Re: Make the git codebase thread-safe

From: David Kastrup <dak@gnu.org>
Date: Feb 12, 2014, 18:50 UTC
Message-ID: <87y51g88sc.fsf@fencepost.gnu.org>
In-Reply-To: <CAHOQ7J8gvwpwJV2mBPDaARu3cQ54-ZDQ6iGOwKuJRr9Z+XBL7g@mail.gmail.com>

Stefan Zager <szager@chromium.org> writes:

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> On Tue, Feb 11, 2014 at 6:11 PM, Duy Nguyen <pclouds@gmail.com> wrote:
>>
>> I have no comments about thread safety improvements (well, not yet).
>> If you have investigated about git performance on chromium
>> repositories, could you please sum it up? Threading may be an option
>> to improve performance, but it's probably not the only option.
>
> Well, the painful operations that we use frequently are pack-objects,
> checkout, status, and blame.

Have you checked the patch in
<URL:http://thread.gmane.org/gmane.comp.version-control.git/241448> and
followups,
Message-ID: <1391454849-26558-1-git-send-email-dak@gnu.org>?

While this does not yet support -M and -C options, it's conceivable that you don't use them in your server/scripts.

> Anything on Windows that touches a lot of files is miserable due to
> the usual file system slowness on Windows, and luafv.sys (the UAC file
> virtualization driver) seems to make it much worse.

There is an obvious solution here... Dedicated hardware is not that expensive. Virtualization will always have a price.

Show 5 quoted lines

> Blame is something that chromium and blink developers use heavily, and
> it is not unusual for a blame invocation on the blink repository to
> run for 30 seconds.  It seems like it should be possible to
> parallelize blame, but it requires pack file operations to be
> thread-safe.

Really, give the above patch a try. I am taking longer to finish it than anticipated (with a lot due to procrastination but that is, unfortunately, a large part of my workflow), and it's cutting into my "paychecks" (voluntary donations which to a good degree depend on timely and nontrivial progress reports for my freely available work on GNU LilyPond).

Note that it looks like the majority of the remaining time on GNU/Linux tends to be spent in system time: I/O time, memory management. And I have an SSD drive. When using packed repositories of considerable size, decompression comes into play as well. I don't think that you can hope to get noticeably higher I/O throughput by multithreading, so really, really, really consider dedicated hardware running on a native Linux file system.

-- 
David Kastrup