Re: Bloom Filters

[Jeff King <peff@peff.net>]

On Thu, Oct 11, 2018 at 08:33:58AM -0400, Derrick Stolee wrote:

> > I don't know if this is a fruitful path at all or not. I was mostly just
> > satisfying my own curiosity on the bitmap encoding question. But I'll
> > post the patches, just to show my work. The first one is the same
> > initial proof of concept I showed earlier.
> > 
> >    [1/3]: initial tree-bitmap proof of concept
> >    [2/3]: test-tree-bitmap: add "dump" mode
> >    [3/3]: test-tree-bitmap: replace ewah with custom rle encoding
> > 
> >   Makefile                    |   1 +
> >   t/helper/test-tree-bitmap.c | 344 ++++++++++++++++++++++++++++++++++++
> >   2 files changed, 345 insertions(+)
> >   create mode 100644 t/helper/test-tree-bitmap.c
> I'm trying to test this out myself, and am having trouble reverse
> engineering how I'm supposed to test it.
> 
> Looks like running "t/helper/test-tree-bitmap gen" will output a lot of
> binary data. Where should I store that? Does any file work?

Yeah, you can do:

  # optionally run with GIT_TRACE=1 to see some per-bitmap stats
  test-tree-bitmap gen >out

  # this should be roughly the same as:
  #  git rev-list --all |
  #  git diff-tree --stdin -t --name-only
  test-tree-bitmap dump <out

> Is this series just for the storage costs, assuming that we would replace
> all TREESAME checks with a query into this database? Or do you have a way to
> test how much this would improve a "git log -- <path>" query?

Right, I was just looking at storage cost here. It's not integrated with
the diff code at all. I left some hypothetical numbers elsewhere in the
thread.

-Peff