view tests/test-convert-darcs.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)

#require darcs

  $ echo "[extensions]" >> $HGRCPATH
  $ echo "convert=" >> $HGRCPATH
  $ DARCS_EMAIL='[e-mail]'; export DARCS_EMAIL

initialize darcs repo

  $ mkdir darcs-repo
  $ cd darcs-repo
  $ darcs init -q
  $ echo a > a
  $ darcs record -a -l -m p0
  Finished recording patch 'p0'
  $ cd ..

branch and update

  $ darcs get -q darcs-repo darcs-clone >/dev/null
  $ cd darcs-clone
  $ echo c >> a
  $ echo c > c
  $ darcs record -a -l -m p1.1
  Finished recording patch 'p1.1'
  $ cd ..

skip if we can't import elementtree

  $ if hg convert darcs-repo darcs-dummy 2>&1 | grep ElementTree > /dev/null; then
  >     echo 'skipped: missing feature: elementtree module'
  >     exit 80
  > fi

update source

  $ cd darcs-repo
  $ echo b >> a
  $ echo b > b
  $ darcs record -a -l -m p1.2
  Finished recording patch 'p1.2'

  $ darcs pull -q -a --no-set-default ../darcs-clone
  Backing up ./a(*) (glob)
  We have conflicts in the following files:
  ./a
   (?)
  $ sleep 1
  $ echo e > a
  $ echo f > f
  $ mkdir dir
  $ echo d > dir/d
  $ echo d > dir/d2
  $ darcs record -a -l -m p2
  Finished recording patch 'p2'

test file and directory move

  $ darcs mv -q f ff

Test remove + move

  $ darcs remove -q dir/d2
  $ rm dir/d2
  $ darcs mv -q dir dir2
  $ darcs record -a -l -m p3
  Finished recording patch 'p3'

The converter does not currently handle patch conflicts very well.
When they occur, it reverts *all* changes and moves forward,
letting the conflict resolving patch fix collisions.
Unfortunately, non-conflicting changes, like the addition of the
"c" file in p1.1 patch are reverted too.
Just to say that manifest not listing "c" here is a bug.

  $ cd ..
  $ hg convert darcs-repo darcs-repo-hg
  initializing destination darcs-repo-hg repository
  scanning source...
  sorting...
  converting...
  4 p0
  3 p1.2
  2 p1.1
  1 p2
  0 p3
  $ hg log -R darcs-repo-hg -g --template '{rev} "{desc|firstline}" ({author}) files: {files}\n' "$@"
  4 "p3" ([e-mail]) files: dir/d dir/d2 dir2/d f ff
  3 "p2" ([e-mail]) files: a dir/d dir/d2 f
  2 "p1.1" ([e-mail]) files: 
  1 "p1.2" ([e-mail]) files: a b
  0 "p0" ([e-mail]) files: a

  $ hg up -q -R darcs-repo-hg
  $ hg -R darcs-repo-hg manifest --debug
  7225b30cdf38257d5cc7780772c051b6f33e6d6b 644   a
  1e88685f5ddec574a34c70af492f95b6debc8741 644   b
  37406831adc447ec2385014019599dfec953c806 644   dir2/d
  b783a337463792a5c7d548ad85a7d3253c16ba8c 644   ff

#if no-outer-repo

try converting darcs1 repository

  $ hg clone -q "$TESTDIR/bundles/darcs1.hg" darcs
  $ hg convert -s darcs darcs/darcs1 2>&1 | grep darcs-1.0
  darcs-1.0 repository format is unsupported, please upgrade

#endif