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

mercurial/configitems.py

@@ -665,7 +665,7 b" coreconfigitem('format', 'manifestcaches"
     default=None,
 )
 coreconfigitem('format', 'maxchainlen',
-    default=None,
+    default=dynamicdefault,
 )
 coreconfigitem('format', 'obsstore-version',
     default=None,

mercurial/localrepo.py

@@ -70,6 +70,10 b' from .utils import ('
     stringutil,
 )
 
+from .revlogutils import (
+    constants as revlogconst,
+)
+
 release = lockmod.release
 urlerr = util.urlerr
 urlreq = util.urlreq
@@ -658,10 +662,6 b' class localrepository(object):'
         chunkcachesize = self.ui.configint('format', 'chunkcachesize')
         if chunkcachesize is not None:
             self.svfs.options['chunkcachesize'] = chunkcachesize
-        # experimental config: format.maxchainlen
-        maxchainlen = self.ui.configint('format', 'maxchainlen')
-        if maxchainlen is not None:
-            self.svfs.options['maxchainlen'] = maxchainlen
         # experimental config: format.manifestcachesize
         manifestcachesize = self.ui.configint('format', 'manifestcachesize')
         if manifestcachesize is not None:
@@ -689,6 +689,13 b' class localrepository(object):'
         self.svfs.options['sparse-revlog'] = sparserevlog
         if sparserevlog:
             self.svfs.options['generaldelta'] = True
+        maxchainlen = None
+        if sparserevlog:
+            maxchainlen = revlogconst.SPARSE_REVLOG_MAX_CHAIN_LENGTH
+        # experimental config: format.maxchainlen
+        maxchainlen = self.ui.configint('format', 'maxchainlen', maxchainlen)
+        if maxchainlen is not None:
+            self.svfs.options['maxchainlen'] = maxchainlen
 
         for r in self.requirements:
             if r.startswith('exp-compression-'):

mercurial/revlogutils/constants.py

@@ -41,3 +41,6 b' REVIDX_FLAGS_ORDER = ['
 REVIDX_KNOWN_FLAGS = util.bitsfrom(REVIDX_FLAGS_ORDER)
 # bitmark for flags that could cause rawdata content change
 REVIDX_RAWTEXT_CHANGING_FLAGS = REVIDX_ISCENSORED | REVIDX_EXTSTORED
+
+SPARSE_REVLOG_MAX_CHAIN_LENGTH = 1000
+

tests/test-sparse-revlog.t

@@ -25,6 +25,7 b' repeatedly while some of it changes rare'
   $ cat >> $HGRCPATH << EOF
   > [format]
   > sparse-revlog = yes
+  > maxchainlen = 15
   > [storage]
   > revlog.optimize-delta-parent-choice = yes
   > EOF
@@ -77,7 +78,7 b' repeatedly while some of it changes rare'
   
 
   $ f -s .hg/store/data/*.d
-  .hg/store/data/_s_p_a_r_s_e-_r_e_v_l_o_g-_t_e_s_t-_f_i_l_e.d: size=59230936
+  .hg/store/data/_s_p_a_r_s_e-_r_e_v_l_o_g-_t_e_s_t-_f_i_l_e.d: size=63002924
   $ hg debugrevlog *
   format : 1
   flags  : generaldelta
@@ -89,45 +90,45 b' repeatedly while some of it changes rare'
       empty     :        0 ( 0.00%)
                      text  :        0 (100.00%)
                      delta :        0 (100.00%)
-      snapshot  :      176 ( 3.52%)
-        lvl-0   :              3 ( 0.06%)
-        lvl-1   :             17 ( 0.34%)
-        lvl-2   :             45 ( 0.90%)
-        lvl-3   :             56 ( 1.12%)
-        lvl-4   :             55 ( 1.10%)
-      deltas    :     4825 (96.48%)
-  revision size : 59230936
-      snapshot  :  5770371 ( 9.74%)
-        lvl-0   :         602962 ( 1.02%)
-        lvl-1   :        1534153 ( 2.59%)
-        lvl-2   :        1604445 ( 2.71%)
-        lvl-3   :        1218174 ( 2.06%)
-        lvl-4   :         810637 ( 1.37%)
-      deltas    : 53460565 (90.26%)
+      snapshot  :      374 ( 7.48%)
+        lvl-0   :              4 ( 0.08%)
+        lvl-1   :             23 ( 0.46%)
+        lvl-2   :             63 ( 1.26%)
+        lvl-3   :            118 ( 2.36%)
+        lvl-4   :            166 ( 3.32%)
+      deltas    :     4627 (92.52%)
+  revision size : 63002924
+      snapshot  :  9888099 (15.69%)
+        lvl-0   :         804262 ( 1.28%)
+        lvl-1   :        1561380 ( 2.48%)
+        lvl-2   :        2096696 ( 3.33%)
+        lvl-3   :        2749539 ( 4.36%)
+        lvl-4   :        2676222 ( 4.25%)
+      deltas    : 53114825 (84.31%)
   
   chunks        :     5001
       0x78 (x)  :     5001 (100.00%)
-  chunks size   : 59230936
-      0x78 (x)  : 59230936 (100.00%)
+  chunks size   : 63002924
+      0x78 (x)  : 63002924 (100.00%)
   
-  avg chain length  :       17
-  max chain length  :       45
-  max chain reach   : 25326012
-  compression ratio :       29
+  avg chain length  :        9
+  max chain length  :       15
+  max chain reach   : 28907121
+  compression ratio :       27
   
   uncompressed data size (min/max/avg) : 346468 / 346472 / 346471
-  full revision size (min/max/avg)     : 200897 / 201050 / 200987
-  inter-snapshot size (min/max/avg)    : 11598 / 171990 / 29869
-      level-1   (min/max/avg)          : 14037 / 171990 / 90244
-      level-2   (min/max/avg)          : 11632 / 84456 / 35654
-      level-3   (min/max/avg)          : 11598 / 41486 / 21753
-      level-4   (min/max/avg)          : 11618 / 19913 / 14738
-  delta size (min/max/avg)             : 10649 / 105209 / 11079
+  full revision size (min/max/avg)     : 201008 / 201141 / 201065
+  inter-snapshot size (min/max/avg)    : 11601 / 157413 / 24550
+      level-1   (min/max/avg)          : 13061 / 157413 / 67886
+      level-2   (min/max/avg)          : 11674 / 85631 / 33280
+      level-3   (min/max/avg)          : 11602 / 42957 / 23301
+      level-4   (min/max/avg)          : 11601 / 21475 / 16121
+  delta size (min/max/avg)             : 10649 / 105465 / 11479
   
-  deltas against prev  : 4156 (86.13%)
-      where prev = p1  : 4120     (99.13%)
+  deltas against prev  : 3966 (85.71%)
+      where prev = p1  : 3922     (98.89%)
       where prev = p2  :    0     ( 0.00%)
-      other            :   36     ( 0.87%)
-  deltas against p1    :  646 (13.39%)
-  deltas against p2    :   23 ( 0.48%)
+      other            :   44     ( 1.11%)
+  deltas against p1    :  611 (13.21%)
+  deltas against p2    :   50 ( 1.08%)
   deltas against other :    0 ( 0.00%)