Re: [PATCH 5/8] commit-graph: check all leading directories in changed path Bloom filters

[Taylor Blau <me@ttaylorr.com>]

Hi Stolee,

On Mon, Jun 15, 2020 at 08:14:50PM +0000, SZEDER Gábor via GitGitGadget wrote:
> From: =?UTF-8?q?SZEDER=20G=C3=A1bor?= <szeder.dev@gmail.com>
>
> The file 'dir/subdir/file' can only be modified if its leading
> directories 'dir' and 'dir/subdir' are modified as well.
>
> So when checking modified path Bloom filters looking for commits
> modifying a path with multiple path components, then check not only
> the full path in the Bloom filters, but all its leading directories as
> well.  Take care to check these paths in "deepest first" order,
> because it's the full path that is least likely to be modified, and
> the Bloom filter queries can short circuit sooner.
>
> This can significantly reduce the average false positive rate, by
> about an order of magnitude or three(!), and can further speed up
> pathspec-limited revision walks.  The table below compares the average
> false positive rate and runtime of
>
>   git rev-list HEAD -- "$path"
>
> before and after this change for 5000+ randomly* selected paths from
> each repository:
>
>                     Average false           Average        Average
>                     positive rate           runtime        runtime
>                   before     after     before     after   difference
>   ------------------------------------------------------------------
>   git             3.220%   0.7853%     0.0558s   0.0387s   -30.6%
>   linux           2.453%   0.0296%     0.1046s   0.0766s   -26.8%
>   tensorflow      2.536%   0.6977%     0.0594s   0.0420s   -29.2%
>
> *Path selection was done with the following pipeline:
>
> 	git ls-tree -r --name-only HEAD | sort -R | head -n 5000
>
> The improvements in runtime are much smaller than the improvements in
> average false positive rate, as we are clearly reaching diminishing
> returns here.  However, all these timings depend on that accessing
> tree objects is reasonably fast (warm caches).  If we had a partial
> clone and the tree objects had to be fetched from a promisor remote,
> e.g.:
>
>   $ git clone --filter=tree:0 --bare file://.../webkit.git webkit.notrees.git
>   $ git -C webkit.git -c core.modifiedPathBloomFilters=1 \
>         commit-graph write --reachable
>   $ cp webkit.git/objects/info/commit-graph webkit.notrees.git/objects/info/
>   $ git -C webkit.notrees.git -c core.modifiedPathBloomFilters=1 \
>         rev-list HEAD -- "$path"
>
> then checking all leading path component can reduce the runtime from
> over an hour to a few seconds (and this is with the clone and the
> promisor on the same machine).
>
> This adjusts the tracing values in t4216-log-bloom.sh, which provides a
> concrete way to notice the improvement.
>
> Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
> Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
> ---
>  revision.c           | 35 ++++++++++++++++++++++++++---------
>  revision.h           |  6 ++++--
>  t/t4216-log-bloom.sh |  2 +-
>  3 files changed, 31 insertions(+), 12 deletions(-)
>
> diff --git a/revision.c b/revision.c
> index c644c660917..027ae3982b4 100644
> --- a/revision.c
> +++ b/revision.c
> @@ -670,9 +670,10 @@ static void prepare_to_use_bloom_filter(struct rev_info *revs)
>  {
>  	struct pathspec_item *pi;
>  	char *path_alloc = NULL;
> -	const char *path;
> +	const char *path, *p;
>  	int last_index;
> -	int len;
> +	size_t len;
> +	int path_component_nr = 0, j;
>
>  	if (!revs->commits)
>  		return;
> @@ -705,8 +706,22 @@ static void prepare_to_use_bloom_filter(struct rev_info *revs)
>
>  	len = strlen(path);
>
> -	revs->bloom_key = xmalloc(sizeof(struct bloom_key));
> -	fill_bloom_key(path, len, revs->bloom_key, revs->bloom_filter_settings);
> +	p = path;
> +	do {
> +		p = strchrnul(p + 1, '/');
> +		path_component_nr++;
> +	} while (p - path < len);
> +
> +	revs->bloom_keys_nr = path_component_nr;
> +	ALLOC_ARRAY(revs->bloom_keys, revs->bloom_keys_nr);
> +
> +	p = path;
> +	for (j = 0; j < revs->bloom_keys_nr; j++) {
> +		p = strchrnul(p + 1, '/');
> +
> +		fill_bloom_key(path, p - path, &revs->bloom_keys[j],
> +			       revs->bloom_filter_settings);
> +	}
>

Somewhat related to our off-list discussion yesterday, there is a bug in
both 2.27 and this patch which produces incorrect results when (1)
Bloom filters are enabled, and (2) we are doing a revision walk from
root with the pathspec '.'.

What appears to be going on is that our normalization takes '.' -> '',
and then we form a Bloom key based on the empty string, which will
return 'definitely not' when querying the Bloom filter some of the time,
which should never happen. This is a consequence of never inserting the
empty key into the Bloom filter upon generation.

As a result, I have patched this in GitHub's fork (which is currently
based on 2.27 and doesn't have these patches yet) by doing an early
return when 'strlen(path) == 0'. Since it looks like these patches are
going to land, here is some clean-up and a fix for the bug that you
should feel free to test with and apply on top:

--- >8 ---

diff --git a/revision.c b/revision.c
index 8bd383b1dd..123e72698d 100644
--- a/revision.c
+++ b/revision.c
@@ -670,10 +670,10 @@ static void prepare_to_use_bloom_filter(struct rev_info *revs)
 {
        struct pathspec_item *pi;
        char *path_alloc = NULL;
-       const char *path, *p;
+       char *path, *p;
        int last_index;
        size_t len;
-       int path_component_nr = 0, j;
+       int path_component_nr = 1, j;

        if (!revs->commits)
                return;
@@ -698,29 +698,33 @@ static void prepare_to_use_bloom_filter(struct rev_info *revs)

        /* remove single trailing slash from path, if needed */
        if (pi->match[last_index] == '/') {
-           path_alloc = xstrdup(pi->match);
-           path_alloc[last_index] = '\0';
-           path = path_alloc;
-       } else
-           path = pi->match;
+               path_alloc = xstrdup(pi->match);
+               path_alloc[last_index] = '\0';
+               path = path_alloc;
+       } else {
+               path = pi->match;
+               len = pi->len;
+       }

-       len = strlen(path);
+       if (!len)
+               return;

-       p = path;
        do {
-               p = strchrnul(p + 1, '/');
-               path_component_nr++;
-       } while (p - path < len);
+               if (is_dir_sep(*p)) {
+                       *p = '\0';
+                       path_component_nr++;
+               }
+       } while (*p++);

        revs->bloom_keys_nr = path_component_nr;
        ALLOC_ARRAY(revs->bloom_keys, revs->bloom_keys_nr);

        p = path;
        for (j = 0; j < revs->bloom_keys_nr; j++) {
-               p = strchrnul(p + 1, '/');
-
-               fill_bloom_key(path, p - path, &revs->bloom_keys[j],
+               size_t plen = strlen(p);
+               fill_bloom_key(p, plen, &revs->bloom_keys[j],
                               revs->bloom_filter_settings);
+               p += plen;
        }

        if (trace2_is_enabled() && !bloom_filter_atexit_registered) {

>  	if (trace2_is_enabled() && !bloom_filter_atexit_registered) {
>  		atexit(trace2_bloom_filter_statistics_atexit);
> @@ -720,7 +735,7 @@ static int check_maybe_different_in_bloom_filter(struct rev_info *revs,
>  						 struct commit *commit)
>  {
>  	struct bloom_filter *filter;
> -	int result;
> +	int result = 1, j;
>
>  	if (!revs->repo->objects->commit_graph)
>  		return -1;
> @@ -740,9 +755,11 @@ static int check_maybe_different_in_bloom_filter(struct rev_info *revs,
>  		return -1;
>  	}
>
> -	result = bloom_filter_contains(filter,
> -				       revs->bloom_key,
> -				       revs->bloom_filter_settings);
> +	for (j = 0; result && j < revs->bloom_keys_nr; j++) {
> +		result = bloom_filter_contains(filter,
> +					       &revs->bloom_keys[j],
> +					       revs->bloom_filter_settings);
> +	}
>
>  	if (result)
>  		count_bloom_filter_maybe++;
> @@ -782,7 +799,7 @@ static int rev_compare_tree(struct rev_info *revs,
>  			return REV_TREE_SAME;
>  	}
>
> -	if (revs->bloom_key && !nth_parent) {
> +	if (revs->bloom_keys_nr && !nth_parent) {
>  		bloom_ret = check_maybe_different_in_bloom_filter(revs, commit);
>
>  		if (bloom_ret == 0)
> diff --git a/revision.h b/revision.h
> index 7c026fe41fc..abbfb4ab59a 100644
> --- a/revision.h
> +++ b/revision.h
> @@ -295,8 +295,10 @@ struct rev_info {
>  	struct topo_walk_info *topo_walk_info;
>
>  	/* Commit graph bloom filter fields */
> -	/* The bloom filter key for the pathspec */
> -	struct bloom_key *bloom_key;
> +	/* The bloom filter key(s) for the pathspec */
> +	struct bloom_key *bloom_keys;
> +	int bloom_keys_nr;
> +
>  	/*
>  	 * The bloom filter settings used to generate the key.
>  	 * This is loaded from the commit-graph being used.
> diff --git a/t/t4216-log-bloom.sh b/t/t4216-log-bloom.sh
> index c7011f33e2c..c13b97d3bda 100755
> --- a/t/t4216-log-bloom.sh
> +++ b/t/t4216-log-bloom.sh
> @@ -142,7 +142,7 @@ test_expect_success 'setup - add commit-graph to the chain with Bloom filters' '
>
>  test_bloom_filters_used_when_some_filters_are_missing () {
>  	log_args=$1
> -	bloom_trace_prefix="statistics:{\"filter_not_present\":3,\"zero_length_filter\":0,\"maybe\":8,\"definitely_not\":6"
> +	bloom_trace_prefix="statistics:{\"filter_not_present\":3,\"zero_length_filter\":0,\"maybe\":6,\"definitely_not\":8"
>  	setup "$log_args" &&
>  	grep -q "$bloom_trace_prefix" "$TRASH_DIRECTORY/trace.perf" &&
>  	test_cmp log_wo_bloom log_w_bloom
> --
> gitgitgadget
>
Thanks,
Taylor