view tests/test-debugbundle.t @ 51681:522b4d729e89

mmap: populate the mapping by default

Without pre-population, accessing all data through a mmap can result in many
pagefault, reducing performance significantly. If the mmap is prepopulated, the
performance can no longer get slower than a full read.
(See benchmark number below)

In some cases were very few data is read, prepopulating can be overkill and
slower than populating on access (through page fault). So that behavior can be
controlled when the caller can pre-determine the best behavior.
(See benchmark number below)

In addition, testing with populating in a secondary thread yield great result
combining the best of each approach. This might be implemented in later
changesets.

In all cases, using mmap has a great effect on memory usage when many processes
run in parallel on the same machine.

### Benchmarks

# What did I run

A couple of month back I ran a large benchmark campaign to assess the impact of
various approach for using mmap with the revlog (and other files), it
highlighted a few benchmarks that capture the impact of the changes well. So to
validate this change I checked the following:

- log command displaying various revisions
  (read the changelog index)
- log command displaying the patch of listed revisions
  (read the changelog index, the manifest index and a few files indexes)
- unbundling a few revisions
  (read and write changelog, manifest and few files indexes, and walk the graph
  to update some cache)
- pushing a few revisions
  (read and write changelog, manifest and few files indexes, walk the graph to
  update some cache, performs various accesses locally and remotely during
  discovery)

Benchmarks were run using the default module policy (c+py) and the rust one. No
significant difference were found between the two implementation, so we will
present result using the default policy (unless otherwise specified).

I ran them on a few repositories :

- mercurial: a "public changeset only" copy of mercurial from 2018-08-01 using
  zstd compression and sparse-revlog

- pypy: a copy of pypy from 2018-08-01 using zstd compression and sparse-revlog

- netbeans: a copy of netbeans from 2018-08-01 using zstd compression and
  sparse-revlog

- mozilla-try: a copy of mozilla-try from 2019-02-18 using zstd compression and
  sparse-revlog

- mozilla-try persistent-nodemap: Same as the above but with a persistent
  nodemap. Used for the log --patch benchmark only

# Results

For the smaller repositories (mercurial, pypy), the impact of mmap is almost
imperceptible, other cost dominating the operation. The impact of prepopulating
is undiscernible in the benchmark we ran.

For larger repositories the benchmark support explanation given above:

On netbeans, the log can be about 1% faster without repopulation (for a
difference < 100ms) but unbundle becomes a bit slower, even when small.

### data-env-vars.name                             = netbeans-2018-08-01-zstd-sparse-revlog
  # benchmark.name                                 = hg.command.unbundle
  # benchmark.variants.issue6528                   = disabled
  # benchmark.variants.reuse-external-delta-parent = yes
  # benchmark.variants.revs                        = any-1-extra-rev
  # benchmark.variants.source                      = unbundle
  # benchmark.variants.verbosity                   = quiet
with-populate: 0.240157
no-populate:   0.265087  (+10.38%, +0.02)
  # benchmark.variants.revs                        = any-100-extra-rev
with-populate: 1.459518
no-populate:   1.481290   (+1.49%, +0.02)
 ## benchmark.name                                 = hg.command.push
  # benchmark.variants.explicit-rev                = none
  # benchmark.variants.issue6528                   = disabled
  # benchmark.variants.protocol                    = ssh
  # benchmark.variants.reuse-external-delta-parent = yes
  # benchmark.variants.revs                        = any-1-extra-rev
with-populate: 0.771919
no-populate:   0.792025   (+2.60%, +0.02)
  # benchmark.variants.revs                        = any-100-extra-rev
with-populate: 1.459518
no-populate:   1.481290   (+1.49%, +0.02)


For mozilla-try, the "slow down" from pre-populate for small `hg log` is more
visible, but still small in absolute time. (using rust value for the persistent
nodemap value to be relevant).

### data-env-vars.name            = mozilla-try-2019-02-18-ds2-pnm
  # benchmark.name                = hg.command.log
  # bin-env-vars.hg.flavor        = rust
  # benchmark.variants.patch      = yes
  # benchmark.variants.limit-rev  = 1
with-populate: 0.237813
no-populate:   0.229452   (-3.52%, -0.01)
  # benchmark.variants.limit-rev  = 10
  # benchmark.variants.patch      = yes
with-populate: 1.213578
no-populate:   1.205189
### data-env-vars.name            = mozilla-try-2019-02-18-zstd-sparse-revlog
  # benchmark.variants.limit-rev  = 1000
  # benchmark.variants.patch      = no
  # benchmark.variants.rev        = tip
with-populate: 0.198607
no-populate:   0.195038   (-1.80%, -0.00)

However pre-populating provide a significant boost on more complex operations
like unbundle or push:

### data-env-vars.name                             = mozilla-try-2019-02-18-zstd-sparse-revlog
  # benchmark.name                                 = hg.command.push
  # benchmark.variants.explicit-rev                = none
  # benchmark.variants.issue6528                   = disabled
  # benchmark.variants.protocol                    = ssh
  # benchmark.variants.reuse-external-delta-parent = yes
  # benchmark.variants.revs                        = any-1-extra-rev
with-populate: 4.798632
no-populate:   4.953295   (+3.22%, +0.15)
  # benchmark.variants.revs                        = any-100-extra-rev
with-populate: 4.903618
no-populate:   5.014963   (+2.27%, +0.11)
 ## benchmark.name                                 = hg.command.unbundle
  # benchmark.variants.revs                        = any-1-extra-rev
with-populate: 1.423411
no-populate:   1.585365  (+11.38%, +0.16)
  # benchmark.variants.revs                        = any-100-extra-rev
with-populate: 1.537909
no-populate:   1.688489   (+9.79%, +0.15)

Create a test repository:

  $ hg init repo
  $ cd repo
  $ touch a ; hg add a ; hg ci -ma
  $ touch b ; hg add b ; hg ci -mb
  $ touch c ; hg add c ; hg ci -mc
  $ hg bundle --base 0 --rev tip bundle.hg -v --type v1
  2 changesets found
  uncompressed size of bundle content:
       332 (changelog)
       282 (manifests)
        93  b
        93  c
  $ hg bundle --base 0 --rev tip bundle2.hg -v --type none-v2
  2 changesets found
  uncompressed size of bundle content:
       344 (changelog)
       322 (manifests)
       113  b
       113  c

Terse output:

  $ hg debugbundle bundle.hg
  0e067c57feba1a5694ca4844f05588bb1bf82342
  991a3460af53952d10ec8a295d3d2cc2e5fa9690

Terse output:

  $ hg debugbundle bundle2.hg
  Stream params: {}
  changegroup -- {nbchanges: 2, version: 02} (mandatory: True)
      0e067c57feba1a5694ca4844f05588bb1bf82342
      991a3460af53952d10ec8a295d3d2cc2e5fa9690
  cache:rev-branch-cache -- {} (mandatory: False)

Quiet output

  $ hg debugbundle --quiet bundle2.hg
  Stream params: {}
  changegroup -- {nbchanges: 2, version: 02} (mandatory: True)
  cache:rev-branch-cache -- {} (mandatory: False)

Verbose output:

  $ hg debugbundle --all bundle.hg
  format: id, p1, p2, cset, delta base, len(delta)
  
  changelog
  0e067c57feba1a5694ca4844f05588bb1bf82342 3903775176ed42b1458a6281db4a0ccf4d9f287a 0000000000000000000000000000000000000000 0e067c57feba1a5694ca4844f05588bb1bf82342 3903775176ed42b1458a6281db4a0ccf4d9f287a 80
  991a3460af53952d10ec8a295d3d2cc2e5fa9690 0e067c57feba1a5694ca4844f05588bb1bf82342 0000000000000000000000000000000000000000 991a3460af53952d10ec8a295d3d2cc2e5fa9690 0e067c57feba1a5694ca4844f05588bb1bf82342 80
  
  manifest
  686dbf0aeca417636fa26a9121c681eabbb15a20 8515d4bfda768e04af4c13a69a72e28c7effbea7 0000000000000000000000000000000000000000 0e067c57feba1a5694ca4844f05588bb1bf82342 8515d4bfda768e04af4c13a69a72e28c7effbea7 55
  ae25a31b30b3490a981e7b96a3238cc69583fda1 686dbf0aeca417636fa26a9121c681eabbb15a20 0000000000000000000000000000000000000000 991a3460af53952d10ec8a295d3d2cc2e5fa9690 686dbf0aeca417636fa26a9121c681eabbb15a20 55
  
  b
  b80de5d138758541c5f05265ad144ab9fa86d1db 0000000000000000000000000000000000000000 0000000000000000000000000000000000000000 0e067c57feba1a5694ca4844f05588bb1bf82342 0000000000000000000000000000000000000000 0
  
  c
  b80de5d138758541c5f05265ad144ab9fa86d1db 0000000000000000000000000000000000000000 0000000000000000000000000000000000000000 991a3460af53952d10ec8a295d3d2cc2e5fa9690 0000000000000000000000000000000000000000 0

  $ hg debugbundle --all bundle2.hg
  Stream params: {}
  changegroup -- {nbchanges: 2, version: 02} (mandatory: True)
      format: id, p1, p2, cset, delta base, len(delta)
  
      changelog
      0e067c57feba1a5694ca4844f05588bb1bf82342 3903775176ed42b1458a6281db4a0ccf4d9f287a 0000000000000000000000000000000000000000 0e067c57feba1a5694ca4844f05588bb1bf82342 0000000000000000000000000000000000000000 66
      991a3460af53952d10ec8a295d3d2cc2e5fa9690 0e067c57feba1a5694ca4844f05588bb1bf82342 0000000000000000000000000000000000000000 991a3460af53952d10ec8a295d3d2cc2e5fa9690 0000000000000000000000000000000000000000 66
  
      manifest
      686dbf0aeca417636fa26a9121c681eabbb15a20 8515d4bfda768e04af4c13a69a72e28c7effbea7 0000000000000000000000000000000000000000 0e067c57feba1a5694ca4844f05588bb1bf82342 8515d4bfda768e04af4c13a69a72e28c7effbea7 55
      ae25a31b30b3490a981e7b96a3238cc69583fda1 686dbf0aeca417636fa26a9121c681eabbb15a20 0000000000000000000000000000000000000000 991a3460af53952d10ec8a295d3d2cc2e5fa9690 686dbf0aeca417636fa26a9121c681eabbb15a20 55
  
      b
      b80de5d138758541c5f05265ad144ab9fa86d1db 0000000000000000000000000000000000000000 0000000000000000000000000000000000000000 0e067c57feba1a5694ca4844f05588bb1bf82342 0000000000000000000000000000000000000000 0
  
      c
      b80de5d138758541c5f05265ad144ab9fa86d1db 0000000000000000000000000000000000000000 0000000000000000000000000000000000000000 991a3460af53952d10ec8a295d3d2cc2e5fa9690 0000000000000000000000000000000000000000 0
  cache:rev-branch-cache -- {} (mandatory: False)

  $ cd ..