantial than a simple 'git status':
> 
>    # without this patch
>    $ time perf record -g ./git log --oneline -m --name-only v2.20.0 >/dev/null
>    [ ... ]
>    
>    real    2m4.320s
>    user    2m0.913s
>    sys     0m2.284s
>    $ perf report -g none |grep -E '(diffcore_rename|register_rename_src)'
>        52.40%     0.79%  git      git                 [.] diffcore_rename
>         0.01%     0.01%  git      git                 [.] register_rename_src
> 
> but it looks like that while more than half of the considerable
> runtime is spent detecting renames, the time spent in
> register_rename_src(), the function optimized in this patch, is
> negligible.
>