sparse-revlog: enabled by default

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

--- a/mercurial/configitems.py	Mon Nov 12 01:22:30 2018 +0100
+++ b/mercurial/configitems.py	Mon Nov 12 01:22:38 2018 +0100
@@ -694,7 +694,7 @@
     default=None,
 )
 coreconfigitem('format', 'sparse-revlog',
-    default=False,
+    default=True,
 )
 coreconfigitem('format', 'usefncache',
     default=True,

--- a/mercurial/upgrade.py	Mon Nov 12 01:22:30 2018 +0100
+++ b/mercurial/upgrade.py	Mon Nov 12 01:22:38 2018 +0100
@@ -269,7 +269,7 @@
 
     _requirement = localrepo.SPARSEREVLOG_REQUIREMENT
 
-    default = False
+    default = True
 
     description = _('in order to limit disk reading and memory usage on older '
                     'version, the span of a delta chain from its root to its '

--- a/tests/test-upgrade-repo.t	Mon Nov 12 01:22:30 2018 +0100
+++ b/tests/test-upgrade-repo.t	Mon Nov 12 01:22:38 2018 +0100
@@ -56,7 +56,7 @@
   fncache:        yes
   dotencode:      yes
   generaldelta:   yes
-  sparserevlog:    no
+  sparserevlog:   yes
   plain-cl-delta: yes
   compression:    zlib
   $ hg debugformat --verbose
@@ -64,7 +64,7 @@
   fncache:        yes    yes     yes
   dotencode:      yes    yes     yes
   generaldelta:   yes    yes     yes
-  sparserevlog:    no     no      no
+  sparserevlog:   yes    yes     yes
   plain-cl-delta: yes    yes     yes
   compression:    zlib   zlib    zlib
   $ hg debugformat --verbose --config format.usefncache=no
@@ -72,7 +72,7 @@
   fncache:        yes     no     yes
   dotencode:      yes     no     yes
   generaldelta:   yes    yes     yes
-  sparserevlog:    no     no      no
+  sparserevlog:   yes    yes     yes
   plain-cl-delta: yes    yes     yes
   compression:    zlib   zlib    zlib
   $ hg debugformat --verbose --config format.usefncache=no --color=debug
@@ -80,7 +80,7 @@
   [formatvariant.name.mismatchconfig|fncache:       ][formatvariant.repo.mismatchconfig| yes][formatvariant.config.special|     no][formatvariant.default|     yes]
   [formatvariant.name.mismatchconfig|dotencode:     ][formatvariant.repo.mismatchconfig| yes][formatvariant.config.special|     no][formatvariant.default|     yes]
   [formatvariant.name.uptodate|generaldelta:  ][formatvariant.repo.uptodate| yes][formatvariant.config.default|    yes][formatvariant.default|     yes]
-  [formatvariant.name.uptodate|sparserevlog:  ][formatvariant.repo.uptodate|  no][formatvariant.config.default|     no][formatvariant.default|      no]
+  [formatvariant.name.uptodate|sparserevlog:  ][formatvariant.repo.uptodate| yes][formatvariant.config.default|    yes][formatvariant.default|     yes]
   [formatvariant.name.uptodate|plain-cl-delta:][formatvariant.repo.uptodate| yes][formatvariant.config.default|    yes][formatvariant.default|     yes]
   [formatvariant.name.uptodate|compression:   ][formatvariant.repo.uptodate| zlib][formatvariant.config.default|   zlib][formatvariant.default|    zlib]
   $ hg debugformat -Tjson
@@ -104,10 +104,10 @@
     "repo": true
    },
    {
-    "config": false,
-    "default": false,
+    "config": true,
+    "default": true,
     "name": "sparserevlog",
-    "repo": false
+    "repo": true
    },
    {
     "config": true,
@@ -127,7 +127,7 @@
   performing an upgrade with "--run" will make the following changes:
   
   requirements
-     preserved: dotencode, fncache, generaldelta, revlogv1, store
+     preserved: dotencode, fncache, generaldelta, revlogv1, sparserevlog, store
   
   additional optimizations are available by specifying "--optimize <name>":
   
@@ -151,7 +151,7 @@
   performing an upgrade with "--run" will make the following changes:
   
   requirements
-     preserved: dotencode, fncache, generaldelta, revlogv1, store
+     preserved: dotencode, fncache, generaldelta, revlogv1, sparserevlog, store
   
   redeltaparent
      deltas within internal storage will choose a new base revision if needed
@@ -188,7 +188,7 @@
   fncache:         no    yes     yes
   dotencode:       no    yes     yes
   generaldelta:    no    yes     yes
-  sparserevlog:    no     no      no
+  sparserevlog:    no    yes     yes
   plain-cl-delta: yes    yes     yes
   compression:    zlib   zlib    zlib
   $ hg debugformat --verbose --config format.usegeneraldelta=no
@@ -196,7 +196,7 @@
   fncache:         no    yes     yes
   dotencode:       no    yes     yes
   generaldelta:    no     no     yes
-  sparserevlog:    no     no      no
+  sparserevlog:    no     no     yes
   plain-cl-delta: yes    yes     yes
   compression:    zlib   zlib    zlib
   $ hg debugformat --verbose --config format.usegeneraldelta=no --color=debug
@@ -204,7 +204,7 @@
   [formatvariant.name.mismatchconfig|fncache:       ][formatvariant.repo.mismatchconfig|  no][formatvariant.config.default|    yes][formatvariant.default|     yes]
   [formatvariant.name.mismatchconfig|dotencode:     ][formatvariant.repo.mismatchconfig|  no][formatvariant.config.default|    yes][formatvariant.default|     yes]
   [formatvariant.name.mismatchdefault|generaldelta:  ][formatvariant.repo.mismatchdefault|  no][formatvariant.config.special|     no][formatvariant.default|     yes]
-  [formatvariant.name.uptodate|sparserevlog:  ][formatvariant.repo.uptodate|  no][formatvariant.config.default|     no][formatvariant.default|      no]
+  [formatvariant.name.mismatchdefault|sparserevlog:  ][formatvariant.repo.mismatchdefault|  no][formatvariant.config.special|     no][formatvariant.default|     yes]
   [formatvariant.name.uptodate|plain-cl-delta:][formatvariant.repo.uptodate| yes][formatvariant.config.default|    yes][formatvariant.default|     yes]
   [formatvariant.name.uptodate|compression:   ][formatvariant.repo.uptodate| zlib][formatvariant.config.default|   zlib][formatvariant.default|    zlib]
   $ hg debugupgraderepo
@@ -219,12 +219,15 @@
   generaldelta
      deltas within internal storage are unable to choose optimal revisions; repository is larger and slower than it could be; interaction with other repositories may require extra network and CPU resources, making "hg push" and "hg pull" slower
   
+  sparserevlog
+     in order to limit disk reading and memory usage on older version, the span of a delta chain from its root to its end is limited, whatever the relevant data in this span. This can severly limit Mercurial ability to build good chain of delta resulting is much more storage space being taken and limit reusability of on disk delta during exchange.
+  
   
   performing an upgrade with "--run" will make the following changes:
   
   requirements
      preserved: revlogv1, store
-     added: dotencode, fncache, generaldelta
+     added: dotencode, fncache, generaldelta, sparserevlog
   
   fncache
      repository will be more resilient to storing certain paths and performance of certain operations should be improved
@@ -235,6 +238,9 @@
   generaldelta
      repository storage will be able to create optimal deltas; new repository data will be smaller and read times should decrease; interacting with other repositories using this storage model should require less network and CPU resources, making "hg push" and "hg pull" faster
   
+  sparserevlog
+     Revlog supports delta chain with more unused data between payload. These gaps will be skipped at read time. This allows for better delta chains, making a better compression and faster exchange with server.
+  
   additional optimizations are available by specifying "--optimize <name>":
   
   redeltaparent
@@ -259,6 +265,9 @@
   generaldelta
      deltas within internal storage are unable to choose optimal revisions; repository is larger and slower than it could be; interaction with other repositories may require extra network and CPU resources, making "hg push" and "hg pull" slower
   
+  sparserevlog
+     in order to limit disk reading and memory usage on older version, the span of a delta chain from its root to its end is limited, whatever the relevant data in this span. This can severly limit Mercurial ability to build good chain of delta resulting is much more storage space being taken and limit reusability of on disk delta during exchange.
+  
   repository lacks features used by the default config options:
   
   dotencode
@@ -269,7 +278,7 @@
   
   requirements
      preserved: revlogv1, store
-     added: fncache, generaldelta
+     added: fncache, generaldelta, sparserevlog
   
   fncache
      repository will be more resilient to storing certain paths and performance of certain operations should be improved
@@ -277,6 +286,9 @@
   generaldelta
      repository storage will be able to create optimal deltas; new repository data will be smaller and read times should decrease; interacting with other repositories using this storage model should require less network and CPU resources, making "hg push" and "hg pull" faster
   
+  sparserevlog
+     Revlog supports delta chain with more unused data between payload. These gaps will be skipped at read time. This allows for better delta chains, making a better compression and faster exchange with server.
+  
   additional optimizations are available by specifying "--optimize <name>":
   
   redeltaparent
@@ -301,7 +313,7 @@
   upgrade will perform the following actions:
   
   requirements
-     preserved: dotencode, fncache, generaldelta, revlogv1, store
+     preserved: dotencode, fncache, generaldelta, revlogv1, sparserevlog, store
   
   beginning upgrade...
   repository locked and read-only
@@ -435,7 +447,7 @@
   upgrade will perform the following actions:
   
   requirements
-     preserved: dotencode, fncache, generaldelta, revlogv1, store
+     preserved: dotencode, fncache, generaldelta, revlogv1, sparserevlog, store
   
   beginning upgrade...
   repository locked and read-only
@@ -466,7 +478,7 @@
   upgrade will perform the following actions:
   
   requirements
-     preserved: dotencode, fncache, generaldelta, revlogv1, store
+     preserved: dotencode, fncache, generaldelta, revlogv1, sparserevlog, store
   
   redeltafulladd
      each revision will be added as new content to the internal storage; this will likely drastically slow down execution time, but some extensions might need it
@@ -523,13 +535,14 @@
   generaldelta
   largefiles
   revlogv1
+  sparserevlog
   store
 
   $ hg debugupgraderepo --run
   upgrade will perform the following actions:
   
   requirements
-     preserved: dotencode, fncache, generaldelta, largefiles, revlogv1, store
+     preserved: dotencode, fncache, generaldelta, largefiles, revlogv1, sparserevlog, store
   
   beginning upgrade...
   repository locked and read-only
@@ -561,6 +574,7 @@
   generaldelta
   largefiles
   revlogv1
+  sparserevlog
   store
 
   $ cat << EOF >> .hg/hgrc
@@ -581,7 +595,7 @@
   upgrade will perform the following actions:
   
   requirements
-     preserved: dotencode, fncache, generaldelta, largefiles, lfs, revlogv1, store
+     preserved: dotencode, fncache, generaldelta, largefiles, lfs, revlogv1, sparserevlog, store
   
   beginning upgrade...
   repository locked and read-only
@@ -667,16 +681,16 @@
   $ hg config format
   format.maxchainlen=9001
   $ hg debugdeltachain file
-      rev  chain# chainlen     prev   delta       size    rawsize  chainsize     ratio   lindist extradist extraratio
-        0       1        1       -1    base         77        182         77   0.42308        77         0    0.00000
-        1       1        2        0      p1         21        191         98   0.51309        98         0    0.00000
-        2       2        1       -1    base         84        200         84   0.42000        84         0    0.00000
+      rev  chain# chainlen     prev   delta       size    rawsize  chainsize     ratio   lindist extradist extraratio   readsize largestblk rddensity srchunks
+        0       1        1       -1    base         77        182         77   0.42308        77         0    0.00000         77         77   1.00000        1
+        1       1        2        0      p1         21        191         98   0.51309        98         0    0.00000         98         98   1.00000        1
+        2       1        2        0   other         30        200        107   0.53500       128        21    0.19626        128        128   0.83594        1
 
   $ hg debugupgraderepo --run --optimize redeltaall
   upgrade will perform the following actions:
   
   requirements
-     preserved: dotencode, fncache, generaldelta, revlogv1, store
+     preserved: dotencode, fncache, generaldelta, revlogv1, sparserevlog, store
   
   redeltaall
      deltas within internal storage will be fully recomputed; this will likely drastically slow down execution time
@@ -686,14 +700,14 @@
   creating temporary repository to stage migrated data: $TESTTMP/localconfig/.hg/upgrade.* (glob)
   (it is safe to interrupt this process any time before data migration completes)
   migrating 9 total revisions (3 in filelogs, 3 in manifests, 3 in changelog)
-  migrating 1.05 KB in store; 882 bytes tracked data
-  migrating 1 filelogs containing 3 revisions (374 bytes in store; 573 bytes tracked data)
-  finished migrating 3 filelog revisions across 1 filelogs; change in size: -63 bytes
+  migrating 1019 bytes in store; 882 bytes tracked data
+  migrating 1 filelogs containing 3 revisions (320 bytes in store; 573 bytes tracked data)
+  finished migrating 3 filelog revisions across 1 filelogs; change in size: -9 bytes
   migrating 1 manifests containing 3 revisions (333 bytes in store; 138 bytes tracked data)
   finished migrating 3 manifest revisions across 1 manifests; change in size: 0 bytes
   migrating changelog containing 3 revisions (366 bytes in store; 171 bytes tracked data)
   finished migrating 3 changelog revisions; change in size: 0 bytes
-  finished migrating 9 total revisions; total change in store size: -63 bytes
+  finished migrating 9 total revisions; total change in store size: -9 bytes
   copying phaseroots
   data fully migrated to temporary repository
   marking source repository as being upgraded; clients will be unable to read from repository
@@ -706,10 +720,10 @@
   copy of old repository backed up at $TESTTMP/localconfig/.hg/upgradebackup.* (glob)
   the old repository will not be deleted; remove it to free up disk space once the upgraded repository is verified
   $ hg debugdeltachain file
-      rev  chain# chainlen     prev   delta       size    rawsize  chainsize     ratio   lindist extradist extraratio
-        0       1        1       -1    base         77        182         77   0.42308        77         0    0.00000
-        1       1        2        0      p1         21        191         98   0.51309        98         0    0.00000
-        2       1        3        1      p1         21        200        119   0.59500       119         0    0.00000
+      rev  chain# chainlen     prev   delta       size    rawsize  chainsize     ratio   lindist extradist extraratio   readsize largestblk rddensity srchunks
+        0       1        1       -1    base         77        182         77   0.42308        77         0    0.00000         77         77   1.00000        1
+        1       1        2        0      p1         21        191         98   0.51309        98         0    0.00000         98         98   1.00000        1
+        2       1        3        1      p1         21        200        119   0.59500       119         0    0.00000        119        119   1.00000        1
   $ cd ..
 
   $ cat << EOF >> $HGRCPATH