Re: [PATCH] sha1_file: introduce close_one_pack() to close packs on fd pressure

From: Jeff King <peff@peff.net>
Date: Jul 30, 2013, 19:52 UTC
Message-ID: <20130730195257.GA16247@sigill.intra.peff.net>
In-Reply-To: <7v61vsxdiz.fsf@alter.siamese.dyndns.org>

On Tue, Jul 30, 2013 at 08:39:48AM -0700, Junio C Hamano wrote:

Show 14 quoted lines

> Brandon Casey <bcasey@nvidia.com> writes:
> 
> > From: Brandon Casey <drafnel@gmail.com>
> >
> > When the number of open packs exceeds pack_max_fds, unuse_one_window()
> > is called repeatedly to attempt to release the least-recently-used
> > pack windows, which, as a side-effect, will also close a pack file
> > after closing its last open window.  If a pack file has been opened,
> > but no windows have been allocated into it, it will never be selected
> > by unuse_one_window() and hence its file descriptor will not be
> > closed.  When this happens, git may exceed the number of file
> > descriptors permitted by the system.
> 
> An interesting find.  The patch from a cursory look reads OK.

Yeah. I wonder if unuse_one_window() should actually leave the pack fd open now in general.

If you close packfile descriptors, you can run into racy situations where somebody else is repacking and deleting packs, and they go away while you are trying to access them. If you keep a descriptor open, you're fine; they last to the end of the process. If you don't, then they disappear from under you.

For normal object access, this isn't that big a deal; we just rescan the packs and retry. But if you are packing yourself (e.g., because you are a pack-objects started by upload-pack for a clone or fetch), it's much harder to recover (and we print some warnings).

We had our core.packedGitWindowSize lowered on GitHub for a while, and we ran into this warning on busy repositories when we were running "git gc" on the server. We solved it by bumping the window size so we never release memory.

But just not closing the descriptor wouldn't work until Brandon's patch, because we used the same function to release memory and descriptor pressure. Now we could do them separately (and progressively if we need to).

Show 8 quoted lines

> > This is not likely to occur during upload-pack since upload-pack
> > reads each object from the pack so that it can peel tags and
> > advertise the exposed object.
> 
> Another interesting find.  Perhaps there is a room for improvements,
> as packed-refs file knows what objects the tags peel to?  I vaguely
> recall Peff was actively reducing the object access during ref
> enumeration in not so distant past...

Yeah, we should be reading almost no objects these days due to the packed-refs peel lines. I just did a double-check on what "git upload-pack . </dev/null >/dev/null" reads on my git.git repo, and it is only three objects: the v1.8.3.3, v1.8.3.4, and v1.8.4-rc0 tag objects. In other words, the tags I got since the last time I ran "git gc". So I think all is working as designed.

We could give receive-pack the same treatment; I've spent less time micro-optimizing it because because we (and most sites, I would think) get an order of magnitude more fetches than pushes.

-Peff