Subject: Re: [PATCH 04/12] bloom: de-duplicate directory entries

From: Taylor Blau <[email-masked]>
Date: Fri, 1 May 2020 17:06:31 -0600

On Fri, May 01, 2020 at 03:30:21PM +0000, Derrick Stolee via GitGitGadget wrote:
> From: Derrick Stolee <[email-masked]>
>
> When computing a changed-path Bloom filter, we need to take the
> files that changed from the diff computation and extract the parent
> directories. That way, a directory pathspec such as "Documentation"
> could match commits that change "Documentation/git.txt".
>
> However, the current code does a poor job of this process. The paths
> are added to a hashmap, but we do not check if an entry already
> exists with that path. This can create many duplicate entries and
> cause the filter to have a much larger length than it should. This
> means that the filter is more sparse than intended, which helps the
> false positive rate, but wastes a lot of space.
>
> Properly use hashmap_get() before hashmap_add(). Also be sure to
> include a comparison function so these can be matched correctly.
>
> This has an effect on a test in t0095-bloom.sh. This makes sense,
> there are ten changes inside "smallDir" so the total number of
> paths in the filter should be 11. This would result in 11 * 10 bits
> required, and with 8 bits per byte, this results in 14 bytes.
>
> Signed-off-by: Derrick Stolee <[email-masked]>
> ---
>  bloom.c          | 35 ++++++++++++++++++++++++++---------
>  t/t0095-bloom.sh |  4 ++--
>  2 files changed, 28 insertions(+), 11 deletions(-)
>
> diff --git a/bloom.c b/bloom.c
> index 2e3e0f5037b..eb08571c628 100644
> --- a/bloom.c
> +++ b/bloom.c
> @@ -158,6 +158,19 @@ void init_bloom_filters(void)
>  	init_bloom_filter_slab(&bloom_filters);
>  }
>
> +static int pathmap_cmp(const void *hashmap_cmp_fn_data,
> +		       const struct hashmap_entry *eptr,
> +		       const struct hashmap_entry *entry_or_key,
> +		       const void *keydata)
> +{
> +	const struct pathmap_hash_entry *e1, *e2;
> +
> +	e1 = container_of(eptr, const struct pathmap_hash_entry, entry);
> +	e2 = container_of(entry_or_key, const struct pathmap_hash_entry, entry);
> +
> +	return strcmp(e1->path, e2->path);
> +}
> +
>  struct bloom_filter *get_bloom_filter(struct repository *r,
>  				      struct commit *c,
>  				      int compute_if_not_present)
> @@ -203,25 +216,29 @@ struct bloom_filter *get_bloom_filter(struct repository *r,
>  		struct hashmap pathmap;
>  		struct pathmap_hash_entry *e;
>  		struct hashmap_iter iter;
> -		hashmap_init(&pathmap, NULL, NULL, 0);
> +		hashmap_init(&pathmap, pathmap_cmp, NULL, 0);
>
>  		for (i = 0; i < diff_queued_diff.nr; i++) {
>  			const char *path = diff_queued_diff.queue[i]->two->path;
>
>  			/*
> -			* Add each leading directory of the changed file, i.e. for
> -			* 'dir/subdir/file' add 'dir' and 'dir/subdir' as well, so
> -			* the Bloom filter could be used to speed up commands like
> -			* 'git log dir/subdir', too.
> -			*
> -			* Note that directories are added without the trailing '/'.
> -			*/
> +			 * Add each leading directory of the changed file, i.e. for
> +			 * 'dir/subdir/file' add 'dir' and 'dir/subdir' as well, so
> +			 * the Bloom filter could be used to speed up commands like
> +			 * 'git log dir/subdir', too.
> +			 *
> +			 * Note that directories are added without the trailing '/'.
> +			 */
>  			do {
>  				char *last_slash = strrchr(path, '/');
>
>  				FLEX_ALLOC_STR(e, path, path);
>  				hashmap_entry_init(&e->entry, strhash(path));
> -				hashmap_add(&pathmap, &e->entry);
> +
> +				if (!hashmap_get(&pathmap, &e->entry, NULL))
> +					hashmap_add(&pathmap, &e->entry);
> +				else
> +					free(e);

Yep; this seems right on: if we have a match in the hashmap already,
there's no need to add another one. In fact, it's much better not to, as
you point out.

In the case that we didn't need to add one, we can free up the
corresponding resources (I had to double-check that 'free()' would
appropriately discard the flex array, but that's fine).

Nice find!

>  				if (!last_slash)
>  					last_slash = (char*)path;
> diff --git a/t/t0095-bloom.sh b/t/t0095-bloom.sh
> index 8f9eef116dc..6defeb544f1 100755
> --- a/t/t0095-bloom.sh
> +++ b/t/t0095-bloom.sh
> @@ -89,8 +89,8 @@ test_expect_success 'get bloom filter for commit with 10 changes' '
>  	git add smallDir &&
>  	git commit -m "commit with 10 changes" &&
>  	cat >expect <<-\EOF &&
> -	Filter_Length:25
> -	Filter_Data:82|a0|65|47|0c|92|90|c0|a1|40|02|a0|e2|40|e0|04|0a|9a|66|cf|80|19|85|42|23|
> +	Filter_Length:14
> +	Filter_Data:02|b3|c4|a0|34|e7|fe|eb|cb|47|fe|a0|e8|72|
>  	EOF
>  	test-tool bloom get_filter_for_commit "$(git rev-parse HEAD)" >actual &&
>  	test_cmp expect actual
> --
> gitgitgadget

Makes sense, I figured that there'd be a little bit of test-fallout
here.

  Reviewed-by: Taylor Blau <[email-masked]>

Thanks,
Taylor