upgrade: add information about sparse-revlog Show information about sparse-revlog in debugformat, just like other requirements.

sparse-revlog: implement algorithm to write sparse delta chains (issue5480) The classic behavior of revlog._isgooddeltainfo is to consider the span size of the whole delta chain, and limit it to 4 * textlen. Once sparse-revlog writing is allowed (and enforced with a requirement), revlog._isgooddeltainfo considers the span of the largest chunk as the distance used in the verification, instead of using the span of the whole delta chain. In order to compute the span of the largest chunk, we need to slice into chunks a chain with the new revision at the top of the revlog, and take the maximal span of these chunks. The sparse read density is a parameter to the slicing, as it will stop when the global read density reaches this threshold. For instance, a density of 50% means that 2 of 4 read bytes are actually used for the reconstruction of the revision (the others are part of other chains). This allows a new revision to be potentially stored with a diff against another revision anywhere in the history, instead of forcing it in the last 4 * textlen. The result is a much better compression on repositories that have many concurrent branches. Here are a comparison between using deltas from current upstream (aggressive-merge-deltas on by default) and deltas from a sparse-revlog Comparison of `.hg/store/` size: mercurial (6.74% merges): before: 46,831,873 bytes after: 46,795,992 bytes (no relevant change) pypy (8.30% merges): before: 333,524,651 bytes after: 308,417,511 bytes -8% netbeans (34.21% merges): before: 1,141,847,554 bytes after: 1,131,093,161 bytes -1% mozilla-central (4.84% merges): before: 2,344,248,850 bytes after: 2,328,459,258 bytes -1% large-private-repo-A (merge 19.73%) before: 41,510,550,163 bytes after: 8,121,763,428 bytes -80% large-private-repo-B (23.77%) before: 58,702,221,709 bytes after: 8,351,588,828 bytes -76% Comparison of `00manifest.d` size: mercurial (6.74% merges): before: 6,143,044 bytes after: 6,107,163 bytes pypy (8.30% merges): before: 52,941,780 bytes after: 27,834,082 bytes -48% netbeans (34.21% merges): before: 130,088,982 bytes after: 119,337,636 bytes -10% mozilla-central (4.84% merges): before: 215,096,339 bytes after: 199,496,863 bytes -8% large-private-repo-A (merge 19.73%) before: 33,725,285,081 bytes after: 390,302,545 bytes -99% large-private-repo-B (23.77%) before: 49,457,701,645 bytes after: 1,366,752,187 bytes -97% The better delta chains provide a performance boost in relevant repositories: pypy, bundling 1000 revisions: before: 1.670s after: 1.149s -31% Unbundling got a bit slower. probably because the sparse algorithm is still pure python. pypy, unbundling 1000 revisions: before: 4.062s after: 4.507s +10% Performance of bundle/unbundle in repository with few concurrent branches (eg: mercurial) are unaffected. No significant differences have been noticed then timing `hg push` and `hg pull` locally. More state timings are being gathered. Same as for aggressive-merge-delta, better delta comes with longer delta chains. Longer chains have a performance impact. For example. The length of the chain needed to get the manifest of pypy's tip moves from 82 item to 1929 items. This moves the restore time from 3.88ms to 11.3ms. Delta chain length is an independent issue that affects repository without this changes. It will be dealt with independently. No significant differences have been observed on repositories where `sparse-revlog` have not much effect (mercurial, unity, netbeans). On pypy, small differences have been observed on some operation affected by delta chain building and retrieval. pypy, perfmanifest before: 0.006162s after: 0.017899s +190% pypy, commit: before: 0.382 after: 0.376 -1% pypy, status: before: 0.157 after: 0.168 +7% More comprehensive and stable timing comparisons are in progress.

sparse-revlog: new requirement enabled with format.sparse-revlog The meaning of the new 'sparse-revlog' requirement is that the revlogs are allowed to contain wider delta chains with larger holes between the interesting chunks. These sparse delta chains should be read in several chunks to avoid a potential explosion of memory usage. Former version won't know how to read a delta chain in several chunks. They would keep reading them in a single read, and therefore would be subject to the potential memory explosion. Hence this new requirement: only versions having support of sparse-revlog reading should be allowed to read such a revlog. Implementation of this new algorithm and tools to enable or disable the requirement will follow in the next changesets.

revlog: extract `deltainfo.distance` for future conditional redefinition This commit exist to make the next one clearer.