Mercurial > hg
view mercurial/setdiscovery.py @ 23813:932f814bf016
setdiscovery: always add exponential sample to the heads
As explained in a previous changeset, prioritizing heads too much behaves
pathologically when there are more heads than the sample size. To counter this,
we always inject exponential samples before reducing to the sample size limit.
This already show some benefit in the test themselves, but on a real-world example
this moves my discovery for push to pathologically headed repo from 45 rounds to
17 of them.
We should maybe ensure that at least 25% of the result sample is heads, but I
think the random sampling will be fine in practice.
author | Pierre-Yves David <pierre-yves.david@fb.com> |
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date | Wed, 07 Jan 2015 17:28:51 -0800 |
parents | db58186dd8e3 |
children | 6a5877a73141 |
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# setdiscovery.py - improved discovery of common nodeset for mercurial # # Copyright 2010 Benoit Boissinot <bboissin@gmail.com> # and Peter Arrenbrecht <peter@arrenbrecht.ch> # # This software may be used and distributed according to the terms of the # GNU General Public License version 2 or any later version. """ Algorithm works in the following way. You have two repository: local and remote. They both contains a DAG of changelists. The goal of the discovery protocol is to find one set of node *common*, the set of nodes shared by local and remote. One of the issue with the original protocol was latency, it could potentially require lots of roundtrips to discover that the local repo was a subset of remote (which is a very common case, you usually have few changes compared to upstream, while upstream probably had lots of development). The new protocol only requires one interface for the remote repo: `known()`, which given a set of changelists tells you if they are present in the DAG. The algorithm then works as follow: - We will be using three sets, `common`, `missing`, `unknown`. Originally all nodes are in `unknown`. - Take a sample from `unknown`, call `remote.known(sample)` - For each node that remote knows, move it and all its ancestors to `common` - For each node that remote doesn't know, move it and all its descendants to `missing` - Iterate until `unknown` is empty There are a couple optimizations, first is instead of starting with a random sample of missing, start by sending all heads, in the case where the local repo is a subset, you computed the answer in one round trip. Then you can do something similar to the bisecting strategy used when finding faulty changesets. Instead of random samples, you can try picking nodes that will maximize the number of nodes that will be classified with it (since all ancestors or descendants will be marked as well). """ from node import nullid, nullrev from i18n import _ import random import util, dagutil def _updatesample(dag, nodes, sample, always, quicksamplesize=0): """update an existing sample to match the expected size The sample is updated with nodes exponentially distant from each head of the <nodes> set. (H~1, H~2, H~4, H~8, etc). If a target size is specified, the sampling will stop once this size is reached. Otherwise sampling will happen until roots of the <nodes> set are reached. :dag: a dag object from dagutil :nodes: set of nodes we want to discover (if None, assume the whole dag) :sample: a sample to update :always: set of notable nodes that will be part of the sample anyway :quicksamplesize: optional target size of the sample""" # if nodes is empty we scan the entire graph if nodes: heads = dag.headsetofconnecteds(nodes) else: heads = dag.heads() dist = {} visit = util.deque(heads) seen = set() factor = 1 while visit: curr = visit.popleft() if curr in seen: continue d = dist.setdefault(curr, 1) if d > factor: factor *= 2 if d == factor: if curr not in always: # need this check for the early exit below sample.add(curr) if quicksamplesize and (len(sample) >= quicksamplesize): return seen.add(curr) for p in dag.parents(curr): if not nodes or p in nodes: dist.setdefault(p, d + 1) visit.append(p) def _setupsample(dag, nodes, size): always = dag.headsetofconnecteds(nodes) desiredlen = size - len(always) if desiredlen <= 0: # This could be bad if there are very many heads, all unknown to the # server. We're counting on long request support here. return always, None, desiredlen return always, set(), desiredlen def _takequicksample(dag, nodes, size): always, sample, desiredlen = _setupsample(dag, nodes, size) if sample is None: return always _updatesample(dag, None, sample, always, quicksamplesize=desiredlen) sample.update(always) return sample def _takefullsample(dag, nodes, size): sample = always = dag.headsetofconnecteds(nodes) # update from heads _updatesample(dag, nodes, sample, always) # update from roots _updatesample(dag.inverse(), nodes, sample, always) assert sample sample.update(always) sample = _limitsample(sample, size) if len(sample) < size: more = size - len(sample) sample.update(random.sample(list(nodes - sample), more)) return sample def _limitsample(sample, desiredlen): """return a random subset of sample of at most desiredlen item""" if len(sample) > desiredlen: sample = set(random.sample(sample, desiredlen)) return sample def findcommonheads(ui, local, remote, initialsamplesize=100, fullsamplesize=200, abortwhenunrelated=True): '''Return a tuple (common, anyincoming, remoteheads) used to identify missing nodes from or in remote. ''' roundtrips = 0 cl = local.changelog dag = dagutil.revlogdag(cl) # early exit if we know all the specified remote heads already ui.debug("query 1; heads\n") roundtrips += 1 ownheads = dag.heads() sample = _limitsample(ownheads, initialsamplesize) # indices between sample and externalized version must match sample = list(sample) if remote.local(): # stopgap until we have a proper localpeer that supports batch() srvheadhashes = remote.heads() yesno = remote.known(dag.externalizeall(sample)) elif remote.capable('batch'): batch = remote.batch() srvheadhashesref = batch.heads() yesnoref = batch.known(dag.externalizeall(sample)) batch.submit() srvheadhashes = srvheadhashesref.value yesno = yesnoref.value else: # compatibility with pre-batch, but post-known remotes during 1.9 # development srvheadhashes = remote.heads() sample = [] if cl.tip() == nullid: if srvheadhashes != [nullid]: return [nullid], True, srvheadhashes return [nullid], False, [] # start actual discovery (we note this before the next "if" for # compatibility reasons) ui.status(_("searching for changes\n")) srvheads = dag.internalizeall(srvheadhashes, filterunknown=True) if len(srvheads) == len(srvheadhashes): ui.debug("all remote heads known locally\n") return (srvheadhashes, False, srvheadhashes,) if sample and len(ownheads) <= initialsamplesize and util.all(yesno): ui.note(_("all local heads known remotely\n")) ownheadhashes = dag.externalizeall(ownheads) return (ownheadhashes, True, srvheadhashes,) # full blown discovery # own nodes I know we both know # treat remote heads (and maybe own heads) as a first implicit sample # response common = cl.incrementalmissingrevs(srvheads) commoninsample = set(n for i, n in enumerate(sample) if yesno[i]) common.addbases(commoninsample) # own nodes where I don't know if remote knows them undecided = set(common.missingancestors(ownheads)) # own nodes I know remote lacks missing = set() full = False while undecided: if sample: missinginsample = [n for i, n in enumerate(sample) if not yesno[i]] missing.update(dag.descendantset(missinginsample, missing)) undecided.difference_update(missing) if not undecided: break if full or common.hasbases(): if full: ui.note(_("sampling from both directions\n")) else: ui.debug("taking initial sample\n") samplefunc = _takefullsample targetsize = fullsamplesize else: # use even cheaper initial sample ui.debug("taking quick initial sample\n") samplefunc = _takequicksample targetsize = initialsamplesize if len(undecided) < targetsize: sample = list(undecided) else: sample = samplefunc(dag, undecided, targetsize) sample = _limitsample(sample, targetsize) roundtrips += 1 ui.progress(_('searching'), roundtrips, unit=_('queries')) ui.debug("query %i; still undecided: %i, sample size is: %i\n" % (roundtrips, len(undecided), len(sample))) # indices between sample and externalized version must match sample = list(sample) yesno = remote.known(dag.externalizeall(sample)) full = True if sample: commoninsample = set(n for i, n in enumerate(sample) if yesno[i]) common.addbases(commoninsample) common.removeancestorsfrom(undecided) # heads(common) == heads(common.bases) since common represents common.bases # and all its ancestors result = dag.headsetofconnecteds(common.bases) # common.bases can include nullrev, but our contract requires us to not # return any heads in that case, so discard that result.discard(nullrev) ui.progress(_('searching'), None) ui.debug("%d total queries\n" % roundtrips) if not result and srvheadhashes != [nullid]: if abortwhenunrelated: raise util.Abort(_("repository is unrelated")) else: ui.warn(_("warning: repository is unrelated\n")) return (set([nullid]), True, srvheadhashes,) anyincoming = (srvheadhashes != [nullid]) return dag.externalizeall(result), anyincoming, srvheadhashes