comparison mercurial/revlog.py @ 29830:92ac2baaea86

revlog: use an LRU cache for delta chain bases

Profiling using statprof revealed a hotspot during changegroup
application calculating delta chain bases on generaldelta repos.
Essentially, revlog._addrevision() was performing a lot of redundant
work tracing the delta chain as part of determining when the chain
distance was acceptable. This was most pronounced when adding
revisions to manifests, which can have delta chains thousands of
revisions long.

There was a delta chain base cache on revlogs before, but it only
captured a single revision. This was acceptable before generaldelta,
when _addrevision would build deltas from the previous revision and
thus we'd pretty much guarantee a cache hit when resolving the delta
chain base on a subsequent _addrevision call. However, it isn't
suitable for generaldelta because parent revisions aren't necessarily
the last processed revision.

This patch converts the delta chain base cache to an LRU dict cache.
The cache can hold multiple entries, so generaldelta repos have a
higher chance of getting a cache hit.

The impact of this change when processing changegroup additions is
significant. On a generaldelta conversion of the "mozilla-unified"
repo (which contains heads of the main Firefox repositories in
chronological order - this means there are lots of transitions between
heads in revlog order), this change has the following impact when
performing an `hg unbundle` of an uncompressed bundle of the repo:

before: 5:42 CPU time
after:  4:34 CPU time

Most of this time is saved when applying the changelog and manifest
revlogs:

before: 2:30 CPU time
after:  1:17 CPU time

That nearly a 50% reduction in CPU time applying changesets and
manifests!

Applying a gzipped bundle of the same repo (effectively simulating a
`hg clone` over HTTP) showed a similar speedup:

before: 5:53 CPU time
after:  4:46 CPU time

Wall time improvements were basically the same as CPU time.

I didn't measure explicitly, but it feels like most of the time
is saved when processing manifests. This makes sense, as large
manifests tend to have very long delta chains and thus benefit the
most from this cache.

So, this change effectively makes changegroup application (which is
used by `hg unbundle`, `hg clone`, `hg pull`, `hg unshelve`, and
various other commands) significantly faster when delta chains are
long (which can happen on repos with large numbers of files and thus
large manifests).

In theory, this change can result in more memory utilization. However,
we're caching a dict of ints. At most we have 200 ints + Python object
overhead per revlog. And, the cache is really only populated when
performing read-heavy operations, such as adding changegroups or
scanning an individual revlog. For memory bloat to be an issue, we'd
need to scan/read several revisions from several revlogs all while
having active references to several revlogs. I don't think there are
many operations that do this, so I don't think memory bloat from the
cache will be an issue.

29829:dae97049345b

        self.indexfile = indexfile
        self.datafile = indexfile[:-2] + ".d"
        self.opener = opener
        # 3-tuple of (node, rev, text) for a raw revision.
        self._cache = None
        # 2-tuple of (rev, baserev) defining the base revision the delta chain
        # begins at for a revision.
        self._basecache = None
        # 2-tuple of (offset, data) of raw data from the revlog at an offset.
        self._chunkcache = (0, '')
        # How much data to read and cache into the raw revlog data cache.
        self._chunkcachesize = 65536
        self._maxchainlen = None

        except KeyError:
            return False

    def clearcaches(self):
        self._cache = None
        self._basecache = None
        self._chunkcache = (0, '')
        self._pcache = {}

        try:
            self._nodecache.clearcaches()

    def end(self, rev):
        return self.start(rev) + self.length(rev)
    def length(self, rev):
        return self.index[rev][1]
    def chainbase(self, rev):
        index = self.index
        base = index[rev][3]
        while base != rev:
            rev = base
            base = index[rev][3]
        return base
    def chainlen(self, rev):
        return self._chaininfo(rev)[0]

    def _chaininfo(self, rev):

                        fh = dfh
                    ptext = self.revision(self.node(rev), _df=fh)
                    delta = mdiff.textdiff(ptext, t)
            data = self.compress(delta)
            l = len(data[1]) + len(data[0])
            if basecache[0] == rev:
                chainbase = basecache[1]
            else:
                chainbase = self.chainbase(rev)
            dist = l + offset - self.start(chainbase)
            if self._generaldelta:
                base = rev
            else:
                base = chainbase

        curr = len(self)
        prev = curr - 1
        offset = self.end(prev)
        delta = None
        if self._basecache is None:
            self._basecache = (prev, self.chainbase(prev))
        basecache = self._basecache
        p1r, p2r = self.rev(p1), self.rev(p2)

        # full versions are inserted when the needed deltas
        # become comparable to the uncompressed text
        if text is None:

        if alwayscache and text is None:
            text = buildtext()

        if type(text) == str: # only accept immutable objects
            self._cache = (node, curr, text)
        self._basecache = (curr, chainbase)
        return node

    def _writeentry(self, transaction, ifh, dfh, entry, data, link, offset):
        # Files opened in a+ mode have inconsistent behavior on various
        # platforms. Windows requires that a file positioning call be made

29830:92ac2baaea86

        self.indexfile = indexfile
        self.datafile = indexfile[:-2] + ".d"
        self.opener = opener
        # 3-tuple of (node, rev, text) for a raw revision.
        self._cache = None
        # Maps rev to chain base rev.
        self._chainbasecache = util.lrucachedict(100)
        # 2-tuple of (offset, data) of raw data from the revlog at an offset.
        self._chunkcache = (0, '')
        # How much data to read and cache into the raw revlog data cache.
        self._chunkcachesize = 65536
        self._maxchainlen = None

        except KeyError:
            return False

    def clearcaches(self):
        self._cache = None
        self._chainbasecache.clear()
        self._chunkcache = (0, '')
        self._pcache = {}

        try:
            self._nodecache.clearcaches()

    def end(self, rev):
        return self.start(rev) + self.length(rev)
    def length(self, rev):
        return self.index[rev][1]
    def chainbase(self, rev):
        base = self._chainbasecache.get(rev)
        if base is not None:
            return base

        index = self.index
        base = index[rev][3]
        while base != rev:
            rev = base
            base = index[rev][3]

        self._chainbasecache[rev] = base
        return base
    def chainlen(self, rev):
        return self._chaininfo(rev)[0]

    def _chaininfo(self, rev):

                        fh = dfh
                    ptext = self.revision(self.node(rev), _df=fh)
                    delta = mdiff.textdiff(ptext, t)
            data = self.compress(delta)
            l = len(data[1]) + len(data[0])
            chainbase = self.chainbase(rev)
            dist = l + offset - self.start(chainbase)
            if self._generaldelta:
                base = rev
            else:
                base = chainbase

        curr = len(self)
        prev = curr - 1
        offset = self.end(prev)
        delta = None
        p1r, p2r = self.rev(p1), self.rev(p2)

        # full versions are inserted when the needed deltas
        # become comparable to the uncompressed text
        if text is None:

        if alwayscache and text is None:
            text = buildtext()

        if type(text) == str: # only accept immutable objects
            self._cache = (node, curr, text)
        self._chainbasecache[curr] = chainbase
        return node

    def _writeentry(self, transaction, ifh, dfh, entry, data, link, offset):
        # Files opened in a+ mode have inconsistent behavior on various
        # platforms. Windows requires that a file positioning call be made