comparison mercurial/revlog.py @ 33207:895ecec31c70

revlog: add an experimental option to mitigated delta issues (issue5480)

The general delta heuristic to select a delta do not scale with the number of
branch. The delta base is frequently too far away to be able to reuse a chain
according to the "distance" criteria. This leads to insertion of larger delta (or
even full text) that themselves push the bases for the next delta further away
leading to more large deltas and full texts. This full text and frequent
recomputation throw Mercurial performance in disarray.

For example of a slightly large repository

  280 000 files (2 150 000 versions)
  430 000 changesets (10 000 topological heads)

Number below compares repository with and without the distance criteria:

manifest size:
    with:    21.4 GB
    without:  0.3 GB

store size:
    with:    28.7 GB
    without   7.4 GB

bundle last 15 00 revisions:
    with:    800 seconds
             971 MB
    without:  50 seconds
              73 MB

unbundle time (of the last 15K revisions):
    with:    1150 seconds (~19 minutes)
    without:   35 seconds

Similar issues has been observed in other repositories.


Adding a new option or "feature" on stable is uncommon. However, given that this
issues is making Mercurial practically unusable, I'm exceptionally targeting
this patch for stable.

What is actually needed is a full rework of the delta building and reading
logic. However, that will be a longer process and churn not suitable for stable.

In the meantime, we introduces a quick and dirty mitigation of this in the
'experimental' config space. The new option introduces a way to set the maximum
amount of memory usable to store a diff in memory. This extend the ability for
Mercurial to create chains without removing all safe guard regarding memory
access. The option should be phased out when core has a more proper solution
available.

Setting the limit to '0' remove all limits, setting it to '-1' use the default
limit (textsize x 4).

33206:45d6e2767a93

        self._pcache = {}
        # Mapping of revision integer to full node.
        self._nodecache = {nullid: nullrev}
        self._nodepos = None
        self._compengine = 'zlib'

        v = REVLOG_DEFAULT_VERSION
        opts = getattr(opener, 'options', None)
        if opts is not None:
            if 'revlogv2' in opts:

            if 'aggressivemergedeltas' in opts:
                self._aggressivemergedeltas = opts['aggressivemergedeltas']
            self._lazydeltabase = bool(opts.get('lazydeltabase', False))
            if 'compengine' in opts:
                self._compengine = opts['compengine']

        if self._chunkcachesize <= 0:
            raise RevlogError(_('revlog chunk cache size %r is not greater '
                                'than 0') % self._chunkcachesize)
        elif self._chunkcachesize & (self._chunkcachesize - 1):

        # - 'dist' is the distance from the base revision -- bounding it limits
        #   the amount of I/O we need to do.
        # - 'compresseddeltalen' is the sum of the total size of deltas we need
        #   to apply -- bounding it limits the amount of CPU we consume.
        dist, l, data, base, chainbase, chainlen, compresseddeltalen = d
        if (dist > textlen * 4 or l > textlen or
            compresseddeltalen > textlen * 2 or
            (self._maxchainlen and chainlen > self._maxchainlen)):
            return False

        return True

33207:895ecec31c70

        self._pcache = {}
        # Mapping of revision integer to full node.
        self._nodecache = {nullid: nullrev}
        self._nodepos = None
        self._compengine = 'zlib'
        self._maxdeltachainspan = -1

        v = REVLOG_DEFAULT_VERSION
        opts = getattr(opener, 'options', None)
        if opts is not None:
            if 'revlogv2' in opts:

            if 'aggressivemergedeltas' in opts:
                self._aggressivemergedeltas = opts['aggressivemergedeltas']
            self._lazydeltabase = bool(opts.get('lazydeltabase', False))
            if 'compengine' in opts:
                self._compengine = opts['compengine']
            if 'maxdeltachainspan' in opts:
                self._maxdeltachainspan = opts['maxdeltachainspan']

        if self._chunkcachesize <= 0:
            raise RevlogError(_('revlog chunk cache size %r is not greater '
                                'than 0') % self._chunkcachesize)
        elif self._chunkcachesize & (self._chunkcachesize - 1):

        # - 'dist' is the distance from the base revision -- bounding it limits
        #   the amount of I/O we need to do.
        # - 'compresseddeltalen' is the sum of the total size of deltas we need
        #   to apply -- bounding it limits the amount of CPU we consume.
        dist, l, data, base, chainbase, chainlen, compresseddeltalen = d

        defaultmax = textlen * 4
        maxdist = self._maxdeltachainspan
        if not maxdist:
            maxdist = dist # ensure the conditional pass
        maxdist = max(maxdist, defaultmax)
        if (dist > maxdist or l > textlen or
            compresseddeltalen > textlen * 2 or
            (self._maxchainlen and chainlen > self._maxchainlen)):
            return False

        return True