Re: [PATCH v2] pack-objects: Use packing_data lock instead of read_mutex

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On Tue, Jan 22, 2019 at 5:26 AM Duy Nguyen <pclouds@xxxxxxxxx> wrote:
>
> On Tue, Jan 22, 2019 at 2:28 PM Jeff King <peff@xxxxxxxx> wrote:
> >
> > On Mon, Jan 21, 2019 at 05:02:33PM +0700, Duy Nguyen wrote:
> >
> > > > As I mentioned in the prior thread I think that it will be simpler
> > > > to simply use the existing lock in packing_data instead of moving
> > > > read_mutex. I can go back to simply moving read_mutex to the
> > > > packing_data struct if that that is preferable, though.
> > >
> > > In early iterations of these changes, I think we hit high contention
> > > when sharing the mutex [1]. I don't know if we will hit the same
> > > performance problem again with this patch. It would be great if Elijah
> > > with his zillion core machine could test this out. Otherwise it may be
> > > just safer to keep the two mutexes separate.
> > >
> > > [1] http://public-inbox.org/git/20180720052829.GA3852@xxxxxxxxxxxxxxxxxxxxx/
> >
> > I haven't been following this thread closely, but I still have access to
> > a 40-core machine if you'd like me to time anything.
> >
> > It sounds like _this_ patch is the more fine-grained one. Is the more
> > coarse-grained one already written?
>
> A more fine-grained one would be 'master' where we use two separate
> mutexes for different code. I guess if repack performance with this
> patch is still the same as 'master', we're good to go. You may need to
> lower $GIT_TEST_OE_SIZE and $GIT_TEST_OE_DELTA_SIZE to force more lock
> contention.

I do have a patch prepared which simply moves read_mutex to the
packing_data struct instead (and renames it read_lock for consistency
with the exiting mutex named "lock"), but I wanted to wait for the
testing regarding lock contention first. I'm prepared either way it
goes.

-Patrick



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