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Re: [PATCH 0/4] Bloom filter experiment

On 10/9/2018 3:34 PM, SZEDER Gábor wrote:
To keep the ball rolling, here is my proof of concept in a somewhat
cleaned-up form, with still plenty of rough edges.

You can play around with it like this:

   $ GIT_USE_POC_BLOOM_FILTER=$((8*1024*1024*8)) git commit-graph write
   Computing commit graph generation numbers: 100% (52801/52801), done.
   Computing bloom filter: 100% (52801/52801), done.
   # Yeah, I even added progress indicator! :)
   $ GIT_TRACE_BLOOM_FILTER=2 GIT_USE_POC_BLOOM_FILTER=y git rev-list --count --full-history HEAD -- t/valgrind/valgrind.sh
   20:40:24.783699 revision.c:486          bloom filter total queries: 66095 definitely not: 64953 maybe: 1142 false positives: 256 fp ratio: 0.003873

The value of $GIT_USE_POC_BLOOM_FILTER only really matters when writing
the Bloom filter, and it specifies the number of bits in the filter's
bitmap, IOW the above command creates a 8MB Bloom filter.  To make use
of the filter the variable can be anything non-empty.

Writing the Bloom filter is very slow as it is (yeah, that's why
bothered with the progress indicator ;).  I wrote about it in patch 2's
commit message: the cause for about half of the slowness is rather
obvious, but I don't (yet) know what's responsible for the other half.

Not a single test...  but I've run loops over all files in git.git
comparing 'git rev-list HEAD -- $file's output with and without the
Bloom filter, and, surprisingly, they match.  My quick'n'dirty
experiments usually don't fare this well...

It's also available at:

   https://github.com/szeder/git bloom-filter-experiment

Thanks! I will take a close look at this tomorrow and start playing with it.