RE: [PATCH] repack: respect gc.pid lock
- Date: Thu, 20 Apr 2017 20:10:24 +0000
- From: David Turner <David.Turner@xxxxxxxxxxxx>
- Subject: RE: [PATCH] repack: respect gc.pid lock
> -----Original Message-----
> From: Jeff King [mailto:peff@xxxxxxxx]
> Sent: Tuesday, April 18, 2017 1:50 PM
> To: David Turner <David.Turner@xxxxxxxxxxxx>
> Cc: git@xxxxxxxxxxxxxxx; christian.couder@xxxxxxxxx; mfick@xxxxxxxxxxxxxx;
> 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?