git/bloom.h, branch v2.45.2

bloom: encode out-of-bounds filters as non-empty

2020-09-18T04:55:50Z

When a changed-path Bloom filter has either zero, or more than a certain number (commonly 512) of entries, the commit-graph machinery encodes it as "missing". More specifically, it sets the indices adjacent in the BIDX chunk as equal to each other to indicate a "length 0" filter; that is, that the filter occupies zero bytes on disk. This has heretofore been fine, since the commit-graph machinery has no need to care about these filters with too few or too many changed paths. Both cases act like no filter has been generated at all, and so there is no need to store them. In a subsequent commit, however, the commit-graph machinery will learn to only compute Bloom filters for some commits in the current commit-graph layer. This is a change from the current implementation which computes Bloom filters for all commits that are in the layer being written. Critically for this patch, only computing some of the Bloom filters means adding a third state for length 0 Bloom filters: zero entries, too many entries, or "hasn't been computed". It will be important for that future patch to distinguish between "not representable" (i.e., zero or too-many changed paths), and "hasn't been computed". In particular, we don't want to waste time recomputing filters that have already been computed. To that end, change how we store Bloom filters in the "computed but not representable" category: - Bloom filters with no entries are stored as a single byte with all bits low (i.e., all queries to that Bloom filter will return "definitely not") - Bloom filters with too many entries are stored as a single byte with all bits set high (i.e., all queries to that Bloom filter will return "maybe"). These rules are sufficient to not incur a behavior change by changing the on-disk representation of these two classes. Likewise, no specification changes are necessary for the commit-graph format, either: - Filters that were previously empty will be recomputed and stored according to the new rules, and - old clients reading filters generated by new clients will interpret the filters correctly and be none the wiser to how they were generated. Clients will invoke the Bloom machinery in more cases than before, but this can be addressed by returning a NULL filter when all bits are set high. This can be addressed in a future patch. Note that this does increase the size of on-disk commit-graphs, but far less than other proposals. In particular, this is generally more efficient than storing a bitmap for which commits haven't computed their Bloom filters. Storing a bitmap incurs a penalty of one bit per commit, whereas storing explicit filters as above incurs a penalty of one byte per too-large or empty commit. In practice, these boundary commits likely occupy a small proportion of the overall number of commits, and so the size penalty is likely smaller than storing a bitmap for all commits. See, for example, these relative proportions of such boundary commits (collected by SZEDER Gábor): | Percentage of | commit-graph | | | commits modifying | file size | | ├────────┬──────────────┼───────────────────┤ pct. | | 0 path | >= 512 paths | before | after | change | ┌────────────────┼────────┼──────────────┼─────────┼─────────┼───────────┤ | android-base | 13.20% | 0.13% | 37.468M | 37.534M | +0.1741 % | | cmssw | 0.15% | 0.23% | 17.118M | 17.119M | +0.0091 % | | cpython | 3.07% | 0.01% | 7.967M | 7.971M | +0.0423 % | | elasticsearch | 0.70% | 1.00% | 8.833M | 8.835M | +0.0128 % | | gcc | 0.00% | 0.08% | 16.073M | 16.074M | +0.0030 % | | gecko-dev | 0.14% | 0.64% | 59.868M | 59.874M | +0.0105 % | | git | 0.11% | 0.02% | 3.895M | 3.895M | +0.0020 % | | glibc | 0.02% | 0.10% | 3.555M | 3.555M | +0.0021 % | | go | 0.00% | 0.07% | 3.186M | 3.186M | +0.0018 % | | homebrew-cask | 0.40% | 0.02% | 7.035M | 7.035M | +0.0065 % | | homebrew-core | 0.01% | 0.01% | 11.611M | 11.611M | +0.0002 % | | jdk | 0.26% | 5.64% | 5.537M | 5.540M | +0.0590 % | | linux | 0.01% | 0.51% | 63.735M | 63.740M | +0.0073 % | | llvm-project | 0.12% | 0.03% | 25.515M | 25.516M | +0.0050 % | | rails | 0.10% | 0.10% | 6.252M | 6.252M | +0.0027 % | | rust | 0.07% | 0.17% | 9.364M | 9.364M | +0.0033 % | | tensorflow | 0.09% | 1.02% | 7.009M | 7.010M | +0.0158 % | | webkit | 0.05% | 0.31% | 17.405M | 17.406M | +0.0047 % | (where the above increase is determined by computing a non-split commit-graph before and after this patch). Given that these projects are all "large" by commit count, the storage cost by writing these filters explicitly is negligible. In the most extreme example, android-base (which has 494,848 commits at the time of writing) would have its commit-graph increase by a modest 68.4 KB. Finally, a test to exercise filters which contain too many changed path entries will be introduced in a subsequent patch. Suggested-by: SZEDER Gábor Suggested-by: Jakub Narębski Helped-by: Derrick Stolee Helped-by: SZEDER Gábor Helped-by: Junio C Hamano Signed-off-by: Taylor Blau Signed-off-by: Junio C Hamano

bloom: use provided 'struct bloom_filter_settings'

2020-09-17T16:31:25Z

When 'get_or_compute_bloom_filter()' needs to compute a Bloom filter from scratch, it looks to the default 'struct bloom_filter_settings' in order to determine the maximum number of changed paths, number of bits per entry, and so on. All of these values have so far been constant, and so there was no need to pass in a pointer from the caller (eg., the one that is stored in the 'struct write_commit_graph_context'). Start passing in a 'struct bloom_filter_settings *' instead of using the default values to respect graph-specific settings (eg., in the case of setting 'GIT_TEST_BLOOM_SETTINGS_MAX_CHANGED_PATHS'). In order to have an initialized value for these settings, move its initialization to earlier in the commit-graph write. Signed-off-by: Taylor Blau Signed-off-by: Junio C Hamano

bloom: split 'get_bloom_filter()' in two

2020-09-17T16:31:25Z

'get_bloom_filter' takes a flag to control whether it will compute a Bloom filter if the requested one is missing. In the next patch, we'll add yet another parameter to this method, which would force all but one caller to specify an extra 'NULL' parameter at the end. Instead of doing this, split 'get_bloom_filter' into two functions: 'get_bloom_filter' and 'get_or_compute_bloom_filter'. The former only looks up a Bloom filter (and does not compute one if it's missing, thus dropping the 'compute_if_not_present' flag). The latter does compute missing Bloom filters, with an additional parameter to store whether or not it needed to do so. This simplifies many call-sites, since the majority of existing callers to 'get_bloom_filter' do not want missing Bloom filters to be computed (so they can drop the parameter entirely and use the simpler version of the function). While we're at it, instrument the new 'get_or_compute_bloom_filter()' with counters in the 'write_commit_graph_context' struct which store the number of filters that we did and didn't compute, as well as filters that were truncated. It would be nice to drop the 'compute_if_not_present' flag entirely, since all remaining callers of 'get_or_compute_bloom_filter' pass it as '1', but this will change in a future patch and hence cannot be removed. Signed-off-by: Taylor Blau Signed-off-by: Junio C Hamano

commit-graph.c: store maximum changed paths

2020-09-17T16:29:22Z

For now, we assume that there is a fixed constant describing the maximum number of changed paths we are willing to store in a Bloom filter. Prepare for that to (at least partially) not be the case by making it a member of the 'struct bloom_filter_settings'. This will be helpful in the subsequent patches by reducing the size of test cases that exercise storing too many changed paths, as well as preparing for an eventual future in which this value might change. This patch alone does not cause newly generated Bloom filters to use a custom upper-bound on the maximum number of changed paths a single Bloom filter can hold, that will occur in a later patch. Signed-off-by: Taylor Blau Signed-off-by: Junio C Hamano

Merge branch 'ds/line-log-on-bloom'

2020-06-09T01:06:26Z

"git log -L..." now takes advantage of the "which paths are touched by this commit?" info stored in the commit-graph system. * ds/line-log-on-bloom: line-log: integrate with changed-path Bloom filters line-log: try to use generation number-based topo-ordering line-log: more responsive, incremental 'git log -L' t4211-line-log: add tests for parent oids line-log: remove unused fields from 'struct line_log_data'

Merge branch 'ds/bloom-cleanup'

2020-05-14T21:39:44Z

Code cleanup and typofixes * ds/bloom-cleanup: completion: offer '--(no-)patch' among 'git log' options bloom: use num_changes not nr for limit detection bloom: de-duplicate directory entries Documentation: changed-path Bloom filters use byte words bloom: parse commit before computing filters test-bloom: fix usage typo bloom: fix whitespace around tab length

line-log: integrate with changed-path Bloom filters

2020-05-11T16:33:56Z

The previous changes to the line-log machinery focused on making the first result appear faster. This was achieved by no longer walking the entire commit history before returning the early results. There is still another way to improve the performance: walk most commits much faster. Let's use the changed-path Bloom filters to reduce time spent computing diffs. Since the line-log computation requires opening blobs and checking the content-diff, there is still a lot of necessary computation that cannot be replaced with changed-path Bloom filters. The part that we can reduce is most effective when checking the history of a file that is deep in several directories and those directories are modified frequently. In this case, the computation to check if a commit is TREESAME to its first parent takes a large fraction of the time. That is ripe for improvement with changed-path Bloom filters. We must ensure that prepare_to_use_bloom_filters() is called in revision.c so that the bloom_filter_settings are loaded into the struct rev_info from the commit-graph. Of course, some cases are still forbidden, but in the line-log case the pathspec is provided in a different way than normal. Since multiple paths and segments could be requested, we compute the struct bloom_key data dynamically during the commit walk. This could likely be improved, but adds code complexity that is not valuable at this time. There are two cases to care about: merge commits and "ordinary" commits. Merge commits have multiple parents, but if we are TREESAME to our first parent in every range, then pass the blame for all ranges to the first parent. Ordinary commits have the same condition, but each is done slightly differently in the process_ranges_[merge|ordinary]_commit() methods. By checking if the changed-path Bloom filter can guarantee TREESAME, we can avoid that tree-diff cost. If the filter says "probably changed", then we need to run the tree-diff and then the blob-diff if there was a real edit. The Linux kernel repository is a good testing ground for the performance improvements claimed here. There are two different cases to test. The first is the "entire history" case, where we output the entire history to /dev/null to see how long it would take to compute the full line-log history. The second is the "first result" case, where we find how long it takes to show the first value, which is an indicator of how quickly a user would see responses when waiting at a terminal. To test, I selected the paths that were changed most frequently in the top 10,000 commits using this command (stolen from StackOverflow [1]): git log --pretty=format: --name-only -n 10000 | sort | \ uniq -c | sort -rg | head -10 which results in 121 MAINTAINERS 63 fs/namei.c 60 arch/x86/kvm/cpuid.c 59 fs/io_uring.c 58 arch/x86/kvm/vmx/vmx.c 51 arch/x86/kvm/x86.c 45 arch/x86/kvm/svm.c 42 fs/btrfs/disk-io.c 42 Documentation/scsi/index.rst (along with a bogus first result). It appears that the path arch/x86/kvm/svm.c was renamed, so we ignore that entry. This leaves the following results for the real command time: | | Entire History | First Result | | Path | Before | After | Before | After | |------------------------------|--------|--------|--------|--------| | MAINTAINERS | 4.26 s | 3.87 s | 0.41 s | 0.39 s | | fs/namei.c | 1.99 s | 0.99 s | 0.42 s | 0.21 s | | arch/x86/kvm/cpuid.c | 5.28 s | 1.12 s | 0.16 s | 0.09 s | | fs/io_uring.c | 4.34 s | 0.99 s | 0.94 s | 0.27 s | | arch/x86/kvm/vmx/vmx.c | 5.01 s | 1.34 s | 0.21 s | 0.12 s | | arch/x86/kvm/x86.c | 2.24 s | 1.18 s | 0.21 s | 0.14 s | | fs/btrfs/disk-io.c | 1.82 s | 1.01 s | 0.06 s | 0.05 s | | Documentation/scsi/index.rst | 3.30 s | 0.89 s | 1.46 s | 0.03 s | It is worth noting that the least speedup comes for the MAINTAINERS file which is * edited frequently, * low in the directory heirarchy, and * quite a large file. All of those points lead to spending more time doing the blob diff and less time doing the tree diff. Still, we see some improvement in that case and significant improvement in other cases. A 2-4x speedup is likely the more typical case as opposed to the small 5% change for that file. Signed-off-by: Derrick Stolee Signed-off-by: Junio C Hamano

bloom: fix `make sparse` warning

2020-05-08T00:08:21Z

* We need a `final_new_line` to make our source code as text file, per POSIX and C specification. * `bloom_filters` should be limited to interal linkage only Signed-off-by: Đoàn Trần Công Danh Signed-off-by: Ramsay Jones Signed-off-by: Junio C Hamano

bloom: fix whitespace around tab length

2020-05-01T18:41:21Z

Fix alignment issues that were likely introduced due to an editor using tab lengths of 4 instead of 8. Signed-off-by: Derrick Stolee Signed-off-by: Junio C Hamano

revision.c: use Bloom filters to speed up path based revision walks

2020-04-06T18:08:37Z

Revision walk will now use Bloom filters for commits to speed up revision walks for a particular path (for computing history for that path), if they are present in the commit-graph file. We load the Bloom filters during the prepare_revision_walk step, currently only when dealing with a single pathspec. Extending it to work with multiple pathspecs can be explored and built on top of this series in the future. While comparing trees in rev_compare_trees(), if the Bloom filter says that the file is not different between the two trees, we don't need to compute the expensive diff. This is where we get our performance gains. The other response of the Bloom filter is '`:maybe', in which case we fall back to the full diff calculation to determine if the path was changed in the commit. We do not try to use Bloom filters when the '--walk-reflogs' option is specified. The '--walk-reflogs' option does not walk the commit ancestry chain like the rest of the options. Incorporating the performance gains when walking reflog entries would add more complexity, and can be explored in a later series. Performance Gains: We tested the performance of `git log -- ` on the git repo, the linux and some internal large repos, with a variety of paths of varying depths. On the git and linux repos: - we observed a 2x to 5x speed up. On a large internal repo with files seated 6-10 levels deep in the tree: - we observed 10x to 20x speed ups, with some paths going up to 28 times faster. Helped-by: Derrick Stolee Helped-by: Jonathan Tan Signed-off-by: Garima Singh Signed-off-by: Junio C Hamano