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).
# 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
import re
from mercurial.i18n import _
from mercurial import (
error,
hg,
util,
)
from . import (
lfutil,
localstore,
wirestore,
)
# During clone this function is passed the src's ui object
# but it needs the dest's ui object so it can read out of
# the config file. Use repo.ui instead.
def openstore(repo, remote=None, put=False):
ui = repo.ui
if not remote:
lfpullsource = getattr(repo, 'lfpullsource', None)
if lfpullsource:
path = ui.expandpath(lfpullsource)
elif put:
path = ui.expandpath('default-push', 'default')
else:
path = ui.expandpath('default')
# ui.expandpath() leaves 'default-push' and 'default' alone if
# they cannot be expanded: fallback to the empty string,
# meaning the current directory.
if path == 'default-push' or path == 'default':
path = ''
remote = repo
else:
path, _branches = hg.parseurl(path)
remote = hg.peer(repo, {}, path)
# The path could be a scheme so use Mercurial's normal functionality
# to resolve the scheme to a repository and use its path
path = util.safehasattr(remote, 'url') and remote.url() or remote.path
match = _scheme_re.match(path)
if not match: # regular filesystem path
scheme = 'file'
else:
scheme = match.group(1)
try:
storeproviders = _storeprovider[scheme]
except KeyError:
raise error.Abort(_('unsupported URL scheme %r') % scheme)
for classobj in storeproviders:
try:
return classobj(ui, repo, remote)
except lfutil.storeprotonotcapable:
pass
raise error.Abort(_('%s does not appear to be a largefile store') %
util.hidepassword(path))
_storeprovider = {
'file': [localstore.localstore],
'http': [wirestore.wirestore],
'https': [wirestore.wirestore],
'ssh': [wirestore.wirestore],
}
_scheme_re = re.compile(r'^([a-zA-Z0-9+-.]+)://')