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@xxxxxxxxx>
>
> 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@xxxxxxxxx>
> Signed-off-by: Derrick Stolee <dstolee@xxxxxxxxxxxxx>
> ---
> 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
