hgext/record.py

revlog: don't flush data file after every added revision

The current behavior of revlogs is to flush the data file when writing
data to it. Tracing system calls revealed that changegroup processing
incurred numerous write(2) calls for values much smaller than the
default buffer size (Python defaults to 4096, but it can be adjusted
based on detected block size at run time by CPython).

The reason we flush revlogs is so readers have all data available.
For example, the current code in revlog.py will re-open the revlog
file (instead of seeking an existing file handle) to read the text
of a revision. This happens when starting a new delta chain when
adding several revisions from changegroups, for example. Yes, this
is likely sub-optimal (we should probably be sharing file descriptors
between readers and writers to avoid the flushing and associated
overhead of re-opening files).

While flushing revlogs is necessary, it appears all callers are
diligent about flushing files before a read is performed (see
buildtext() in _addrevision()), making the flush in
_writeentry() redundant and unncessary. So, we remove it. In practice,
this means we incur a write(2) a) when the buffer is full (typically
4096 bytes) b) when a new delta chain is created rather than after
every added revision. This applies to every revlog, but by volume
it mostly impacts filelogs.

Removing the redundant flush from _writeentry() significantly
reduces the number of write(2) calls during changegroup processing on
my Linux machine. When applying a changegroup of the hg repo based on
my local repo, the total number of write(2) calls during application
of the mercurial/localrepo.py revlogs dropped from 1,320 to 217 with
this patch applied. Total I/O related system calls dropped from 1,577
to 474.

When unbundling a mozilla-central gzipped bundle (264,403 changesets
with 1,492,215 changes to 222,507 files), total write(2) calls
dropped from 1,252,881 to 827,106 and total system calls dropped from
3,601,259 to 3,178,636 - a reduction of 425,775!

While the system call reduction is significant, it appears
to have no impact on wall time on my Linux and Windows machines. Still,
fewer syscalls is fewer syscalls. Surely this can't hurt. If nothing
else, it makes examining remaining system call usage simpler and opens
the door to experimenting with the performance impact of different
buffer sizes.

# record.py
#
# Copyright 2007 Bryan O'Sullivan <[e-mail]>
#
# This software may be used and distributed according to the terms of the
# GNU General Public License version 2 or any later version.

'''commands to interactively select changes for commit/qrefresh'''

from mercurial.i18n import _
from mercurial import cmdutil, commands, extensions
from mercurial import util

cmdtable = {}
command = cmdutil.command(cmdtable)
# Note for extension authors: ONLY specify testedwith = 'internal' for
# extensions which SHIP WITH MERCURIAL. Non-mainline extensions should
# be specifying the version(s) of Mercurial they are tested with, or
# leave the attribute unspecified.
testedwith = 'internal'


@command("record",
         # same options as commit + white space diff options
        [c for c in commands.table['^commit|ci'][1][:]
            if c[1] != "interactive"] + commands.diffwsopts,
          _('hg record [OPTION]... [FILE]...'))
def record(ui, repo, *pats, **opts):
    '''interactively select changes to commit

    If a list of files is omitted, all changes reported by :hg:`status`
    will be candidates for recording.

    See :hg:`help dates` for a list of formats valid for -d/--date.

    You will be prompted for whether to record changes to each
    modified file, and for files with multiple changes, for each
    change to use. For each query, the following responses are
    possible::

      y - record this change
      n - skip this change
      e - edit this change manually

      s - skip remaining changes to this file
      f - record remaining changes to this file

      d - done, skip remaining changes and files
      a - record all changes to all remaining files
      q - quit, recording no changes

      ? - display help

    This command is not available when committing a merge.'''

    if not ui.interactive():
        raise util.Abort(_('running non-interactively, use %s instead') %
                         'commit')

    opts["interactive"] = True
    backup = ui.backupconfig('experimental', 'crecord')
    try:
        ui.setconfig('experimental', 'crecord', False, 'record')
        commands.commit(ui, repo, *pats, **opts)
    finally:
        ui.restoreconfig(backup)

def qrefresh(origfn, ui, repo, *pats, **opts):
    if not opts['interactive']:
        return origfn(ui, repo, *pats, **opts)

    mq = extensions.find('mq')

    def committomq(ui, repo, *pats, **opts):
        # At this point the working copy contains only changes that
        # were accepted. All other changes were reverted.
        # We can't pass *pats here since qrefresh will undo all other
        # changed files in the patch that aren't in pats.
        mq.refresh(ui, repo, **opts)

    # backup all changed files
    cmdutil.dorecord(ui, repo, committomq, None, True,
                    cmdutil.recordfilter, *pats, **opts)

# This command registration is replaced during uisetup().
@command('qrecord',
    [],
    _('hg qrecord [OPTION]... PATCH [FILE]...'),
    inferrepo=True)
def qrecord(ui, repo, patch, *pats, **opts):
    '''interactively record a new patch

    See :hg:`help qnew` & :hg:`help record` for more information and
    usage.
    '''
    return _qrecord('qnew', ui, repo, patch, *pats, **opts)

def _qrecord(cmdsuggest, ui, repo, patch, *pats, **opts):
    try:
        mq = extensions.find('mq')
    except KeyError:
        raise util.Abort(_("'mq' extension not loaded"))

    repo.mq.checkpatchname(patch)

    def committomq(ui, repo, *pats, **opts):
        opts['checkname'] = False
        mq.new(ui, repo, patch, *pats, **opts)

    backup = ui.backupconfig('experimental', 'crecord')
    try:
        ui.setconfig('experimental', 'crecord', False, 'record')
        cmdutil.dorecord(ui, repo, committomq, cmdsuggest, False,
                         cmdutil.recordfilter, *pats, **opts)
    finally:
        ui.restoreconfig(backup)

def qnew(origfn, ui, repo, patch, *args, **opts):
    if opts['interactive']:
        return _qrecord(None, ui, repo, patch, *args, **opts)
    return origfn(ui, repo, patch, *args, **opts)


def uisetup(ui):
    try:
        mq = extensions.find('mq')
    except KeyError:
        return

    cmdtable["qrecord"] = \
        (qrecord,
         # same options as qnew, but copy them so we don't get
         # -i/--interactive for qrecord and add white space diff options
         mq.cmdtable['^qnew'][1][:] + commands.diffwsopts,
         _('hg qrecord [OPTION]... PATCH [FILE]...'))

    _wrapcmd('qnew', mq.cmdtable, qnew, _("interactively record a new patch"))
    _wrapcmd('qrefresh', mq.cmdtable, qrefresh,
             _("interactively select changes to refresh"))

def _wrapcmd(cmd, table, wrapfn, msg):
    entry = extensions.wrapcommand(table, cmd, wrapfn)
    entry[1].append(('i', 'interactive', None, msg))