Re: [PATCH v2] packfile: freshen the mtime of packfile by configuration

[Taylor Blau <ttaylorr@github.com>]

On Thu, Jul 15, 2021 at 12:46:47AM +0800, Sun Chao wrote:
> > Stepping back, I'm not sure I understand why freshening a pack is so
> > slow for you. freshen_file() just calls utime(2), and any sync back to
> > the disk shouldn't need to update the pack itself, just a couple of
> > fields in its inode. Maybe you could help explain further.
> >
> > [ ... ]
>
> The reason why we want to avoid freshen the mtime of ".pack" file is to
> improve the reading speed of Git Servers.
>
> We have some large repositories in our Git Severs (some are bigger than 10GB),
> and we created '.keep' files for large ".pack" files, we want the big files
> unchanged to speed up git upload-pack, because in our mind the file system
> cache will reduce the disk IO if a file does not changed.
>
> However we find the mtime of ".pack" files changes over time which makes the
> file system always reload the big files, that takes a lot of IO time and result
> in lower speed of git upload-pack and even further the disk IOPS is exhausted.

That's surprising behavior to me. Are you saying that calling utime(2)
causes the *page* cache to be invalidated and that most reads are
cache-misses lowering overall IOPS?

If so, then I am quite surprised ;). The only state that should be
dirtied by calling utime(2) is the inode itself, so the blocks referred
to by the inode corresponding to a pack should be left in-tact.

If you're on Linux, you can try observing the behavior of evicting
inodes, blocks, or both from the disk cache by changing "2" in the
following:

    hyperfine 'git pack-objects --all --stdout --delta-base-offset >/dev/null'
      --prepare='sync; echo 2 | sudo tee /proc/sys/vm/drop_caches'

where "1" drops the page cache, "2" drops the inodes, and "3" evicts
both.

I wonder if you could share the results of running the above varying
the value of "1", "2", and "3", as well as swapping the `--prepare` for
`--warmup=3` to warm your caches (and give us an idea of what your
expected performance is probably like).

Thanks,
Taylor