Subject: Re: [PATCH 0/4] Bloom filter experiment

From: Derrick Stolee <[email-masked]>
Date: Tue, 16 Oct 2018 09:03:14 -0400

On 10/16/2018 8:57 AM, Ævar Arnfjörð Bjarmason wrote:
> On Tue, Oct 16 2018, Derrick Stolee wrote:
>
>> On 10/16/2018 12:45 AM, Junio C Hamano wrote:
>>> Derrick Stolee <[email-masked]> writes:
>>>
>>>> 2. The filters are sized according to the number of changes in each
>>>> commit, with a minimum of one 64-bit word.
>>>> ...
>>>> 6. When we compute the Bloom filters, we don't store a filter for
>>>> commits whose first-parent diff has more than 512 paths.
>>> Just being curious but was 512 taken out of thin air or is there
>>> some math behind it, e.g. to limit false positive rate down to
>>> certain threshold?  With a wide-enough bitset, you could store
>>> arbitrary large number of paths with low enough false positive, I
>>> guess, but is there a point where there is too many paths in the
>>> change that gives us diminishing returns and not worth having a
>>> filter in the first place?
>> 512 is somewhat arbitrary, but having a maximum size is not.
>>> In a normal source-code-control context, the set of paths modified
>>> by any single commit ought to be a small subset of the entire paths,
>>> and whole-tree changes ought to be fairly rare.  In a project for
>>> which that assumption does not hold, it might help to have a
>>> negative bloom filter (i.e. "git log -- A" asks "does the commit
>>> modify A?" and the filter would say "we know it does not, because we
>>> threw all the paths that are not touched to the bloom filter"), but
>>> I think that would optimize for a wrong case.
>> A commit with many changed paths is very rare. The 512 I picked above
>> is enough to cover 99% of commits in each of the repos I sampled when
>> first investigating Bloom filters.
>>
>> When a Bloom filter response says "maybe yes" (in our case, "maybe not
>> TREESAME"), then we need to verify that it is correct. In the extreme
>> case that every path is changed, then the Bloom filter does nothing
>> but add extra work.
>>
>> These extreme cases are also not unprecedented: in our Azure Repos
>> codebase, we were using core.autocrlf to smudge CRLFs to LFs, but
>> when it was time to dogfood VFS for Git, we needed to turn off the
>> smudge filter. So, there is one commit that converts every LF to a
>> CRLF in every text file. Storing a Bloom filter for those ~250,000
>> entries would take ~256KB for essentially no value. By not storing a
>> filter for this commit, we go immediately to the regular TREESAME
>> check, which would happen for most pathspecs.
>>
>> This is all to say: having a maximum size is good. 512 is big enough
>> to cover _most_ commits, but not so big that we may store _really_ big
>> filters.
> Makes sense. 512 is good enough to hardcode initially, but I couldn't
> tell from briefly skimming the patches if it was possible to make this
> size dynamic per-repo when the graph/filter is written.
My proof-of-concept has it as a constant, but part of my plan is to make 
these all config options, as of this item in my message:

 >>> 2. We need config values for writing and consuming bloom filters, 
but also to override the default settings.

> I.e. we might later add some discovery step where we look at N number of
> commits at random, until we're satisfied that we've come up with some
> average/median number of total (recursive) tree entries & how many tend
> to be changed per-commit.
>
> I.e. I can imagine repositories (with some automated changes) where we
> have 10k files and tend to change 1k per commit, or ones with 10k files
> where we tend to change just 1-10 per commit, which would mean a
> larger/smaller filter would be needed / would do.
I'm not sure a dynamic approach would be worth the effort, but I'm open 
to hearing the results of an experiment.

Thanks,
-Stolee