On Fri, Jun 19, 2020 at 11:17:17AM -0600, Taylor Blau wrote:
> 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;
I should note that _this_ is the critical fix, and it should fix the bug
if you only applied just this hunk.
Everything else is purely style clean-ups on top (ranging from the four
spaces used instead of a tab, to some string processing niceties that I
_think_ should address Rene's concern, although I'm not sure if an
actual bug is lurking there or not...)
> - 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
Thanks,
Taylor
