revlog: add an experimental option to mitigated delta issues (issue5480)

The general delta heuristic to select a delta do not scale with the number of
branch. The delta base is frequently too far away to be able to reuse a chain
according to the "distance" criteria. This leads to insertion of larger delta (or
even full text) that themselves push the bases for the next delta further away
leading to more large deltas and full texts. This full text and frequent
recomputation throw Mercurial performance in disarray.

For example of a slightly large repository

  280 000 files (2 150 000 versions)
  430 000 changesets (10 000 topological heads)

Number below compares repository with and without the distance criteria:

manifest size:
    with:    21.4 GB
    without:  0.3 GB

store size:
    with:    28.7 GB
    without   7.4 GB

bundle last 15 00 revisions:
    with:    800 seconds
             971 MB
    without:  50 seconds
              73 MB

unbundle time (of the last 15K revisions):
    with:    1150 seconds (~19 minutes)
    without:   35 seconds

Similar issues has been observed in other repositories.


Adding a new option or "feature" on stable is uncommon. However, given that this
issues is making Mercurial practically unusable, I'm exceptionally targeting
this patch for stable.

What is actually needed is a full rework of the delta building and reading
logic. However, that will be a longer process and churn not suitable for stable.

In the meantime, we introduces a quick and dirty mitigation of this in the
'experimental' config space. The new option introduces a way to set the maximum
amount of memory usable to store a diff in memory. This extend the ability for
Mercurial to create chains without removing all safe guard regarding memory
access. The option should be phased out when core has a more proper solution
available.

Setting the limit to '0' remove all limits, setting it to '-1' use the default
limit (textsize x 4).

mercurial/localrepo.py

@@ -442,6 +442,9 b' class localrepository(object):'
             'aggressivemergedeltas', False)
         self.svfs.options['aggressivemergedeltas'] = aggressivemergedeltas
         self.svfs.options['lazydeltabase'] = not scmutil.gddeltaconfig(self.ui)
+        chainspan = self.ui.configbytes('experimental', 'maxdeltachainspan', -1)
+        if 0 <= chainspan:
+            self.svfs.options['maxdeltachainspan'] = chainspan
 
         for r in self.requirements:
             if r.startswith('exp-compression-'):

mercurial/revlog.py

@@ -292,6 +292,7 b' class revlog(object):'
         self._nodecache = {nullid: nullrev}
         self._nodepos = None
         self._compengine = 'zlib'
+        self._maxdeltachainspan = -1
 
         v = REVLOG_DEFAULT_VERSION
         opts = getattr(opener, 'options', None)
@@ -313,6 +314,8 b' class revlog(object):'
             self._lazydeltabase = bool(opts.get('lazydeltabase', False))
             if 'compengine' in opts:
                 self._compengine = opts['compengine']
+            if 'maxdeltachainspan' in opts:
+                self._maxdeltachainspan = opts['maxdeltachainspan']
 
         if self._chunkcachesize <= 0:
             raise RevlogError(_('revlog chunk cache size %r is not greater '
@@ -1659,7 +1662,13 b' class revlog(object):'
         # - 'compresseddeltalen' is the sum of the total size of deltas we need
         #   to apply -- bounding it limits the amount of CPU we consume.
         dist, l, data, base, chainbase, chainlen, compresseddeltalen = d
-        if (dist > textlen * 4 or l > textlen or
+
+        defaultmax = textlen * 4
+        maxdist = self._maxdeltachainspan
+        if not maxdist:
+            maxdist = dist # ensure the conditional pass
+        maxdist = max(maxdist, defaultmax)
+        if (dist > maxdist or l > textlen or
             compresseddeltalen > textlen * 2 or
             (self._maxchainlen and chainlen > self._maxchainlen)):
             return False

tests/test-generaldelta.t

@@ -161,3 +161,191 b' Test that strip bundle use bundle2'
       1c5d4dc9a8b8d6e1750966d343e94db665e7a1e9 draft
 
   $ cd ..
+
+test maxdeltachainspan
+
+  $ hg init source-repo
+  $ cd source-repo
+  $ hg debugbuilddag --new-file '.+5:brancha$.+11:branchb$.+30:branchc<brancha+2<branchb+2'
+  $ cd ..
+  $ hg -R source-repo debugindex -m
+     rev    offset  length  delta linkrev nodeid       p1           p2
+       0         0      46     -1       0 19deeef41503 000000000000 000000000000
+       1        46      57      0       1 fffc37b38c40 19deeef41503 000000000000
+       2       103      57      1       2 5822d75c83d9 fffc37b38c40 000000000000
+       3       160      57      2       3 19cf2273e601 5822d75c83d9 000000000000
+       4       217      57      3       4 d45ead487afe 19cf2273e601 000000000000
+       5       274      57      4       5 96e0c2ce55ed d45ead487afe 000000000000
+       6       331      46     -1       6 0c2ea5222c74 000000000000 000000000000
+       7       377      57      6       7 4ca08a89134d 0c2ea5222c74 000000000000
+       8       434      57      7       8 c973dbfd30ac 4ca08a89134d 000000000000
+       9       491      57      8       9 d81d878ff2cd c973dbfd30ac 000000000000
+      10       548      58      9      10 dbee7f0dd760 d81d878ff2cd 000000000000
+      11       606      58     10      11 474be9f1fd4e dbee7f0dd760 000000000000
+      12       664      58     11      12 594a27502c85 474be9f1fd4e 000000000000
+      13       722      58     12      13 a7d25307d6a9 594a27502c85 000000000000
+      14       780      58     13      14 3eb53082272e a7d25307d6a9 000000000000
+      15       838      58     14      15 d1e94c85caf6 3eb53082272e 000000000000
+      16       896      58     15      16 8933d9629788 d1e94c85caf6 000000000000
+      17       954      58     16      17 a33416e52d91 8933d9629788 000000000000
+      18      1012      47     -1      18 4ccbf31021ed 000000000000 000000000000
+      19      1059      58     18      19 dcad7a25656c 4ccbf31021ed 000000000000
+      20      1117      58     19      20 617c4f8be75f dcad7a25656c 000000000000
+      21      1175      58     20      21 975b9c1d75bb 617c4f8be75f 000000000000
+      22      1233      58     21      22 74f09cd33b70 975b9c1d75bb 000000000000
+      23      1291      58     22      23 54e79bfa7ef1 74f09cd33b70 000000000000
+      24      1349      58     23      24 c556e7ff90af 54e79bfa7ef1 000000000000
+      25      1407      58     24      25 42daedfe9c6b c556e7ff90af 000000000000
+      26      1465      58     25      26 f302566947c7 42daedfe9c6b 000000000000
+      27      1523      58     26      27 2346959851cb f302566947c7 000000000000
+      28      1581      58     27      28 ca8d867106b4 2346959851cb 000000000000
+      29      1639      58     28      29 fd9152decab2 ca8d867106b4 000000000000
+      30      1697      58     29      30 3fe34080a79b fd9152decab2 000000000000
+      31      1755      58     30      31 bce61a95078e 3fe34080a79b 000000000000
+      32      1813      58     31      32 1dd9ba54ba15 bce61a95078e 000000000000
+      33      1871      58     32      33 3cd9b90a9972 1dd9ba54ba15 000000000000
+      34      1929      58     33      34 5db8c9754ef5 3cd9b90a9972 000000000000
+      35      1987      58     34      35 ee4a240cc16c 5db8c9754ef5 000000000000
+      36      2045      58     35      36 9e1d38725343 ee4a240cc16c 000000000000
+      37      2103      58     36      37 3463f73086a8 9e1d38725343 000000000000
+      38      2161      58     37      38 88af72fab449 3463f73086a8 000000000000
+      39      2219      58     38      39 472f5ce73785 88af72fab449 000000000000
+      40      2277      58     39      40 c91b8351e5b8 472f5ce73785 000000000000
+      41      2335      58     40      41 9c8289c5c5c0 c91b8351e5b8 000000000000
+      42      2393      58     41      42 a13fd4a09d76 9c8289c5c5c0 000000000000
+      43      2451      58     42      43 2ec2c81cafe0 a13fd4a09d76 000000000000
+      44      2509      58     43      44 f27fdd174392 2ec2c81cafe0 000000000000
+      45      2567      58     44      45 a539ec59fe41 f27fdd174392 000000000000
+      46      2625      58     45      46 5e98b9ecb738 a539ec59fe41 000000000000
+      47      2683      58     46      47 31e6b47899d0 5e98b9ecb738 000000000000
+      48      2741      58     47      48 2cf25d6636bd 31e6b47899d0 000000000000
+      49      2799     197     -1      49 9fff62ea0624 96e0c2ce55ed 000000000000
+      50      2996      58     49      50 467f8e30a066 9fff62ea0624 000000000000
+      51      3054     356     50      51 346db97283df a33416e52d91 000000000000
+      52      3410      58     51      52 4e003fd4d5cd 346db97283df 000000000000
+  $ hg clone --pull source-repo --config experimental.maxdeltachainspan=2800 relax-chain --config format.generaldelta=yes
+  requesting all changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 53 changesets with 53 changes to 53 files (+2 heads)
+  updating to branch default
+  14 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg -R relax-chain debugindex -m
+     rev    offset  length  delta linkrev nodeid       p1           p2
+       0         0      46     -1       0 19deeef41503 000000000000 000000000000
+       1        46      57      0       1 fffc37b38c40 19deeef41503 000000000000
+       2       103      57      1       2 5822d75c83d9 fffc37b38c40 000000000000
+       3       160      57      2       3 19cf2273e601 5822d75c83d9 000000000000
+       4       217      57      3       4 d45ead487afe 19cf2273e601 000000000000
+       5       274      57      4       5 96e0c2ce55ed d45ead487afe 000000000000
+       6       331      46     -1       6 0c2ea5222c74 000000000000 000000000000
+       7       377      57      6       7 4ca08a89134d 0c2ea5222c74 000000000000
+       8       434      57      7       8 c973dbfd30ac 4ca08a89134d 000000000000
+       9       491      57      8       9 d81d878ff2cd c973dbfd30ac 000000000000
+      10       548      58      9      10 dbee7f0dd760 d81d878ff2cd 000000000000
+      11       606      58     10      11 474be9f1fd4e dbee7f0dd760 000000000000
+      12       664      58     11      12 594a27502c85 474be9f1fd4e 000000000000
+      13       722      58     12      13 a7d25307d6a9 594a27502c85 000000000000
+      14       780      58     13      14 3eb53082272e a7d25307d6a9 000000000000
+      15       838      58     14      15 d1e94c85caf6 3eb53082272e 000000000000
+      16       896      58     15      16 8933d9629788 d1e94c85caf6 000000000000
+      17       954      58     16      17 a33416e52d91 8933d9629788 000000000000
+      18      1012      47     -1      18 4ccbf31021ed 000000000000 000000000000
+      19      1059      58     18      19 dcad7a25656c 4ccbf31021ed 000000000000
+      20      1117      58     19      20 617c4f8be75f dcad7a25656c 000000000000
+      21      1175      58     20      21 975b9c1d75bb 617c4f8be75f 000000000000
+      22      1233      58     21      22 74f09cd33b70 975b9c1d75bb 000000000000
+      23      1291      58     22      23 54e79bfa7ef1 74f09cd33b70 000000000000
+      24      1349      58     23      24 c556e7ff90af 54e79bfa7ef1 000000000000
+      25      1407      58     24      25 42daedfe9c6b c556e7ff90af 000000000000
+      26      1465      58     25      26 f302566947c7 42daedfe9c6b 000000000000
+      27      1523      58     26      27 2346959851cb f302566947c7 000000000000
+      28      1581      58     27      28 ca8d867106b4 2346959851cb 000000000000
+      29      1639      58     28      29 fd9152decab2 ca8d867106b4 000000000000
+      30      1697      58     29      30 3fe34080a79b fd9152decab2 000000000000
+      31      1755      58     30      31 bce61a95078e 3fe34080a79b 000000000000
+      32      1813      58     31      32 1dd9ba54ba15 bce61a95078e 000000000000
+      33      1871      58     32      33 3cd9b90a9972 1dd9ba54ba15 000000000000
+      34      1929      58     33      34 5db8c9754ef5 3cd9b90a9972 000000000000
+      35      1987      58     34      35 ee4a240cc16c 5db8c9754ef5 000000000000
+      36      2045      58     35      36 9e1d38725343 ee4a240cc16c 000000000000
+      37      2103      58     36      37 3463f73086a8 9e1d38725343 000000000000
+      38      2161      58     37      38 88af72fab449 3463f73086a8 000000000000
+      39      2219      58     38      39 472f5ce73785 88af72fab449 000000000000
+      40      2277      58     39      40 c91b8351e5b8 472f5ce73785 000000000000
+      41      2335      58     40      41 9c8289c5c5c0 c91b8351e5b8 000000000000
+      42      2393      58     41      42 a13fd4a09d76 9c8289c5c5c0 000000000000
+      43      2451      58     42      43 2ec2c81cafe0 a13fd4a09d76 000000000000
+      44      2509      58     43      44 f27fdd174392 2ec2c81cafe0 000000000000
+      45      2567      58     44      45 a539ec59fe41 f27fdd174392 000000000000
+      46      2625      58     45      46 5e98b9ecb738 a539ec59fe41 000000000000
+      47      2683      58     46      47 31e6b47899d0 5e98b9ecb738 000000000000
+      48      2741      58     47      48 2cf25d6636bd 31e6b47899d0 000000000000
+      49      2799     197     -1      49 9fff62ea0624 96e0c2ce55ed 000000000000
+      50      2996      58     49      50 467f8e30a066 9fff62ea0624 000000000000
+      51      3054      58     17      51 346db97283df a33416e52d91 000000000000
+      52      3112     369     -1      52 4e003fd4d5cd 346db97283df 000000000000
+  $ hg clone --pull source-repo --config experimental.maxdeltachainspan=0 noconst-chain --config format.generaldelta=yes
+  requesting all changes
+  adding changesets
+  adding manifests
+  adding file changes
+  added 53 changesets with 53 changes to 53 files (+2 heads)
+  updating to branch default
+  14 files updated, 0 files merged, 0 files removed, 0 files unresolved
+  $ hg -R noconst-chain debugindex -m
+     rev    offset  length  delta linkrev nodeid       p1           p2
+       0         0      46     -1       0 19deeef41503 000000000000 000000000000
+       1        46      57      0       1 fffc37b38c40 19deeef41503 000000000000
+       2       103      57      1       2 5822d75c83d9 fffc37b38c40 000000000000
+       3       160      57      2       3 19cf2273e601 5822d75c83d9 000000000000
+       4       217      57      3       4 d45ead487afe 19cf2273e601 000000000000
+       5       274      57      4       5 96e0c2ce55ed d45ead487afe 000000000000
+       6       331      46     -1       6 0c2ea5222c74 000000000000 000000000000
+       7       377      57      6       7 4ca08a89134d 0c2ea5222c74 000000000000
+       8       434      57      7       8 c973dbfd30ac 4ca08a89134d 000000000000
+       9       491      57      8       9 d81d878ff2cd c973dbfd30ac 000000000000
+      10       548      58      9      10 dbee7f0dd760 d81d878ff2cd 000000000000
+      11       606      58     10      11 474be9f1fd4e dbee7f0dd760 000000000000
+      12       664      58     11      12 594a27502c85 474be9f1fd4e 000000000000
+      13       722      58     12      13 a7d25307d6a9 594a27502c85 000000000000
+      14       780      58     13      14 3eb53082272e a7d25307d6a9 000000000000
+      15       838      58     14      15 d1e94c85caf6 3eb53082272e 000000000000
+      16       896      58     15      16 8933d9629788 d1e94c85caf6 000000000000
+      17       954      58     16      17 a33416e52d91 8933d9629788 000000000000
+      18      1012      47     -1      18 4ccbf31021ed 000000000000 000000000000
+      19      1059      58     18      19 dcad7a25656c 4ccbf31021ed 000000000000
+      20      1117      58     19      20 617c4f8be75f dcad7a25656c 000000000000
+      21      1175      58     20      21 975b9c1d75bb 617c4f8be75f 000000000000
+      22      1233      58     21      22 74f09cd33b70 975b9c1d75bb 000000000000
+      23      1291      58     22      23 54e79bfa7ef1 74f09cd33b70 000000000000
+      24      1349      58     23      24 c556e7ff90af 54e79bfa7ef1 000000000000
+      25      1407      58     24      25 42daedfe9c6b c556e7ff90af 000000000000
+      26      1465      58     25      26 f302566947c7 42daedfe9c6b 000000000000
+      27      1523      58     26      27 2346959851cb f302566947c7 000000000000
+      28      1581      58     27      28 ca8d867106b4 2346959851cb 000000000000
+      29      1639      58     28      29 fd9152decab2 ca8d867106b4 000000000000
+      30      1697      58     29      30 3fe34080a79b fd9152decab2 000000000000
+      31      1755      58     30      31 bce61a95078e 3fe34080a79b 000000000000
+      32      1813      58     31      32 1dd9ba54ba15 bce61a95078e 000000000000
+      33      1871      58     32      33 3cd9b90a9972 1dd9ba54ba15 000000000000
+      34      1929      58     33      34 5db8c9754ef5 3cd9b90a9972 000000000000
+      35      1987      58     34      35 ee4a240cc16c 5db8c9754ef5 000000000000
+      36      2045      58     35      36 9e1d38725343 ee4a240cc16c 000000000000
+      37      2103      58     36      37 3463f73086a8 9e1d38725343 000000000000
+      38      2161      58     37      38 88af72fab449 3463f73086a8 000000000000
+      39      2219      58     38      39 472f5ce73785 88af72fab449 000000000000
+      40      2277      58     39      40 c91b8351e5b8 472f5ce73785 000000000000
+      41      2335      58     40      41 9c8289c5c5c0 c91b8351e5b8 000000000000
+      42      2393      58     41      42 a13fd4a09d76 9c8289c5c5c0 000000000000
+      43      2451      58     42      43 2ec2c81cafe0 a13fd4a09d76 000000000000
+      44      2509      58     43      44 f27fdd174392 2ec2c81cafe0 000000000000
+      45      2567      58     44      45 a539ec59fe41 f27fdd174392 000000000000
+      46      2625      58     45      46 5e98b9ecb738 a539ec59fe41 000000000000
+      47      2683      58     46      47 31e6b47899d0 5e98b9ecb738 000000000000
+      48      2741      58     47      48 2cf25d6636bd 31e6b47899d0 000000000000
+      49      2799      58      5      49 9fff62ea0624 96e0c2ce55ed 000000000000
+      50      2857      58     49      50 467f8e30a066 9fff62ea0624 000000000000
+      51      2915      58     17      51 346db97283df a33416e52d91 000000000000
+      52      2973      58     51      52 4e003fd4d5cd 346db97283df 000000000000