Re: Slow git pack-refs --all
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> From: Patrick Steinhardt <ps@pks.im> Sent: Monday, January 5, 2026 11:53 PM
> On Mon, Jan 05, 2026 at 11:45:41PM +0000, Martin Fick wrote:
> > OK, after discovering the strace -r and -T options, I have determined that
> > the 29K writes were all very fast in themselves. However, most of the
> > writes seem to follow each other with no other system calls in between.
> > This explains why it looks like the writes are slow, even though they aren't.
>
> > If I tally up the time between the previous system call, and each write(),
> > it adds up to the bulk of the time (4mins out of 4m15s) that it takes to
> > pack refs. This tells me that no visible I/O or system calls are the problem,
> > but rather that the program itself is taking a long time between writes.
> > I very much doubt that this is heavy CPU time, but rather I am going to
> > guess that this is hidden system time spent accessing mmaped memory.
> > Could it be really slow reading the packed-refs file? I can see the
> > packed-refs file is mmaped() before the writes start, and then
> > munmapped after the writes are completed. If I had to guess, that likely
> > means that the packed-refs file is being read in small increments by the
> > kernel via mmap, and that is what is making things very slow over NFS.
>
> ... Do you use any special flags for mounting the NFS filesystem?
Oh sorry, I forgot to reply to this last time. We use the following mount flags:
rw,intr,retrans=10,timeo=600,hard,rsize=32768,wsize=32768,tcp,noacl,_netdev
-Martin
Martin Fick <mfick@nvidia.com>