From: Jeff King <peff@peff.net>
To: Derrick Stolee <stolee@gmail.com>
Cc: "SZEDER Gábor" <szeder.dev@gmail.com>,
"Ævar Arnfjörð Bjarmason" <avarab@gmail.com>,
"Stefan Beller" <sbeller@google.com>, git <git@vger.kernel.org>,
"Duy Nguyen" <pclouds@gmail.com>
Subject: Re: Bloom Filters (was Re: We should add a "git gc --auto" after "git clone" due to commit graph)
Date: Tue, 9 Oct 2018 17:14:50 -0400 [thread overview]
Message-ID: <20181009211449.GB9563@sigill.intra.peff.net> (raw)
In-Reply-To: <ec3ca377-0434-322e-4ab9-49e27f96f4af@gmail.com>
On Tue, Oct 09, 2018 at 03:03:08PM -0400, Derrick Stolee wrote:
> > I wonder if Roaring does better here.
>
> In these sparse cases, usually Roaring will organize the data as "array
> chunks" which are simply lists of the values. The thing that makes this
> still compressible is that we store two bytes per entry, as the entries are
> grouped by a common most-significant two bytes. SInce you say ~120k unique
> paths, the Roaring bitmap would have two or three chunks per bitmap (and
> those chunks could be empty). The overhead to store the chunk positions,
> types, and lengths does come at a cost, but it's more like 32 bytes _per
> commit_.
Hmph. It really sounds like we could do better with a custom RLE
solution. But that makes me feel like I'm missing something, because
surely I can't invent something better than the state of the art in a
simple thought experiment, right?
I know what I'm proposing would be quite bad for random access, but my
impression is that EWAH is the same. For the scale of bitmaps we're
talking about, I think linear/streaming access through the bitmap would
be OK.
> > So at any rate, I do think it would not be out of the question to store
> > bitmaps like this. I'm much more worried about the maintenance cost of
> > adding new entries incrementally. I think it's only feasible if we give
> > up sorting, and then I wonder what other problems that might cause.
> The patch below gives me a starting point to try the Bloom filter approach
> and see what the numbers are like. You did all the "git" stuff like
> computing the changed paths, so thanks!
Great, I hope it can be useful. I almost wrote it as perl consuming the
output of "log --format=%h --name-only", but realized I didn't have a
perl ewah implementation handy.
You'll probably want to tweak this part:
> > + prepare_revision_walk(&revs);
> > + while ((commit = get_revision(&revs))) {
> > + data.commit = commit;
> > + diff_tree_combined_merge(commit, 0, &revs);
> > + }
...to handle merges in a particular way. This will actually ignore
merges totally. You could add "-m" to the revision arguments to get a
per-parent diff, but of course you'd see those in your callback
individually. If you want to do _just_ the first parent diff, I think
you'll have to pick it apart manually, like:
while ((commit = get_revision(&revs))) {
struct object_id *parent_oid;
/* ignore non-first parents, but handle root commits like --root */
if (commit->parents)
parent = &commit->parents->item->object.oid;
else
parent = the_hash_algo->empty_tree;
diff_tree_oid(parent, &commit->oid, ...);
}
-Peff
next prev parent reply other threads:[~2018-10-09 21:14 UTC|newest]
Thread overview: 78+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-10-03 13:23 We should add a "git gc --auto" after "git clone" due to commit graph Ævar Arnfjörð Bjarmason
2018-10-03 13:36 ` SZEDER Gábor
2018-10-03 13:42 ` Derrick Stolee
2018-10-03 14:18 ` Ævar Arnfjörð Bjarmason
2018-10-03 14:01 ` Ævar Arnfjörð Bjarmason
2018-10-03 14:17 ` SZEDER Gábor
2018-10-03 14:22 ` Ævar Arnfjörð Bjarmason
2018-10-03 14:53 ` SZEDER Gábor
2018-10-03 15:19 ` Ævar Arnfjörð Bjarmason
2018-10-03 16:59 ` SZEDER Gábor
2018-10-05 6:09 ` Junio C Hamano
2018-10-10 22:07 ` SZEDER Gábor
2018-10-10 23:01 ` Ævar Arnfjörð Bjarmason
2018-10-03 19:08 ` Stefan Beller
2018-10-03 19:21 ` Jeff King
2018-10-03 20:35 ` Ævar Arnfjörð Bjarmason
2018-10-03 17:47 ` Stefan Beller
2018-10-03 18:47 ` Ævar Arnfjörð Bjarmason
2018-10-03 18:51 ` Jeff King
2018-10-03 18:59 ` Derrick Stolee
2018-10-03 19:18 ` Jeff King
2018-10-08 16:41 ` SZEDER Gábor
2018-10-08 16:57 ` Derrick Stolee
2018-10-08 18:10 ` SZEDER Gábor
2018-10-08 18:29 ` Derrick Stolee
2018-10-09 3:08 ` Jeff King
2018-10-09 13:48 ` Bloom Filters (was Re: We should add a "git gc --auto" after "git clone" due to commit graph) Derrick Stolee
2018-10-09 18:45 ` Ævar Arnfjörð Bjarmason
2018-10-09 18:46 ` Jeff King
2018-10-09 19:03 ` Derrick Stolee
2018-10-09 21:14 ` Jeff King [this message]
2018-10-09 23:12 ` Bloom Filters Jeff King
2018-10-09 23:13 ` [PoC -- do not apply 1/3] initial tree-bitmap proof of concept Jeff King
2018-10-09 23:14 ` [PoC -- do not apply 2/3] test-tree-bitmap: add "dump" mode Jeff King
2018-10-10 0:48 ` Junio C Hamano
2018-10-11 3:13 ` Jeff King
2018-10-09 23:14 ` [PoC -- do not apply 3/3] test-tree-bitmap: replace ewah with custom rle encoding Jeff King
2018-10-10 0:58 ` Junio C Hamano
2018-10-11 3:20 ` Jeff King
2018-10-11 12:33 ` Bloom Filters Derrick Stolee
2018-10-11 13:43 ` Jeff King
2018-10-09 21:30 ` We should add a "git gc --auto" after "git clone" due to commit graph SZEDER Gábor
2018-10-09 19:34 ` [PATCH 0/4] Bloom filter experiment SZEDER Gábor
2018-10-09 19:34 ` [PATCH 1/4] Add a (very) barebones Bloom filter implementation SZEDER Gábor
2018-10-09 19:34 ` [PATCH 2/4] commit-graph: write a Bloom filter containing changed paths for each commit SZEDER Gábor
2018-10-09 21:06 ` Jeff King
2018-10-09 21:37 ` SZEDER Gábor
2018-10-09 19:34 ` [PATCH 3/4] revision.c: use the Bloom filter to speed up path-limited revision walks SZEDER Gábor
2018-10-09 19:34 ` [PATCH 4/4] revision.c: add GIT_TRACE_BLOOM_FILTER for a bit of statistics SZEDER Gábor
2018-10-09 19:47 ` [PATCH 0/4] Bloom filter experiment Derrick Stolee
2018-10-11 1:21 ` [PATCH 0/2] Per-commit filter proof of concept Jonathan Tan
2018-10-11 1:21 ` [PATCH 1/2] One filter per commit Jonathan Tan
2018-10-11 12:49 ` Derrick Stolee
2018-10-11 19:11 ` [PATCH] Per-commit and per-parent filters for 2 parents Jonathan Tan
2018-10-11 1:21 ` [PATCH 2/2] Only make bloom filter for first parent Jonathan Tan
2018-10-11 7:37 ` [PATCH 0/2] Per-commit filter proof of concept Ævar Arnfjörð Bjarmason
2018-10-15 14:39 ` [PATCH 0/4] Bloom filter experiment Derrick Stolee
2018-10-16 4:45 ` Junio C Hamano
2018-10-16 11:13 ` Derrick Stolee
2018-10-16 12:57 ` Ævar Arnfjörð Bjarmason
2018-10-16 13:03 ` Derrick Stolee
2018-10-18 2:00 ` Junio C Hamano
2018-10-16 23:41 ` Jonathan Tan
2018-10-08 23:02 ` We should add a "git gc --auto" after "git clone" due to commit graph Junio C Hamano
2018-10-03 14:32 ` Duy Nguyen
2018-10-03 16:45 ` Duy Nguyen
2018-10-04 21:42 ` [RFC PATCH] " Ævar Arnfjörð Bjarmason
2018-10-05 12:05 ` Derrick Stolee
2018-10-05 13:05 ` Ævar Arnfjörð Bjarmason
2018-10-05 13:45 ` Derrick Stolee
2018-10-05 14:04 ` Ævar Arnfjörð Bjarmason
2018-10-05 19:21 ` Jeff King
2018-10-05 19:41 ` Derrick Stolee
2018-10-05 19:47 ` Jeff King
2018-10-05 20:00 ` Derrick Stolee
2018-10-05 20:02 ` Jeff King
2018-10-05 20:01 ` Ævar Arnfjörð Bjarmason
2018-10-05 20:09 ` Jeff King
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