On 6/25/2020 3:25 AM, René Scharfe wrote:
> Am 23.06.20 um 19:47 schrieb SZEDER Gábor via GitGitGadget:
>> 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.
>>
>> Helped-by: Taylor Blau <me@xxxxxxxxxxxx>
>> Helped-by: René Scharfe <l.s.r@xxxxxx>
>> Signed-off-by: SZEDER Gábor <szeder.dev@xxxxxxxxx>
>> Signed-off-by: Derrick Stolee <dstolee@xxxxxxxxxxxxx>
>> ---
>> revision.c | 41 ++++++++++++++++++++++++++++++++---------
>> revision.h | 6 ++++--
>> t/t4216-log-bloom.sh | 2 +-
>> 3 files changed, 37 insertions(+), 12 deletions(-)
>>
>> diff --git a/revision.c b/revision.c
>> index b53377cd52..077888ee51 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 = 1;
>>
>> if (!revs->commits)
>> return;
>> @@ -709,8 +710,28 @@ static void prepare_to_use_bloom_filter(struct rev_info *revs)
>> return;
>> }
>>
>> - revs->bloom_key = xmalloc(sizeof(struct bloom_key));
>> - fill_bloom_key(path, len, revs->bloom_key, revs->bloom_filter_settings);
>> + p = path;
>> + while (*p) {
>> + if (is_dir_sep(*p))
>> + path_component_nr++;
>> + p++;
>> + }
>> +
>> + revs->bloom_keys_nr = path_component_nr;
>> + ALLOC_ARRAY(revs->bloom_keys, revs->bloom_keys_nr);
>> +
>> + fill_bloom_key(path, len, &revs->bloom_keys[0],
>> + revs->bloom_filter_settings);
>> + path_component_nr = 1;
>> +
>> + p = path + len - 1;
>
> len cannot be 0 at this point, as patch 9 made sure, so this is safe.
> Good.
>
>> + while (p > path) {
>> + if (is_dir_sep(*p))
>> + fill_bloom_key(path, p - path,
>> + &revs->bloom_keys[path_component_nr++],
>> + revs->bloom_filter_settings);
>> + p--;
>> + }
>
> This walks the directory hierarchy upwards and adds bloom filters for
> shorter and shorter paths, ("deepest first"). Good.
>
> And it supports all directory separators. On Windows that would be
> slash (/) and backslash (\). I assume paths are normalized to use
> only slashes when bloom filters are written, correct? Then the lookup
> side needs to normalize a given path to only use slashes as well,
> otherwise paths with backslashes cannot be found. This part seems to
> be missing.
Yes, that's a good point. We _require_ the paths to be normalized
here to be Unix-style paths or else the Bloom filter keys are
incorrect. Thankfully, they are. Let's make that clear in-code by
using '/' instead of is_dir_sep().
Thanks,
-Stolee
