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snapshot: search for unrelated but reusable full-snapshot...
snapshot: search for unrelated but reusable full-snapshot # New Strategy Step: Reusing Snapshot Outside Of Parents' Chain. If no suitable bases were found in the parent's chains, see if we could reuse a full snapshot not directly related to the current revision. Such search can be expensive, so we only search for snapshots appended to the revlog *after* the bases used by the parents of the current revision (the one we just tested). We assume the parent's bases were created because the previous snapshots were unsuitable, so there are low odds they would be useful now. This search gives a chance to reuse a delta chain unrelated to the current revision. Without this re-use, topological branches would keep reopening new full chains. Creating more and more snapshots as the repository grow. In repositories with many topological branches, the lack of delta reuse can create too many snapshots reducing overall compression to nothing. This results in a very large repository and other usability issues. For now, we still focus on creating level-1 snapshots. However, this principle will play a large part in how we avoid snapshot explosion once we have more snapshot levels. # Effects On The Test Repository In the test repository we created, we can see the beneficial effect of such reuse. We need very few level-0 snapshots and the overall revlog size has decreased. The `hg debugrevlog` call, show a "lvl-2" snapshot. It comes from the existing delta logic using the `prev` revision (revlog's tip) as the base. In this specific case, it turns out the tip was a level-1 snapshot. This is a coincidence that can be ignored. Finding and testing against all these unrelated snapshots can have a performance impact at write time. We currently focus on building good deltas chain we build. Performance concern will be dealt with later in another series.
Pulkit Goyal - - Load All Authors

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rewriteutil.py
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# rewriteutil.py - utility functions for rewriting changesets
#
# Copyright 2017 Octobus <contact@octobus.net>
#
# This software may be used and distributed according to the terms of the
# GNU General Public License version 2 or any later version.
from __future__ import absolute_import
from .i18n import _
from . import (
error,
node,
obsolete,
revset,
)
def precheck(repo, revs, action='rewrite'):
"""check if revs can be rewritten
action is used to control the error message.
Make sure this function is called after taking the lock.
"""
if node.nullrev in revs:
msg = _("cannot %s null changeset") % (action)
hint = _("no changeset checked out")
raise error.Abort(msg, hint=hint)
publicrevs = repo.revs('%ld and public()', revs)
if len(repo[None].parents()) > 1:
raise error.Abort(_("cannot %s while merging") % action)
if publicrevs:
msg = _("cannot %s public changesets") % (action)
hint = _("see 'hg help phases' for details")
raise error.Abort(msg, hint=hint)
newunstable = disallowednewunstable(repo, revs)
if newunstable:
raise error.Abort(_("cannot %s changeset with children") % action)
def disallowednewunstable(repo, revs):
"""Checks whether editing the revs will create new unstable changesets and
are we allowed to create them.
To allow new unstable changesets, set the config:
`experimental.evolution.allowunstable=True`
"""
allowunstable = obsolete.isenabled(repo, obsolete.allowunstableopt)
if allowunstable:
return revset.baseset()
return repo.revs("(%ld::) - %ld", revs, revs)