treemanifest: use visitchildrenset when filtering a manifest to a matcher Differential Revision: https://phab.mercurial-scm.org/D4370

tests: stabilize test-no-symlink This goes with 89630d0b3e23.

shelve: use the internal phase when possible If the repository support it, use the internal phase for all changesets created by shelve.

treemanifest: avoid loading everything just to get their nodeid Differential Revision: https://phab.mercurial-scm.org/D4369

treemanifest: avoid unnecessary copies/processing when using alwaysmatcher Differential Revision: https://phab.mercurial-scm.org/D4368

treemanifest: attempt to avoid loading all lazily-loaded subdirs in _isempty Differential Revision: https://phab.mercurial-scm.org/D4367

treemanifest: introduce lazy loading of subdirs An earlier patch series made it so that what to load was up to the calling code, which works fine until manifests are copied - when they're copied, they're loaded completely and thus we lose the entire benefit. By lazy loading everything, we can avoid having to pass in the matcher to ~every manifest function, and handle copies correctly as well. This changeset doesn't go as far as it could with loading only the necessary subsets, that will happen in later changes in this series; at the moment, except in a few situations, we just load everything the moment we want to interact with treemanifest._dirs. This is thus most likely to be a small slowdown if treemanifests is in use regardless of whether narrow is in use, but hopefully easier to verify correctness and review. This is part of a series of speedups, it is not expected to produce any real speed improvements itself, but the numbers show that it doesn't produce a large speed penalty in any common case, and for the cases it does provide a penalty in, it is not a large absolute amount (even if it is a large percentage amount). Timing numbers according to command: hyperfine --prepare <preparation_script> 'hg status' HGRCPATH points to a file with the following contents: [extensions] narrow = strip = rebase = mozilla-unified (called m-u below) was at revision #468856. regular hash: eb39298e432d treemanifests hash: 0553b7f29eaf large-dir-repo (called l-d-r below) was generated with the following script: #!/bin/bash hg init large-dir-repo mkdir -p large-dir-repo/third_party/rust/log touch large-dir-repo/third_party/rust/log/foo.txt for i in $(seq 1 30000); do d=$(mktemp -d large-dir-repo/third_party/XXXXXXXXX) touch $d/file.txt done hg -R large-dir-repo ci -Am 'rev0' --user test --date '0 0' echo hi > large-dir-repo/third_party/rust/log/bar.txt hg -R large-dir-repo ci -Am 'rev1' --user test --date '0 0' echo hi > large-dir-repo/third_party/rust/log/baz.txt hg -R large-dir-repo ci -Am 'rev2' --user test --date '0 0' for the repos that use narrow, the narrowspec was this: [include] rootfilesin:accessible/jsat rootfilesin:accessible/tests/mochitest/jsat rootfilesin:mobile/android/chrome/content rootfilesin:mobile/android/modules/geckoview rootfilesin:third_party/rust/log [exclude] This narrowspec was chosen due to the size of the third_party/rust directory (this directory was *not* modified in revision #468856 in mozilla-unified), plus all the directories that *were* modified in revision #468856 of mozilla-unified. Importantly, when using narrow, these repos had everything checked out (in the case of large-dir-repo, that means all 30,001 directories), *before* adding the narrowspec. This is to simulate the behavior when using a virtual filesystem that shows everything for the user even if they haven't added it to the narrowspec yet. This is not a supported configuration, and `hg update` and `hg rebase` will not really do the "correct" thing if there are mutations outside of the narrowspec (which is not the case in these tests, due to a carefully crafted narrowspec), but non-mutating commands should behave correctly. I'm not claiming anything less than a 5% speed win as improvements due to this change; these are probably eiter measurement artifacts or constant time improvements. The numbers that aren't changing are shown primarily to prove that this doesn't make anything worse in any case I plan on testing during this series. 'before' is hg from commit 6268fed3 'N' indicates narrow in use 'T' indicates treemanifest in use Please note that these commands and the narrowspec are a little different than the ones in a similar table that I made in a3cabe9415e1. Important: it is my understanding that these numbers below are *not super reliable*, the large slowdowns may be artifacts of some odd interaction between GC and python module/code complexity. Another changeset of mine (D4351) had shown large timing differences when ~empty, uncalled functions were added to match.py, though only when using --color=never or redirecting to /dev/null. We seem to be on some cusp of complexity or code size that is causing, at my best guess (according to linux `perf` benchmarks) GC to alter behavior and cause a 200-400ms difference in timings. I haven't had a chance to replicate these results on another machine. diff --git: repo | N | T | before (mean +- stdev) | after (mean +- stdev) | % of before ------+---+---+------------------------+-----------------------+------------ m-u | | | 1.580 s +- 0.034 s | 1.576 s +- 0.022 s | 99.7% m-u | | x | 1.568 s +- 0.025 s | 1.584 s +- 0.044 s | 101.0% m-u | x | | 1.569 s +- 0.031 s | 1.554 s +- 0.025 s | 99.0% m-u | x | x | 107.3 ms +- 1.6 ms | 106.3 ms +- 1.5 ms | 99.1% l-d-r | | | 232.5 ms +- 5.9 ms | 233.5 ms +- 5.3 ms | 100.4% l-d-r | | x | 236.6 ms +- 6.3 ms | 233.6 ms +- 7.0 ms | 98.7% l-d-r | x | | 118.4 ms +- 2.1 ms | 118.4 ms +- 1.4 ms | 100.0% l-d-r | x | x | 116.8 ms +- 1.5 ms | 118.9 ms +- 1.6 ms | 101.8% diff -c . --git: repo | N | T | before (mean +- stdev) | after (mean +- stdev) | % of before ------+---+---+------------------------+-----------------------+------------ m-u | | | 354.4 ms +- 16.6 ms | 351.0 ms +- 6.9 ms | 99.0% m-u | | x | 207.2 ms +- 3.0 ms | 206.2 ms +- 2.7 ms | 99.5% m-u | x | | 422.0 ms +- 26.0 ms | 351.2 ms +- 6.4 ms | 83.2% <-- m-u | x | x | 166.7 ms +- 2.1 ms | 169.5 ms +- 4.1 ms | 101.7% l-d-r | | | 98.4 ms +- 4.5 ms | 98.5 ms +- 2.1 ms | 100.1% l-d-r | | x | 5.519 s +- 0.060 s | 5.149 s +- 0.042 s | 93.3% <-- l-d-r | x | | 99.1 ms +- 3.2 ms | 102.6 ms +- 9.7 ms | 103.5% <--? l-d-r | x | x | 994.9 ms +- 10.7 ms | 1.026 s +- 0.012 s | 103.1% <--? rebase -r . --keep -d .^^: repo | N | T | before (mean +- stdev) | after (mean +- stdev) | % of before ------+---+---+------------------------+-----------------------+------------ m-u | | | 6.639 s +- 0.168 s | 6.559 s +- 0.097 s | 98.8% m-u | | x | 6.601 s +- 0.143 s | 6.640 s +- 0.207 s | 100.6% m-u | x | | 6.582 s +- 0.098 s | 6.543 s +- 0.098 s | 99.4% m-u | x | x | 678.4 ms +- 57.7 ms | 703.7 ms +- 52.4 ms | 103.7% <--? l-d-r | | | 780.0 ms +- 23.9 ms | 776.0 ms +- 12.6 ms | 99.5% l-d-r | | x | 7.520 s +- 0.255 s | 7.395 s +- 0.044 s | 98.3% l-d-r | x | | 331.9 ms +- 16.5 ms | 327.0 ms +- 3.4 ms | 98.5% l-d-r | x | x | 6.228 s +- 0.113 s | 5.924 s +- 0.044 s | 95.1% status --change . --copies: repo | N | T | before (mean +- stdev) | after (mean +- stdev) | % of before ------+---+---+------------------------+-----------------------+------------ m-u | | | 330.8 ms +- 7.2 ms | 329.0 ms +- 7.1 ms | 99.5% m-u | | x | 182.9 ms +- 2.7 ms | 183.5 ms +- 2.7 ms | 100.3% m-u | x | | 330.0 ms +- 7.6 ms | 327.1 ms +- 5.4 ms | 99.1% m-u | x | x | 146.2 ms +- 2.4 ms | 147.1 ms +- 1.3 ms | 100.6% l-d-r | | | 95.3 ms +- 1.4 ms | 95.9 ms +- 1.5 ms | 100.6% l-d-r | | x | 5.157 s +- 0.035 s | 5.166 s +- 0.058 s | 100.2% l-d-r | x | | 99.7 ms +- 3.0 ms | 100.2 ms +- 4.4 ms | 100.5% l-d-r | x | x | 993.6 ms +- 13.1 ms | 1.025 s +- 0.015 s | 103.2% <--? status --copies: repo | N | T | before (mean +- stdev) | after (mean +- stdev) | % of before ------+---+---+------------------------+-----------------------+------------ m-u | | | 2.348 s +- 0.031 s | 2.329 s +- 0.019 s | 99.2% m-u | | x | 2.337 s +- 0.026 s | 2.346 s +- 0.034 s | 100.4% m-u | x | | 2.354 s +- 0.015 s | 2.342 s +- 0.021 s | 99.5% m-u | x | x | 120.6 ms +- 4.3 ms | 119.2 ms +- 2.1 ms | 98.8% l-d-r | | | 731.5 ms +- 11.1 ms | 719.6 ms +- 9.8 ms | 98.4% l-d-r | | x | 729.0 ms +- 15.5 ms | 725.7 ms +- 10.6 ms | 99.5% l-d-r | x | | 211.0 ms +- 3.9 ms | 212.8 ms +- 3.7 ms | 100.9% l-d-r | x | x | 211.5 ms +- 4.2 ms | 211.0 ms +- 3.3 ms | 99.8% update $rev^; ~/src/hg/hg{hg}/hg update $rev: repo | N | T | before (mean +- stdev) | after (mean +- stdev) | % of before ------+---+---+------------------------+-----------------------+------------ m-u | | | 3.910 s +- 0.055 s | 3.920 s +- 0.075 s | 100.3% m-u | | x | 3.613 s +- 0.056 s | 3.630 s +- 0.056 s | 100.5% m-u | x | | 3.873 s +- 0.055 s | 3.864 s +- 0.049 s | 99.8% m-u | x | x | 400.4 ms +- 7.4 ms | 403.6 ms +- 5.0 ms | 100.8% l-d-r | | | 531.6 ms +- 10.0 ms | 528.8 ms +- 9.6 ms | 99.5% l-d-r | | x | 10.377 s +- 0.049 s | 9.955 s +- 0.046 s | 95.9% l-d-r | x | | 308.3 ms +- 4.4 ms | 306.8 ms +- 3.7 ms | 99.5% l-d-r | x | x | 1.805 s +- 0.015 s | 1.834 s +- 0.020 s | 101.6% Differential Revision: https://phab.mercurial-scm.org/D4366

contrib: use a monotonic timer in catapipe As spotted by Gregory, we should use a monotonic timer to get better timings. Differential Revision: https://phab.mercurial-scm.org/D4516

contrib: fix catapipe output argument documentation Differential Revision: https://phab.mercurial-scm.org/D4515

tracing: trace command function execution Differential Revision: https://phab.mercurial-scm.org/D4514

extension: add a summary of total loading time per extension Differential Revision: https://phab.mercurial-scm.org/D4513

extensions: trace the total time of running all reposetup callbacks Differential Revision: https://phab.mercurial-scm.org/D4512

extensions: trace the total time of running all extsetup callbacks Differential Revision: https://phab.mercurial-scm.org/D4511

extensions: trace the total time of running all uisetup callbacks Differential Revision: https://phab.mercurial-scm.org/D4510

extensions: add timing for extensions reposetup Differential Revision: https://phab.mercurial-scm.org/D4509

sparse-revlog: set max delta chain length to on thousand The new snapshot system used in the sparse-revlog case gave us some small size benefit so far. However its most important property is to gracefully handle harder limit on delta chainlength. Long delta chain has a very detrimental impact on read (and write) performance in revlog. Being able to shorter them provide a great boost. However, shorting delta used to result significantly lower compression ratio. The intermediate snapshots effectively suppress most of this effect (even all in some case). # Effect on the test repository The repository we use for test is not "realistic" but can still show this in action using an unreasonably low chain limit. Limiting the chain length show a sizeable increase but stay under control: +6% for limit=15; +15% for limit=10. Without the snapshot system the increase is significantly bigger: +45% for limit=15; +80% for limit=10. Even slightly larger than without delta chain limit, the resulting size is still smaller than before we started doing snapshots. Here is a table for comparison. *Since the repository is not branchy, the initial sparse-revlog version does not bring much benefit compare to the non-sparse one): chain length limit | none | limit=15 | limit=10 | without sparse-revlog | 62 818 987 | 112 664 615 | 131 222 574 | without snapshot | 74 365 490 | 108 211 410 | 133 857 764 | with snapshot | 59 230 936 | 63 002 924 | 68 415 329 | # Effect On Real Life Repositories The series provides significant benefits on all kind of repositories. Using `hg debugupgraderepo -o redeltaparent --run`, we recomputed delta chain for various repositories with different settings: - delta chain length: unlimited or 1000 limit - sparse-revlog: enabled or disabled - this series: applied or not applied We can observe multiple types of effect: - On very branchy repositories: * The delta chain limit as low impact on the repo size. * Intermediate snapshot greatly reduces manifest size: - pypy: -80% - netbeans: -95% * The delta chain limit is effective, without a size impact: - netbeans average: 613 -> 282 - private #1 average: 1 068 -> 307 - On more linear repository: * Intermediate snapshot limit the impact of delta chain limit: - mozilla: without the series: +360% with the series: +25% * The delta chain limit provides large improvement: - mozilla's average chain length: unlimited: 15 338 limited: 469 * Despite the chain length limit, the manifest size is reduced: - mercurial: -25% - mozilla: -30% It is clear that the use of chains of intermediate snapshots provide large benefits both in storage size and delta chains quality. We should now switch our effort toward making sure the write performance are acceptable. Then, `sparse-revlog` will be a suitable format for all new repository. # Raw Statistic * no-sparse: general delta repository not using sparse-revlog * no-snapshot: sparse-revlog repository not using this series * snapshot: sparse-revlog repository using this series mercurial Manifest Size: limit | none | 1000 ------------|-------------|------------ no-sparse | 8 021 373 | 8 199 366 no-snapshot | 8 103 561 | 8 259 719 snapshot | 6 137 116 | 6 126 433 Manifest Chain length data limit || none || 1000 || value || average | max || average | max || ------------||---------|---------||---------|---------|| no-sparse || 307 | 1456 || 279 | 1000 || no-snapshot || 312 | 1456 || 283 | 1000 || snapshot || 248 | 1208 || 241 | 1000 || Full Store Size limit | none | 1000 ------------|-------------|------------ no-sparse | 51 013 198 | 51 201 574 no-snapshot | 50 930 795 | 51 141 006 snapshot | 48 072 037 | 48 093 572 pypy Manifest Size: limit | none | 1000 ------------|-------------|------------ no-sparse | 193 987 784 | 193 987 784 no-snapshot | 163 171 745 | 163 312 229 snapshot | 34 605 900 | 34 600 750 Manifest Chain length data limit || none || 1000 || value || average | max || average | max || ------------||---------|---------||---------|---------|| no-sparse || 101 | 692 || 101 | 692 || no-snapshot || 151 | 1307 || 148 | 1000 || snapshot || 128 | 1309 || 125 | 1000 || Full Store Size limit | none | 1000 ------------|-------------|------------ no-sparse | 495 931 473 | 495 931 473 no-snapshot | 465 441 017 | 465 581 501 snapshot | 355 467 301 | 355 472 451 Mozilla Manifest Size: limit | none | 1000 ------------|----------------|--------------- no-sparse | 416 757 148 | 1 869 009 668 no-snapshot | 401 592 370 | 1 843 493 795 snapshot | 224 359 521 | 284 615 500 Manifest Chain length data limit || none || 1000 || value || average | max || average | max || ------------||---------|---------||---------|---------|| no-sparse || 15 333 | 58 980 || 468 | 1 000 || no-snapshot || 15 336 | 58 980 || 469 | 1 000 || snapshot || 15 338 | 58 983 || 469 | 1 000 || Full Store Size limit | none | 1000 ------------|----------------|--------------- no-sparse | 2 712 477 887 | 4 164 995 451 no-snapshot | 2 698 887 835 | 4 141 054 304 snapshot | 2 518 130 385 | 2 578 587 596 Netbeans Manifest Size: limit | none | 1000 ------------|----------------|--------------- no-sparse | 4 766 794 101 | 4 870 642 687 no-snapshot | 4 334 806 082 | 4 428 681 309 snapshot | 232 659 666 | 240 330 665 Manifest Chain length data limit || none || 1000 || value || average | max || average | max || ------------||---------|---------||---------|---------|| no-sparse || 597 | 6802 || 254 | 1 000 || no-snapshot || 648 | 6 802 || 305 | 1 000 || snapshot || 613 | 6 804 || 282 | 1 000 || Full Store Size limit | none | 1000 ------------|----------------|--------------- no-sparse | 5 807 347 998 | 5 911 196 584 no-snapshot | 5 375 398 602 | 5 469 273 829 snapshot | 1 282 519 928 | 1 290 190 927 Private repo #1 Manifest Size: limit | none | 1000 ------------|-----------------|--------------- no-sparse | 41 389 010 840 | 41 398 162 091 no-snapshot | 9 737 319 435 | 10 223 773 150 snapshot | 744 215 807 | 747 961 822 Manifest Chain length data limit || none || 1000 || value || average | max || average | max || ------------||---------|---------||---------|---------|| no-sparse || 245 | 8 885 || 81 | 1 000 || no-snapshot || 1 225 | 8 885 || 336 | 1 000 || snapshot || 1 068 | 7 909 || 307 | 1 000 || Full Store Size limit | none | 1000 ------------|----------------|--------------- no-sparse | 49 646 065 126 | 49 655 216 377 no-snapshot | 17 924 862 856 | 18 411 316 571 snapshot | 9 009 024 710 | 9 012 770 725 Private repo #2 We currently have less data available for that repository. * Before is a sparse-revlog repository without this series * After is a sparse-revlog repository with this series + 1000 chain limit Manifest Size: Before: 1 531 485 040 bytes After: 1 091 422 451 bytes Manifest Chain: Before: 2 218 avg; 6 575 Max After: 442 avg; 1 000 Max Full Store Size Before: 15 203 955 615 after: 8 207 180 693

snapshot: also consider the snapshot chain of one unrelated revision To maximize the chance of good delta chain reuse, we inject an unrelated delta chain into our search. To do so, we search for the highest revision unrelated to the parents of the current revision and use its snapshot chain too. Adding this extra snapshot into the mix can have a performance impact. We'll deal with performance impact in a later series.

snapshot: extract parent chain computation The final step of this series is to include chain related to "prev" in the search. Before adding that code we do some simple code movement to clarify the next diff.

snapshot: refine candidate snapshot base upward Once we found a suitable snapshot base it is useful to check if it has a "children" snapshot that would provide a better diff. This is useful when base not directly related to stored revision are picked. In those case, we "jumped" to this new chain at an arbitrary point, checking if a higher point is more appropriate will help to provide better results and increase snapshot reuse.

snapshot: try to refine new snapshot base down the chain There are cases where doing a diff against a snapshot's parent will be shorter than against the snapshot itself. Reusing snapshot not directly related to the revision we are trying to store increase this odd. So once we found a possible candidate, we check the snapshots lower in the chain. This will involve extra processing, but this extra processing will only happen when we are doing building a snapshot, a rare situation.

snapshot: make sure we'll never refine delta base from a reused source The point of reusing delta from the source is to avoid doing computation when applying a bundle. Refining such delta would go against that spirit. We do not have refining logic in place yet. This code needed to be moved out of the way before we could start adding such logic.

snapshot: turn _refinedgroups into a coroutine We are now almost ready to start adding refining logic.

snapshot: also use None as a stop value for `_refinegroup` This is yet another small step toward turning `_refinegroups` into a co-routine.

snapshot: add refining logic at the findeltainfo level Once we found a delta, we want to have the candidates logic challenge it, searching for a better candidate. The logic at the lower level is still missing. We'll introduce it later. Adding small changes in individual commits make it simpler to explain the code change. This is another small step toward turning `_refinegroups` into a co-routine.

snapshot: use None as a stop value when looking for a good delta Having clear stop value should help keep clear logic around the co-routine. The alternative of using a StopIteration exception give a messier result. This is one small step toward turning `_refinegroups` into a co-routine.

snapshot: introduce an intermediate `_refinedgroups` generator This method will be used to improve the search for a good snapshot base. To keep things simpler, we introduce the necessary function before doing any delta base logic change. The next handful of commits will focus on refactoring the code to let that new logic land as clearly as possible. # General Idea Right now, the search for a good delta base stop whenever we found a good one. However, when using sparse-revlog, we should probably try a bit harder. We do significant effort to increase delta re-use by jumping on "unrelated" delta chains that provide better results. Moving to another chain for a better result is good, but we have no guarantee we jump at a reasonable point in that new chain. When we consider over the chains related to the parents, we start from the higher-level snapshots. This is a way to consider the snapshot closer to the current revision that has the best chance to produce a small delta. We do benefit from this walk order when jumping to a better "unrelated" stack. To counter-balance this, we'll introduce a way to "refine" the result. After a good delta have been found, we'll keep searching for a better delta, using the current best one as a starting point. # Target Setup The `finddeltainfo` method is responsible for the general search for a good delta. It requests candidates base from `_candidategroups` and decides which one are usable. The `_candidategroups` generator act as a top-level filter, it does not care about how we pick candidates, it just does basic filtering, excluding revisions that have been tested already or that are an obvious misfit. The `_rawgroups` generator is the one with the actual ancestors walking logic, It does not care about what would do a good delta and what was already tested, it just issues the initial candidates. We introduce a new `_refinedgroup` function to bridge the gap between `_candidategroups` and `_rawgroups`. It delegates the initial iteration logic and then performing relevant refining of the valid base once found. (This logic is yet to be added to function) All these logics are fairly independent and easier to understand when standing alone, not mixed with each other. It also makes it easy to test and try different approaches for one of those four layers without affecting the other ones. # Technical details To communicate `finddeltainfo` choice of "current best delta base" to the `_refinegroup` logic, we plan to use python co-routine feature. The `_candidategroups` and `_refinegroup` generators will become co-routine. This will allow `_refinegroup` to detect when a good delta have been found and triggers various refining steps. For now, `_candidategroups` will just pass the value down the stack. After poking at various option, the co-routine appears the best to keep each layers focus on its duty, without the need to spread implementation details across layers.

snapshot: consider unrelated snapshots at a similar level first This new step is inserted before considering using a level-N snapshot as a base for a level-N+1 snapshot. We first check if existing level-N+1 snapshots using the same base would be a suitable base for a level-N+2 snapshot. This increases snapshot reuse and limits the risk of snapshot explosion in very branchy repositories. Using a "deeper" snapshot as the base also results in a smaller snapshot since it builds a level-N+2 intermediate snapshot instead of an N+1 one. This logic is similar for the one we added in a previous commit. In that previous commit is only applied to level-0 "siblings". We can see this effect in the test repository. Snapshots moved from lower levels to higher levels.

snapshot: consider all snapshots in the parents' chains There are no reasons to only consider full snapshot as a possible base for an intermediate snapshot. Now that the basic principles have been set, we can start adding more levels of snapshots. We now consider all snapshots in the parent's chains (full or intermediate). This creates a chain of intermediate snapshots, each smaller than the previous one. # Effect On The Test Repository In the test repository, we can see a decrease in the revlog size and slightly shorter delta chain. However, that approach creates snapshots more frequently, increasing the risk of ending into problematic cases in very branchy repositories (not triggered by the test repository). The next changesets will remove that risk by adding logic that increases deltas reuse.

snapshot: search for unrelated but reusable full-snapshot # New Strategy Step: Reusing Snapshot Outside Of Parents' Chain. If no suitable bases were found in the parent's chains, see if we could reuse a full snapshot not directly related to the current revision. Such search can be expensive, so we only search for snapshots appended to the revlog *after* the bases used by the parents of the current revision (the one we just tested). We assume the parent's bases were created because the previous snapshots were unsuitable, so there are low odds they would be useful now. This search gives a chance to reuse a delta chain unrelated to the current revision. Without this re-use, topological branches would keep reopening new full chains. Creating more and more snapshots as the repository grow. In repositories with many topological branches, the lack of delta reuse can create too many snapshots reducing overall compression to nothing. This results in a very large repository and other usability issues. For now, we still focus on creating level-1 snapshots. However, this principle will play a large part in how we avoid snapshot explosion once we have more snapshot levels. # Effects On The Test Repository In the test repository we created, we can see the beneficial effect of such reuse. We need very few level-0 snapshots and the overall revlog size has decreased. The `hg debugrevlog` call, show a "lvl-2" snapshot. It comes from the existing delta logic using the `prev` revision (revlog's tip) as the base. In this specific case, it turns out the tip was a level-1 snapshot. This is a coincidence that can be ignored. Finding and testing against all these unrelated snapshots can have a performance impact at write time. We currently focus on building good deltas chain we build. Performance concern will be dealt with later in another series.

snapshot: try intermediate snapshot against parents' base # Regarding The Series Started By This Changeset This is the first changesets of a group adjusting delta chain strategy to build a useful chain of intermediate snapshots. The series will introduce a full strategy to produce chains of multiple snapshots on top of which a "usual" delta chain will be built. That strategy will have multiple steps to maximize snapshot reuse, avoiding pathological cases and improving overall compression in very branchy repositories. An important property of sparse-revlog using such snapshot-chain is that they can use very short delta chain without problematic impact on the resulting compression. Shorter delta chains are important to achieve good performance. To make each step clear, we'll introduce them one by one. See the end of this series for full details. # Regarding This Changeset Before this change, if we cannot store the current revision as a delta against a "simple" candidate (p1, p2, prev), we created a new level-0 snapshot (also called full snapshot). As the first step, we introduce a simple strategy: try an intermediate level-1 snapshot against the chain base of the "current revision" parents. The "current revision" is the one we are currently trying to store in the revlog, triggering this search for a good delta base. The first item in the chain is always a level-0 snapshot. # Effect On The Test Repository We can already see the effect on the test-repository. Most of the snapshots have shifted from level 0 to level 1. The overall size has slightly decreased. (However, keep in mind that this repository only emulates real data) # Regarding Statistic The current series focuses on improving the chain built. Improving the performance of this logic will be done as a second step. Sparse-revlog is still experimental and disabled by default. We'll provide more statistic about resulting size and delta chain at the end of this series.