Re: [PATCH] read-cache.c: index format v5 -- 30% smaller/faster than v4

[Duy Nguyen <pclouds@gmail.com>]

On Thu, Feb 14, 2019 at 5:02 PM Ævar Arnfjörð Bjarmason
<avarab@gmail.com> wrote:
> > Take a look at stat data, st_dev, st_uid, st_gid and st_mode are the
> > same most of the time. ctime should often be the same (or differs just
> > slightly). And sometimes mtime is the same as well. st_ino is also
> > always zero on Windows. We're storing a lot of duplicate values.
> >
> > Index v5 handles this
>
> This looks really promising.

I was going to reply to Junio. But it turns out I underestimated
"varint" encoding overhead and it increases read time too much. I
might get back and try some optimization when I'm bored, but until
then this is yet another failed experiment.

> > As a result of this, v5 reduces file size from 30% (git.git) to
> > 36% (webkit.git) compared to v4. Comparing to v2, webkit.git index file
> > size is reduced by 63%! A 8.4MB index file is _almost_ acceptable.
> >
> > Of course we trade off storage with cpu. We now need to spend more
> > cycles writing or even reading (but still plenty fast compared to
> > zlib). For reading, I'm counting on multi thread to hide away all this
> > even if it becomes significant.
>
> This would be a bigger change, but have we/you ever done a POC
> experiment to see how much of this time is eaten up by zlib that
> wouldn't be eaten up with some of the newer "fast but good enough"
> compression algorithms, e.g. Snappy and Zstandard?

I'm quite sure I tried zlib at some point, the only lasting impression
I have is "not good enough". Other algorithms might improve a bit,
perhaps on the uncompress/read side, but I find it unlikely we could
reasonably compress like a hundred megabytes in a few dozen
milliseconds (a quick google says Snappy compresses 250MB/s, so about
400ms per 100MB, too long). Splitting the files and compressing in
parallel might help. But I will probably focus on "sparse index"
approach before going that direction.
-- 
Duy