sparse-revlog: enabled by default The feature provides large benefits. It now seems mature enough to be enabled by default. * It solves catastrophic issues regarding delta storage in revlog, * It allows for shorter delta chain in all repositories, improving performances. Running benchmark of a wide range of operation did not reveal problematic impact. Performance gains are observed where expected. The format is supported by Mercurial version 4.7. So it seems safe to enable it by default now. Here is a reminder of key numbers regarding this delta strategy effect on repository size and performance. Effect on Size: =============== For repositories with a lot of branches, sparse-revlog significantly improve size, fixing limitation associated with the span of a delta chain. In addition, sparse-revlog, deal well with limitations of the delta chain length. For large repositories, this allows for a stiff reduction of the delta chain without a problematic impact on the repository size. This delta chain length improvement helps all repositories, not just the ones with many branches. As a reminder, here are the default chain limits for each "format": * no-sparse: none * sparse: 1000 Mercurial --------- Manifest Size: limit | none | 1000 ------------|-------------|------------ no-sparse | 6 143 044 | 6 269 496 sparse | 5 798 796 | 5 827 025 Manifest Chain length data limit || none || 1000 value || average | max || average | max ------------||---------|---------||---------|--------- no-sparse || 429 | 1 397 || 397 | 1 000 sparse || 326 | 1 290 || 313 | 1 000 Full Store Size limit | none | 1000 ------------|-------------|------------ no-sparse | 46 944 775 | 47 166 129 sparse | 46 622 445 | 46 723 774 pypy ---- Manifest Size: limit | none | 1000 ------------|-------------|------------ no-sparse | 52 941 760 | 56 200 970 sparse | 26 348 229 | 27 384 133 Manifest Chain length data limit || none || 1000 value || average | max || average | max ------------||---------|---------||---------|--------- no-sparse || 769 | 3 889 || 390 | 1 000 sparse || 1 223 | 3 846 || 495 | 1 000 Full Store Size limit | none | 1000 ------------|-------------|------------ no-sparse | 336 050 203 | 339 309 413 sparse | 338 673 985 | 339 709 889 Mozilla ------- Manifest Size: limit | none | 1000 ------------|----------------|--------------- no-sparse | 215 096 339 | 1 708 853 525 sparse | 188 947 271 | 278 894 170 Manifest Chain length data limit || none || 1000 value || average | max || average | max ------------||---------|---------||---------|-------- no-sparse || 20 454 | 59 562 || 491 | 1 000 sparse || 23 509 | 69 891 || 489 | 1 000 Full Store Size limit | none | 1000 ------------|----------------|--------------- no-sparse | 2 377 578 715 | 3 876 258 798 sparse | 2 441 677 137 | 2 535 997 381 Netbeans -------- Manifest Size: limit | none | 1000 ------------|----------------|--------------- no-sparse | 130 088 982 | 741 590 565 sparse | 118 836 887 | 159 161 207 Manifest Chain length data limit || none || 1000 value || average | max || average | max ------------||---------|---------||---------|--------- no-sparse || 19 321 | 61 397 || 510 | 1 000 sparse || 21 240 | 61 583 || 503 | 1 000 Full Store Size limit | none | 1000 ------------|----------------|--------------- no-sparse | 1 160 013 008 | 1 771 514 591 sparse | 1 164 959 988 | 1 205 284 308 Private repo #1 --------------- Manifest Size: limit | none | 1000 ------------|-----------------|--------------- no-sparse | 33 725 285 081 | 33 724 834 190 sparse | 350 542 420 | 423 470 579 Manifest Chain length data limit || none || 1000 value || average | max || average | max ------------||---------|---------||---------|--------- no-sparse || 282 | 8 885 || 113 | 1 000 snapshot || 3 655 | 8 951 || 530 | 1 000 Full Store Size limit | none | 1000 ------------|----------------|--------------- no-sparse | 41 544 149 652 | 41 543 698 761 sparse | 8 448 037 300 | 8 520 965 459 Effect on speed: ================ Performances are strongly impacted by the delta chain length. Longer chain results in slower revision restoration. For this reason, the 1000 chain limit introduced by sparse-revlog helps repository with previously large chains a lot. In our corpus, this means `netbeans` and `mozilla-central` who suffered from unreasonable manifest delta chain length. Another way sparse revlog helps, is by producing better delta's. For repositories with many branches, the pathological patterns that resulted in many sub-optimal deltas are gone. Smaller delta help with operations where deltas are directly relevant, like bundle. However, the sparse-revlog logic introduces some extra processing and a more throughout testing of possible delta candidates. Adding an extra cost in some cases. This cost is usually counterbalanced by the other performance gain. However, for smaller repositories not affected by delta chain length issues or branching related issues, this might make things a bit slower. However, these are also repository where revlog performance is dwarfed by other costs. Below are the summary of some timing from the performance test suite running at `http://perf.octobus.net/` for a handful of key commands and operation. It is important to keep in mind that most of this command works on the tip part of the repository. The non-sparse and sparse version produce different delta chains and the tip revision can end up at an arbitrary point of these chains. This will impact some performance number listed in this summary. For the record: here is the delta chain length for the tip revision of manifest log in the benchmarked repository: | no-sparse | sparse | mercurial | 94 | 904 | pypy | 23 | 673 | netbeans | 4158 | 258 | mozilla | 63263 | 781 | As you can see, the chain length for mercurial and pypy turn out to be significantly longer. The netbeans and mozilla one get shorter because these repositories benefit from the maximum chain length. Timing for `hg commit`: ----------------------- The time taken by `hg commit` does not varies significantly, no drawback for using sparse here. | no-sparse | sparse | mercurial | 68.1ms | 66.7ms | pypy | 95.0ms | 94.1ms | netbeans | 614.0ms | 611.0ms | mozilla | 1340.0ms | 1.320.0ms | Check the final section for statistics on a wider array of write. Timing for bundling 10 000 changesets ------------------------------------- The repository that benefits from better delta see a good performance boost. The other ones are not significantly affected. | no-sparse | sparse | mercurial | 3.1s | 3.0s | pypy | 25.1s | 7.5s | netbeans | 24.2s | 17.0s | mozilla | 23.7s | 25.0s | Timing for unbundling 1 000 changesets -------------------------------------- Mercurial and mozilla are unaffected. The pypy repository benefit well from the better delta. However, the netbeans repository takes a visible hit. Digging that difference reveals that it comes from the sparse-revlog bundle having to deal with a snapshot that was re-encoded in the bundle. The slow path for adding new a revision had to be triggered for it, slowing things down. The Sparse versions do not have such snapshot to handle similar cases in the tested configuration. | no-sparse | sparse | mercurial | 519ms | 502ms | pypy | 1.270ms | 886ms | netbeans | 1.370ms | 2.250ms | mozilla | 3.230ms | 3.210ms | Netbeans benefits from the better deltas in other dimensions too. For example, the produced bundle is significantly smaller: * netbeans-no-sparse.hg: 2.3MB * netbeans-sparse.hg: 1.9MB Timing to restore the tip most manifest entry: ---------------------------------------------- Nothing surprising here. The timing for mercurial and pypy are within a small range where they won't affect performance much. In our tested case, they are slower as they use a longer chain. Timing for netbeans and mozilla improves a lot. Removing a significant amount of time. | no-sparse | sparse | mercurial | 1.09ms | 3.15ms | pypy | 4.11ms | 10.70ms | netbeans | 239.00ms | 112.00ms | mozilla | 688.00ms | 198.00ms | Reading 100 revision in descending order: ----------------------------------------- We see the same kind of effect when reading the last 100 revisions. Large boost for netbeans and mozilla, as they use much smaller delta chain. Mercurial and pypy longer chain means slower reads, but nothing gets out of control. | no-sparse | sparse | mercurial | 0.089s | 0.268s | pypy | 0.259s | 0.698s | netbeans | 125.000s | 20.600s | mozilla | 23.000s | 11.400s | Writing from full text: statistic for the last 30K revisions ------------------------------------------------------------ This benchmark adds revisions to revlog from their full text. This is similar to the work done during a commit, but for a large amount of revisions so that we get a more relevant view. We see better overall performances with sparse-revlog. The very worst case is usually slower with sparse-revlog, but does not gets out of control. For the vast majorities of the other writes, sparse-revlog is significantly faster for larger repositories. This is reflected in the accumulated rewrite time for netbeans and mozilla. The notable exception is the pypy repository where things get slower. The extra processing is not balanced by shorter delta chain. However, this is to be seen as a blocking issue. First, the overall time spend dealing with revlog for the repository pypy size is small compared to the other costs, so we get slower on operations that matter less than for other larger repository. Second, we still get nice size benefit from using sparse-revlog, smaller repo size brings other usability and speed benefit (eg: bundle size). max time | no-sparse | sparse | mercurial | 0.010143s | 0.045280s | pypy | 0.034924s | 0.243288s | netbeans | 0.605371s | 2.130876s | mozilla | 1.478342s | 3.424541s | 99% time | no-sparse | sparse | mercurial | 0.003774s | 0.003758s | pypy | 0.017387s | 0.025310s | netbeans | 0.576913s | 0.271195s | mozilla | 1.478342s | 0.449661s | 95% time | no-sparse | sparse | mercurial | 0.002069s | 0.002120s | pypy | 0.010141s | 0.014797s | netbeans | 0.540202s | 0.258644s | mozilla | 0.654830s | 0.243440s | full time | no-sparse | sparse | mercurial | 14.15s | 14.87s | pypy | 90.50s | 137.12s | netbeans | 6401.06s | 3411.14s | mozilla | 3086.89s | 1991.97s | Differential Revision: https://phab.mercurial-scm.org/D5345