Re: [PATCH 2/2] use zstd zlib wrapper

From: Stefan Beller <sbeller@google.com>
Date: Sep 15, 2016, 01:22 UTC
Message-ID: <CAGZ79kYcB-x40_w1fcWL3NSp8JU9SPrTAEiru-6Jpb7fDM1Y0w@mail.gmail.com>
In-Reply-To: <20160914235843.nacr54ekvl6rjipk@sigill.intra.peff.net>

On Wed, Sep 14, 2016 at 4:58 PM, Jeff King <peff@peff.net> wrote:

Show 29 quoted lines

> There's a fancy new compression algorithm called "zstd". The
> idea is that it's supposed to get similar compression ratios
> to zlib, but with much faster compression and decompression
> times. And on top of that, a nice sliding scale to trade off
> size versus time on the compression side.
>
> The zstd site at https://facebook.github.io/zstd/ claims
> close to 3x speedup for both compression and decompression
> versus zlib, with similar compression ratios. There are
> other fast algorithms (like lz4), but they usually compress
> much worse (follow the link above for a nice table of
> results).
>
> Since any git operations that have to access objects need to
> do a zlib inflate, in theory we can speed up everything by
> using zstd. And then on the packing side, use higher
> compression levels when making on-disk packfiles (which will
> be accessed many times) and lower ones when making loose
> objects, or deflating packed objects on the fly when serving
> fetches.
>
> The catch, of course, is that it's a new incompatible
> format. This would be a pretty huge change and totally break
> backwards compatibility for git, not just on disk but
> on-the-wire as well. So my goal here was not a finished
> product but just a quick experiment to see if it did indeed
> bring the promise speedups.
>
> Disappointingly, the answer seems to be "no".

After having looked at the data, I disagree with the conclusion. And for that I think we need to reason about the frequency of the operations happening.

* As an enduser, happily hacking away at one repository,
  I probably do not care about the pack size on disk as much
  as I care about timing of the local operations. And I assume
  that for each repack we have about 1000 reads (log/rev-list)
  The 1000 is a wild speculation without any data to back it up.
  So as an end user I'd be happy about [zstd, ~5]
  For the end user LZ4 seems to be the best solution if it were available.

* As a service provider, I know we have a lot more reads than
  writes, and repacking is annoying. Also at that scale the disk
  isn't negligible cheap. So we need to weigh the numbers differently,
  but how? I suspect depending on the weighting it could still be
  considered beneficial to go with zstd5. (No hard numbers here)