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From: "SZEDER Gábor" <szeder.dev@gmail.com>
To: Taylor Blau <me@ttaylorr.com>
Cc: git@vger.kernel.org, dstolee@microsoft.com, gitster@pobox.com,
	peff@peff.net
Subject: Re: [PATCH v2 12/13] builtin/commit-graph.c: introduce '--max-new-filters=<n>'
Date: Fri, 18 Sep 2020 11:23:24 +0200	[thread overview]
Message-ID: <20200918092036.GA24813@szeder.dev> (raw)
In-Reply-To: <ccb6482feb8d8606d82b5ab97e33184f26d6c5b6.1600279373.git.me@ttaylorr.com>

On Wed, Sep 16, 2020 at 02:08:10PM -0400, Taylor Blau wrote:
> Introduce a command-line flag to specify the maximum number of new Bloom
> filters that a 'git commit-graph write' is willing to compute from
> scratch.
> 
> Prior to this patch, a commit-graph write with '--changed-paths' would
> compute Bloom filters for all selected commits which haven't already
> been computed (i.e., by a previous commit-graph write with '--split'
> such that a roll-up or replacement is performed).
> 
> This behavior can cause prohibitively-long commit-graph writes for a
> variety of reasons:
> 
>   * There may be lots of filters whose diffs take a long time to
>     generate (for example, they have close to the maximum number of
>     changes, diffing itself takes a long time, etc).
> 
>   * Old-style commit-graphs (which encode filters with too many entries
>     as not having been computed at all) cause us to waste time
>     recomputing filters that appear to have not been computed only to
>     discover that they are too-large.
> 
> This can make the upper-bound of the time it takes for 'git commit-graph
> write --changed-paths' to be rather unpredictable.
> 
> To make this command behave more predictably, introduce
> '--max-new-filters=<n>' to allow computing at most '<n>' Bloom filters
> from scratch. This lets "computing" already-known filters proceed
> quickly, while bounding the number of slow tasks that Git is willing to
> do.
> 
> Signed-off-by: Taylor Blau <me@ttaylorr.com>
> ---
>  Documentation/git-commit-graph.txt |  5 +++
>  bloom.c                            |  7 ++-
>  builtin/commit-graph.c             | 27 +++++++++++-
>  commit-graph.c                     |  9 +++-
>  commit-graph.h                     |  1 +
>  t/t4216-log-bloom.sh               | 70 ++++++++++++++++++++++++++++++
>  6 files changed, 111 insertions(+), 8 deletions(-)
> 
> diff --git a/Documentation/git-commit-graph.txt b/Documentation/git-commit-graph.txt
> index 17405c73a9..8357846d30 100644
> --- a/Documentation/git-commit-graph.txt
> +++ b/Documentation/git-commit-graph.txt
> @@ -67,6 +67,11 @@ this option is given, future commit-graph writes will automatically assume
>  that this option was intended. Use `--no-changed-paths` to stop storing this
>  data.
>  +
> +With the `--max-new-filters=<n>` option, generate at most `n` new Bloom
> +filters (if `--changed-paths` is specified).

If '--max-new-filters=42' is specified without '--changed-paths', then
it is silently ignored instead of erroring out.  Is that intentional?

> If `n` is `-1`, no limit is
> +enforced. Commits whose filters are not calculated are stored as a
> +length zero Bloom filter.

This last sentence goes into implementation details of the file
format, so I don't think it should be included here.

What should be documented here instead is what happens later with
those commits for which no modified path Bloom filters were computed.
Will they ever be computed?  If yes, which commands will compute them
(even implicitly), and thus will run slower?  What command(s) should
the users run if they want to compute the missing filters?

Let's suppose that running

  git commit-graph write --reachable --changed-paths --max-new-filters=<L> --split

writes a commit-graph layer containing N1 commits, and N1 > L, so N1-L
commits won't have an associated modified path Bloom filter.  After
creating and fetching new commits, this command is executed again to
add information about N2 commits to the commit-graph, and N2 < L.

  - If this second invocation writes a new commit-graph layer, then it
    will only compute Bloom filters for commits in this new layer,
    even though it still has some "Bloom-computing-capacity" left.
    (This will happen even if this second write were invoked without
    '--max-new-filters'.)

  - If the merge strategy decides that, instead of writing a new
    layer, the new N2 commits should be merged with the layer
    containing the previous N1 commits, then it will compute Bloom
    filters for some or all of those N1 commits without filters,
    depending on whether N2-L is smaller or larger than N1-L.

Is this how it is supposed to work?  We can't tell, because neither
the commit messages nor the documentation updates talk about this.

> ++
>  With the `--split[=<strategy>]` option, write the commit-graph as a
>  chain of multiple commit-graph files stored in
>  `<dir>/info/commit-graphs`. Commit-graph layers are merged based on the

  reply	other threads:[~2020-09-18  9:23 UTC|newest]

Thread overview: 75+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-09-09 15:22 [PATCH 00/12] more miscellaneous Bloom filter improvements, redux Taylor Blau
2020-09-09 15:22 ` [PATCH 01/12] commit-graph: introduce 'get_bloom_filter_settings()' Taylor Blau
2020-09-09 15:22 ` [PATCH 02/12] t4216: use an '&&'-chain Taylor Blau
2020-09-09 15:22 ` [PATCH 03/12] commit-graph: pass a 'struct repository *' in more places Taylor Blau
2020-09-09 15:23 ` [PATCH 04/12] t/helper/test-read-graph.c: prepare repo settings Taylor Blau
2020-09-09 15:23 ` [PATCH 05/12] commit-graph: respect 'commitGraph.readChangedPaths' Taylor Blau
2020-09-09 15:23 ` [PATCH 06/12] commit-graph.c: store maximum changed paths Taylor Blau
2020-09-09 15:23 ` [PATCH 07/12] bloom: split 'get_bloom_filter()' in two Taylor Blau
2020-09-09 15:23 ` [PATCH 08/12] bloom: use provided 'struct bloom_filter_settings' Taylor Blau
2020-09-09 15:23 ` [PATCH 09/12] bloom/diff: properly short-circuit on max_changes Taylor Blau
2020-09-09 15:23 ` [PATCH 10/12] bloom: encode out-of-bounds filters as non-empty Taylor Blau
2020-09-10  3:35   ` Taylor Blau
2020-09-10 15:45     ` Taylor Blau
2020-09-11 18:15       ` Derrick Stolee
2020-09-09 15:23 ` [PATCH 11/12] commit-graph: rename 'split_commit_graph_opts' Taylor Blau
2020-09-09 15:24 ` [PATCH 12/12] builtin/commit-graph.c: introduce '--max-new-filters=<n>' Taylor Blau
2020-09-11 17:52   ` Jeff King
2020-09-11 18:59     ` Taylor Blau
2020-09-11 19:25       ` Taylor Blau
2020-09-14 20:12         ` Taylor Blau
2020-09-14 20:31           ` Derrick Stolee
2020-09-14 20:36             ` Taylor Blau
2020-09-15  0:59               ` Derrick Stolee
2020-09-15  4:31                 ` Taylor Blau
2020-09-15 21:49               ` Junio C Hamano
2020-09-15 21:53                 ` Taylor Blau
2020-09-11 19:47       ` Jeff King
2020-09-11 19:31     ` Junio C Hamano
2020-09-16 18:06 ` [PATCH v2 00/13] more miscellaneous Bloom filter improvements, redux Taylor Blau
2020-09-16 18:06   ` [PATCH v2 01/13] commit-graph: introduce 'get_bloom_filter_settings()' Taylor Blau
2020-09-16 18:07   ` [PATCH v2 02/13] t4216: use an '&&'-chain Taylor Blau
2020-09-16 18:07   ` [PATCH v2 03/13] commit-graph: pass a 'struct repository *' in more places Taylor Blau
2020-09-16 18:07   ` [PATCH v2 04/13] t/helper/test-read-graph.c: prepare repo settings Taylor Blau
2020-09-16 18:07   ` [PATCH v2 05/13] commit-graph: respect 'commitGraph.readChangedPaths' Taylor Blau
2020-09-16 18:07   ` [PATCH v2 06/13] commit-graph.c: store maximum changed paths Taylor Blau
2020-09-16 18:07   ` [PATCH v2 07/13] bloom: split 'get_bloom_filter()' in two Taylor Blau
2020-09-16 18:07   ` [PATCH v2 08/13] bloom: use provided 'struct bloom_filter_settings' Taylor Blau
2020-09-16 18:07   ` [PATCH v2 09/13] bloom/diff: properly short-circuit on max_changes Taylor Blau
2020-09-16 18:07   ` [PATCH v2 10/13] bloom: encode out-of-bounds filters as non-empty Taylor Blau
2020-09-17 22:13     ` SZEDER Gábor
2020-09-17 23:13       ` Taylor Blau
2020-09-18  0:52         ` Junio C Hamano
2020-09-18  1:15           ` Taylor Blau
2020-09-16 18:08   ` [PATCH v2 11/13] commit-graph: rename 'split_commit_graph_opts' Taylor Blau
2020-09-16 18:08   ` [PATCH v2 12/13] builtin/commit-graph.c: introduce '--max-new-filters=<n>' Taylor Blau
2020-09-18  9:23     ` SZEDER Gábor [this message]
2020-09-18 13:27       ` Taylor Blau
2020-09-16 18:08   ` [PATCH v2 13/13] commit-graph: introduce 'commitGraph.maxNewFilters' Taylor Blau
2020-09-16 22:51   ` [PATCH v2 00/13] more miscellaneous Bloom filter improvements, redux Derrick Stolee
2020-09-16 23:07     ` Junio C Hamano
2020-09-17  0:45       ` Taylor Blau
2020-09-17  0:59         ` Junio C Hamano
2020-09-17  1:10           ` Taylor Blau
2020-09-17 13:34             ` Taylor Blau
2020-09-17 13:38               ` Derrick Stolee
2020-09-18  2:58 ` [PATCH v3 " Taylor Blau
2020-09-18  2:58   ` [PATCH v3 01/13] commit-graph: introduce 'get_bloom_filter_settings()' Taylor Blau
2020-09-18  2:58   ` [PATCH v3 02/13] t4216: use an '&&'-chain Taylor Blau
2020-09-18  2:59   ` [PATCH v3 03/13] commit-graph: pass a 'struct repository *' in more places Taylor Blau
2020-09-18  2:59   ` [PATCH v3 04/13] t/helper/test-read-graph.c: prepare repo settings Taylor Blau
2020-09-18  2:59   ` [PATCH v3 05/13] commit-graph: respect 'commitGraph.readChangedPaths' Taylor Blau
2020-09-18  2:59   ` [PATCH v3 06/13] commit-graph.c: store maximum changed paths Taylor Blau
2020-09-18  2:59   ` [PATCH v3 07/13] bloom: split 'get_bloom_filter()' in two Taylor Blau
2020-09-18  2:59   ` [PATCH v3 08/13] bloom: use provided 'struct bloom_filter_settings' Taylor Blau
2020-09-18 16:27     ` SZEDER Gábor
2020-09-18 16:32       ` Taylor Blau
2020-09-18  2:59   ` [PATCH v3 09/13] bloom/diff: properly short-circuit on max_changes Taylor Blau
2020-09-18  2:59   ` [PATCH v3 10/13] bloom: encode out-of-bounds filters as non-empty Taylor Blau
2020-09-18  2:59   ` [PATCH v3 11/13] commit-graph: rename 'split_commit_graph_opts' Taylor Blau
2020-09-18  2:59   ` [PATCH v3 12/13] builtin/commit-graph.c: introduce '--max-new-filters=<n>' Taylor Blau
2020-09-18  2:59   ` [PATCH v3 13/13] commit-graph: introduce 'commitGraph.maxNewFilters' Taylor Blau
2020-09-18 13:29     ` Taylor Blau
2020-09-18 17:43       ` Junio C Hamano
2020-09-18 13:31   ` [PATCH v3 00/13] more miscellaneous Bloom filter improvements, redux Taylor Blau
2020-09-18 13:34     ` Taylor Blau

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