view tests/test-histedit-no-change.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)

test for old histedit issue #6:
editing a changeset without any actual change would corrupt the repository

  $ . "$TESTDIR/histedit-helpers.sh"

  $ cat >> $HGRCPATH <<EOF
  > [extensions]
  > histedit=
  > EOF

  $ initrepo ()
  > {
  >     dir="$1"
  >     comment="$2"
  >     if [ -n "${comment}" ]; then
  >         echo % ${comment}
  >         echo % ${comment} | sed 's:.:-:g'
  >     fi
  >     hg init ${dir}
  >     cd ${dir}
  >     for x in a b c d e f ; do
  >         echo $x > $x
  >         hg add $x
  >         hg ci -m $x
  >     done
  >     cd ..
  > }

  $ geneditor ()
  > {
  >     # generate an editor script for selecting changesets to be edited
  >     choice=$1  # changesets that should be edited (using sed line ranges)
  >     cat <<EOF | sed 's:^....::'
  >     # editing the rules, replacing 'pick' with 'edit' for the chosen lines
  >     sed '${choice}s:^pick:edit:' "\$1" > "\${1}.tmp"
  >     mv "\${1}.tmp" "\$1"
  >     # displaying the resulting rules, minus comments and empty lines
  >     sed '/^#/d;/^$/d;s:^:| :' "\$1" >&2
  > EOF
  > }

  $ startediting ()
  > {
  >     # begin an editing session
  >     choice="$1"  # changesets that should be edited
  >     number="$2"  # number of changesets considered (from tip)
  >     comment="$3"
  >     geneditor "${choice}" > edit.sh
  >     echo % start editing the history ${comment}
  >     HGEDITOR="sh ./edit.sh" hg histedit -- -${number} 2>&1 | fixbundle
  > }

  $ continueediting ()
  > {
  >     # continue an edit already in progress
  >     editor="$1"  # message editor when finalizing editing
  >     comment="$2"
  >     echo % finalize changeset editing ${comment}
  >     HGEDITOR=${editor} hg histedit --continue 2>&1 | fixbundle
  > }

  $ graphlog ()
  > {
  >     comment="${1:-log}"
  >     echo % "${comment}"
  >     hg log -G --template '{rev} {node} \"{desc|firstline}\"\n'
  > }


  $ initrepo r1 "test editing with no change"
  % test editing with no change
  -----------------------------
  $ cd r1
  $ graphlog "log before editing"
  % log before editing
  @  5 652413bf663ef2a641cab26574e46d5f5a64a55a "f"
  |
  o  4 e860deea161a2f77de56603b340ebbb4536308ae "e"
  |
  o  3 055a42cdd88768532f9cf79daa407fc8d138de9b "d"
  |
  o  2 177f92b773850b59254aa5e923436f921b55483b "c"
  |
  o  1 d2ae7f538514cd87c17547b0de4cea71fe1af9fb "b"
  |
  o  0 cb9a9f314b8b07ba71012fcdbc544b5a4d82ff5b "a"
  
  $ startediting 2 3 "(not changing anything)" # edit the 2nd of 3 changesets
  % start editing the history (not changing anything)
  | pick 055a42cdd887 3 d
  | edit e860deea161a 4 e
  | pick 652413bf663e 5 f
  0 files updated, 0 files merged, 2 files removed, 0 files unresolved
  Editing (e860deea161a), commit as needed now to split the change
  (to edit e860deea161a, `hg histedit --continue` after making changes)
  $ continueediting true "(leaving commit message unaltered)"
  % finalize changeset editing (leaving commit message unaltered)


check state of working copy
  $ hg id
  794fe033d0a0 tip

  $ graphlog "log after history editing"
  % log after history editing
  @  5 794fe033d0a030f8df77c5de945fca35c9181c30 "f"
  |
  o  4 04d2fab980779f332dec458cc944f28de8b43435 "e"
  |
  o  3 055a42cdd88768532f9cf79daa407fc8d138de9b "d"
  |
  o  2 177f92b773850b59254aa5e923436f921b55483b "c"
  |
  o  1 d2ae7f538514cd87c17547b0de4cea71fe1af9fb "b"
  |
  o  0 cb9a9f314b8b07ba71012fcdbc544b5a4d82ff5b "a"
  

  $ cd ..

  $ initrepo r2 "test editing with no change, then abort"
  % test editing with no change, then abort
  -----------------------------------------
  $ cd r2
  $ graphlog "log before editing"
  % log before editing
  @  5 652413bf663ef2a641cab26574e46d5f5a64a55a "f"
  |
  o  4 e860deea161a2f77de56603b340ebbb4536308ae "e"
  |
  o  3 055a42cdd88768532f9cf79daa407fc8d138de9b "d"
  |
  o  2 177f92b773850b59254aa5e923436f921b55483b "c"
  |
  o  1 d2ae7f538514cd87c17547b0de4cea71fe1af9fb "b"
  |
  o  0 cb9a9f314b8b07ba71012fcdbc544b5a4d82ff5b "a"
  
  $ startediting 1,2 3 "(not changing anything)" # edit the 1st two of 3 changesets
  % start editing the history (not changing anything)
  | edit 055a42cdd887 3 d
  | edit e860deea161a 4 e
  | pick 652413bf663e 5 f
  0 files updated, 0 files merged, 3 files removed, 0 files unresolved
  Editing (055a42cdd887), commit as needed now to split the change
  (to edit 055a42cdd887, `hg histedit --continue` after making changes)
  $ continueediting true "(leaving commit message unaltered)"
  % finalize changeset editing (leaving commit message unaltered)
  Editing (e860deea161a), commit as needed now to split the change
  (to edit e860deea161a, `hg histedit --continue` after making changes)
  $ graphlog "log after first edit"
  % log after first edit
  @  6 e5ae3ca2f1ffdbd89ec41ebc273a231f7c3022f2 "d"
  |
  | o  5 652413bf663ef2a641cab26574e46d5f5a64a55a "f"
  | |
  | o  4 e860deea161a2f77de56603b340ebbb4536308ae "e"
  | |
  | o  3 055a42cdd88768532f9cf79daa407fc8d138de9b "d"
  |/
  o  2 177f92b773850b59254aa5e923436f921b55483b "c"
  |
  o  1 d2ae7f538514cd87c17547b0de4cea71fe1af9fb "b"
  |
  o  0 cb9a9f314b8b07ba71012fcdbc544b5a4d82ff5b "a"
  

abort editing session, after first forcibly updating away
  $ hg up 0
  abort: histedit in progress
  (use 'hg histedit --continue' or 'hg histedit --abort')
  [20]
  $ mv .hg/histedit-state .hg/histedit-state-ignore
  $ hg up 0
  0 files updated, 0 files merged, 3 files removed, 0 files unresolved
  $ mv .hg/histedit-state-ignore .hg/histedit-state
  $ hg sum
  parent: 0:cb9a9f314b8b 
   a
  branch: default
  commit: 1 added, 1 unknown (new branch head)
  update: 6 new changesets (update)
  phases: 7 draft
  hist:   2 remaining (histedit --continue)

  $ hg histedit --abort 2>&1 | fixbundle

modified files should survive the abort when we've moved away already
  $ hg st
  A e
  ? edit.sh

  $ graphlog "log after abort"
  % log after abort
  o  5 652413bf663ef2a641cab26574e46d5f5a64a55a "f"
  |
  o  4 e860deea161a2f77de56603b340ebbb4536308ae "e"
  |
  o  3 055a42cdd88768532f9cf79daa407fc8d138de9b "d"
  |
  o  2 177f92b773850b59254aa5e923436f921b55483b "c"
  |
  o  1 d2ae7f538514cd87c17547b0de4cea71fe1af9fb "b"
  |
  @  0 cb9a9f314b8b07ba71012fcdbc544b5a4d82ff5b "a"
  
aborting and not changing files can skip mentioning updating (no) files
  $ hg up
  5 files updated, 0 files merged, 0 files removed, 0 files unresolved
  $ hg commit --close-branch -m 'closebranch'
  $ startediting 1 1 "(not changing anything)" # edit the 3rd of 3 changesets
  % start editing the history (not changing anything)
  | edit 292aec348d9e 6 closebranch
  Editing (292aec348d9e), commit as needed now to split the change
  (to edit 292aec348d9e, `hg histedit --continue` after making changes)
  $ hg histedit --abort

  $ cd ..