Subject: Re: [PATCH v2 09/11] commit-graph: add --changed-paths option to write subcommand

From: Jakub Narebski <[email-masked]>
Date: Thu, 20 Feb 2020 21:28:06 +0100

"Garima Singh via GitGitGadget" <[email-masked]> writes:

> From: Garima Singh <[email-masked]>
>
> Add --changed-paths option to git commit-graph write. This option will
> allow users to compute information about the paths that have changed
> between a commit and its first parent, and write it into the commit graph
> file. If the option is passed to the write subcommand we set the
> COMMIT_GRAPH_WRITE_BLOOM_FILTERS flag and pass it down to the
> commit-graph logic.

In the manpage you write that this operation (computing Bloom filters)
can take a while on large repositories.  Could you perhaps provide some
numbers: how much longer does it take to write commit-graph file with
and without '--changed-paths' for example for Linux kernel, or some
other large repository?  Thanks in advance.

>
> Helped-by: Derrick Stolee <[email-masked]>
> Signed-off-by: Garima Singh <[email-masked]>
> ---
>  Documentation/git-commit-graph.txt | 5 +++++
>  builtin/commit-graph.c             | 9 +++++++--
>  2 files changed, 12 insertions(+), 2 deletions(-)

What is missing is some sanity tests: that bloom index and bloom data
chunks are not present without '--changed-paths', and that they are
added with '--changed-paths'.

If possible, maybe also check in a separate test that the size of
bloom_index chunk agrees with the number of commits in the commit graph.

Also, we can now add those tests I have wrote about in my review of
previous patch, that is:

1. If you write commit-graph with --changed-paths, and either add some
   commits later or exclude some commits from the commit graph, then:

   a.) commit(s) in commit-graph have Bloom filter
   b.) commit(s) not in commit-graph do not have Bloom filter

2. If you write commit-graph without --changed-paths as base layer,
   and then write next layer with --changed-paths and --split, then:

   a.) commit(s) in top layer have Bloom filter(s)
   b.) commit(s) in bottom layer don't have Bloom filter(s)

>
> diff --git a/Documentation/git-commit-graph.txt b/Documentation/git-commit-graph.txt
> index bcd85c1976..907d703b30 100644
> --- a/Documentation/git-commit-graph.txt
> +++ b/Documentation/git-commit-graph.txt
> @@ -54,6 +54,11 @@ or `--stdin-packs`.)
>  With the `--append` option, include all commits that are present in the
>  existing commit-graph file.
>  +
> +With the `--changed-paths` option, compute and write information about the
> +paths changed between a commit and it's first parent. This operation can
> +take a while on large repositories. It provides significant performance gains
> +for getting history of a directory or a file with `git log -- <path>`.
> ++

Should we write about limitation that the topmost layer in the split
commit graph needs to be written with '--changed-paths' for Git to use
this information?  Or perhaps we should try (in the future) to remove
this limitation??

>  With the `--split` option, write the commit-graph as a chain of multiple
>  commit-graph files stored in `<dir>/info/commit-graphs`. The new commits
>  not already in the commit-graph are added in a new "tip" file. This file
> diff --git a/builtin/commit-graph.c b/builtin/commit-graph.c
> index e0c6fc4bbf..261dcce091 100644
> --- a/builtin/commit-graph.c
> +++ b/builtin/commit-graph.c
> @@ -9,7 +9,7 @@
>  
>  static char const * const builtin_commit_graph_usage[] = {
>  	N_("git commit-graph verify [--object-dir <objdir>] [--shallow] [--[no-]progress]"),
> -	N_("git commit-graph write [--object-dir <objdir>] [--append|--split] [--reachable|--stdin-packs|--stdin-commits] [--[no-]progress] <split options>"),
> +	N_("git commit-graph write [--object-dir <objdir>] [--append|--split] [--reachable|--stdin-packs|--stdin-commits] [--changed-paths] [--[no-]progress] <split options>"),
>  	NULL
>  };
>  
> @@ -19,7 +19,7 @@ static const char * const builtin_commit_graph_verify_usage[] = {
>  };
>  
>  static const char * const builtin_commit_graph_write_usage[] = {
> -	N_("git commit-graph write [--object-dir <objdir>] [--append|--split] [--reachable|--stdin-packs|--stdin-commits] [--[no-]progress] <split options>"),
> +	N_("git commit-graph write [--object-dir <objdir>] [--append|--split] [--reachable|--stdin-packs|--stdin-commits] [--changed-paths] [--[no-]progress] <split options>"),
>  	NULL
>  };
>

All right.

> @@ -32,6 +32,7 @@ static struct opts_commit_graph {
>  	int split;
>  	int shallow;
>  	int progress;
> +	int enable_changed_paths;

Bikeshed painting: should this field be called enable_changed_paths or
simply changed_paths?

>  } opts;
>  
>  static int graph_verify(int argc, const char **argv)
> @@ -110,6 +111,8 @@ static int graph_write(int argc, const char **argv)
>  			N_("start walk at commits listed by stdin")),
>  		OPT_BOOL(0, "append", &opts.append,
>  			N_("include all commits already in the commit-graph file")),
> +		OPT_BOOL(0, "changed-paths", &opts.enable_changed_paths,
> +			N_("enable computation for changed paths")),
>  		OPT_BOOL(0, "progress", &opts.progress, N_("force progress reporting")),
>  		OPT_BOOL(0, "split", &opts.split,
>  			N_("allow writing an incremental commit-graph file")),

All right.

> @@ -143,6 +146,8 @@ static int graph_write(int argc, const char **argv)
>  		flags |= COMMIT_GRAPH_WRITE_SPLIT;
>  	if (opts.progress)
>  		flags |= COMMIT_GRAPH_WRITE_PROGRESS;
> +	if (opts.enable_changed_paths)
> +		flags |= COMMIT_GRAPH_WRITE_BLOOM_FILTERS;
>  
>  	read_replace_refs = 0;

All right.  This actually turns on calculation Bloom filters for changed
paths, thanks to

 	ctx->changed_paths = flags & COMMIT_GRAPH_WRITE_BLOOM_FILTERS ? 1 : 0;

that was added by the "[PATCH v2 04/11] commit-graph: compute Bloom
filters for changed paths" patch.

Though... should this enabling be split into two separate patches like
this?

Best,
-- 
Jakub Narębski