RE: [PATCH] repack: respect gc.pid lock

[David Turner <David.Turner@twosigma.com>]

> -----Original Message-----
> From: Jeff King [mailto:peff@peff.net]
> Sent: Tuesday, April 18, 2017 1:50 PM
> To: David Turner <David.Turner@twosigma.com>
> Cc: git@vger.kernel.org; christian.couder@gmail.com; mfick@codeaurora.org;
> jacob.keller@gmail.com
> Subject: Re: [PATCH] repack: respect gc.pid lock
> 
> On Tue, Apr 18, 2017 at 05:43:29PM +0000, David Turner wrote:
> 
> > > A lock can catch the racy cases where both run at the same time. But
> > > I think that
> > > even:
> > >
> > >   git -c repack.writeBitmaps=true repack -Ad
> > >   [...wait...]
> > >   git gc
> > >
> > > is questionable, because that gc will erase your bitmaps. How does
> > > git-gc know that it's doing a bad thing by repacking without
> > > bitmaps, and that you didn't simply change your configuration or want to get
> rid of them?
> >
> > Sorry, the gc in Gitlab does keep bitmaps.  The one I quoted in a
> > previous message  doesn't, because the person typing the command was
> > just doing some manual  testing and I guess didn't realize that
> > bitmaps were important.  Or perhaps he knew that repack.writeBitmaps was
> already set in the config.
> 
> Sure, but I guess I'd just wonder what _else_ is different between the commands
> (and if nothing, why are both running).

Presumably, repack is faster, and they're not intended to run concurrently (but 
there's a Gitlab bug causing them to do so).  But you'll have to ask the Gitlab 
folks for more details.

> > So given that the lock will catch the races, might it be a good idea
> > (if Implemented to avoid locking on repack -d)?
> 
> I'm mildly negative just because it increases complexity, and I don't think it's
> actually buying very much. It's not clear to me which invocations of repack
> would want to lock and which ones wouldn't.
> 
> Is "-a" or "-A" the key factor? Are there current callers who prefer the current
> behavior of "possibly duplicate some work, but never report failure" versus "do
> not duplicate work, but sometimes fail due to lock contention"?

One problem with failing is that it can leave a temp pack behind.

I think the correct fix is to change the default code.packedGitLimit on 64-bit 
machines to 32 terabytes (2**45 bytes).  That's because on modern Intel 
processors, there are 48 bits of address space actually available, but the kernel 
is going to probably reserve a few bits.  My machine claims to have 2**46 bytes 
of virtual address space available.  It's also several times bigger than any 
repo that I know of or can easily imagine.

Does that seem reasonable to you?