git/commit-graph.c, branch v2.29.2

Merge branch 'tb/bloom-improvements'

2020-09-29T21:01:20Z

"git commit-graph write" learned to limit the number of bloom filters that are computed from scratch with the --max-new-filters option. * tb/bloom-improvements: commit-graph: introduce 'commitGraph.maxNewFilters' builtin/commit-graph.c: introduce '--max-new-filters=' commit-graph: rename 'split_commit_graph_opts' bloom: encode out-of-bounds filters as non-empty bloom/diff: properly short-circuit on max_changes bloom: use provided 'struct bloom_filter_settings' bloom: split 'get_bloom_filter()' in two commit-graph.c: store maximum changed paths commit-graph: respect 'commitGraph.readChangedPaths' t/helper/test-read-graph.c: prepare repo settings commit-graph: pass a 'struct repository *' in more places t4216: use an '&&'-chain commit-graph: introduce 'get_bloom_filter_settings()'

Merge branch 'ds/maintenance-part-1'

2020-09-25T22:25:38Z

A "git gc"'s big brother has been introduced to take care of more repository maintenance tasks, not limited to the object database cleaning. * ds/maintenance-part-1: maintenance: add trace2 regions for task execution maintenance: add auto condition for commit-graph task maintenance: use pointers to check --auto maintenance: create maintenance..enabled config maintenance: take a lock on the objects directory maintenance: add --task option maintenance: add commit-graph task maintenance: initialize task array maintenance: replace run_auto_gc() maintenance: add --quiet option maintenance: create basic maintenance runner

builtin/commit-graph.c: introduce '--max-new-filters='

2020-09-18T17:35:39Z

Introduce a command-line flag to specify the maximum number of new Bloom filters that a 'git commit-graph write' is willing to compute from scratch. Prior to this patch, a commit-graph write with '--changed-paths' would compute Bloom filters for all selected commits which haven't already been computed (i.e., by a previous commit-graph write with '--split' such that a roll-up or replacement is performed). This behavior can cause prohibitively-long commit-graph writes for a variety of reasons: * There may be lots of filters whose diffs take a long time to generate (for example, they have close to the maximum number of changes, diffing itself takes a long time, etc). * Old-style commit-graphs (which encode filters with too many entries as not having been computed at all) cause us to waste time recomputing filters that appear to have not been computed only to discover that they are too-large. This can make the upper-bound of the time it takes for 'git commit-graph write --changed-paths' to be rather unpredictable. To make this command behave more predictably, introduce '--max-new-filters=' to allow computing at most '' Bloom filters from scratch. This lets "computing" already-known filters proceed quickly, while bounding the number of slow tasks that Git is willing to do. Helped-by: Junio C Hamano Signed-off-by: Taylor Blau Signed-off-by: Junio C Hamano

commit-graph: rename 'split_commit_graph_opts'

2020-09-18T04:55:50Z

In the subsequent commit, additional options will be added to the commit-graph API which have nothing to do with splitting. Rename the 'split_commit_graph_opts' structure to the more-generic 'commit_graph_opts' to encompass both. Likewise, rename the 'flags' member to instead be 'split_flags' to clarify that it only has to do with the behavior implied by '--split'. Suggested-by: Derrick Stolee Signed-off-by: Taylor Blau Signed-off-by: Junio C Hamano

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

maintenance: add commit-graph task

2020-09-17T18:30:05Z

The first new task in the 'git maintenance' builtin is the 'commit-graph' task. This updates the commit-graph file incrementally with the command git commit-graph write --reachable --split By writing an incremental commit-graph file using the "--split" option we minimize the disruption from this operation. The default behavior is to merge layers until the new "top" layer is less than half the size of the layer below. This provides quick writes most of the time, with the longer writes following a power law distribution. Most importantly, concurrent Git processes only look at the commit-graph-chain file for a very short amount of time, so they will verly likely not be holding a handle to the file when we try to replace it. (This only matters on Windows.) If a concurrent process reads the old commit-graph-chain file, but our job expires some of the .graph files before they can be read, then those processes will see a warning message (but not fail). This could be avoided by a future update to use the --expire-time argument when writing the commit-graph. Signed-off-by: Derrick Stolee 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

commit-graph: respect 'commitGraph.readChangedPaths'

2020-09-09T19:51:48Z

Git uses the 'core.commitGraph' configuration value to control whether or not the commit graph is used when parsing commits or performing a traversal. Now that commit-graphs can also contain a section for changed-path Bloom filters, administrators that already have commit-graphs may find it convenient to use those graphs without relying on their changed-path Bloom filters. This can happen, for example, during a staged roll-out, or in the event of an incident. Introduce 'commitGraph.readChangedPaths' to control whether or not Bloom filters are read. Note that this configuration is independent from both: - 'core.commitGraph', to allow flexibility in using all parts of a commit-graph _except_ for its Bloom filters. - The '--changed-paths' option for 'git commit-graph write', to allow reading and writing Bloom filters to be controlled independently. When the variable is set, pretend as if no Bloom data was specified at all. This avoids adding additional special-casing outside of the commit-graph internals. Suggested-by: Derrick Stolee Signed-off-by: Taylor Blau Signed-off-by: Junio C Hamano