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.
# reproduce issue2264, issue2516
create test repo
$ cat <<EOF >> $HGRCPATH
> [extensions]
> transplant =
> EOF
$ hg init repo
$ cd repo
$ template="{rev} {desc|firstline} [{branch}]\n"
# we need to start out with two changesets on the default branch
# in order to avoid the cute little optimization where transplant
# pulls rather than transplants
add initial changesets
$ echo feature1 > file1
$ hg ci -Am"feature 1"
adding file1
$ echo feature2 >> file2
$ hg ci -Am"feature 2"
adding file2
# The changes to 'bugfix' are enough to show the bug: in fact, with only
# those changes, it's a very noisy crash ("RuntimeError: nothing
# committed after transplant"). But if we modify a second file in the
# transplanted changesets, the bug is much more subtle: transplant
# silently drops the second change to 'bugfix' on the floor, and we only
# see it when we run 'hg status' after transplanting. Subtle data loss
# bugs are worse than crashes, so reproduce the subtle case here.
commit bug fixes on bug fix branch
$ hg branch fixes
marked working directory as branch fixes
(branches are permanent and global, did you want a bookmark?)
$ echo fix1 > bugfix
$ echo fix1 >> file1
$ hg ci -Am"fix 1"
adding bugfix
$ echo fix2 > bugfix
$ echo fix2 >> file1
$ hg ci -Am"fix 2"
$ hg log -G --template="$template"
@ 3 fix 2 [fixes]
|
o 2 fix 1 [fixes]
|
o 1 feature 2 [default]
|
o 0 feature 1 [default]
transplant bug fixes onto release branch
$ hg update 0
1 files updated, 0 files merged, 2 files removed, 0 files unresolved
$ hg branch release
marked working directory as branch release
$ hg transplant 2 3
applying [0-9a-f]{12} (re)
[0-9a-f]{12} transplanted to [0-9a-f]{12} (re)
applying [0-9a-f]{12} (re)
[0-9a-f]{12} transplanted to [0-9a-f]{12} (re)
$ hg log -G --template="$template"
@ 5 fix 2 [release]
|
o 4 fix 1 [release]
|
| o 3 fix 2 [fixes]
| |
| o 2 fix 1 [fixes]
| |
| o 1 feature 2 [default]
|/
o 0 feature 1 [default]
$ hg status
$ hg status --rev 0:4
M file1
A bugfix
$ hg status --rev 4:5
M bugfix
M file1
now test that we fixed the bug for all scripts/extensions
$ cat > $TESTTMP/committwice.py <<__EOF__
> from mercurial import ui, hg, match, node
> from time import sleep
>
> def replacebyte(fn, b):
> f = open(fn, "rb+")
> f.seek(0, 0)
> f.write(b)
> f.close()
>
> def printfiles(repo, rev):
> print "revision %s files: %s" % (rev, repo[rev].files())
>
> repo = hg.repository(ui.ui(), '.')
> assert len(repo) == 6, \
> "initial: len(repo): %d, expected: 6" % len(repo)
>
> replacebyte("bugfix", "u")
> sleep(2)
> try:
> print "PRE: len(repo): %d" % len(repo)
> wlock = repo.wlock()
> lock = repo.lock()
> replacebyte("file1", "x")
> repo.commit(text="x", user="test", date=(0, 0))
> replacebyte("file1", "y")
> repo.commit(text="y", user="test", date=(0, 0))
> print "POST: len(repo): %d" % len(repo)
> finally:
> lock.release()
> wlock.release()
> printfiles(repo, 6)
> printfiles(repo, 7)
> __EOF__
$ $PYTHON $TESTTMP/committwice.py
PRE: len(repo): 6
POST: len(repo): 8
revision 6 files: ['bugfix', 'file1']
revision 7 files: ['file1']
Do a size-preserving modification outside of that process
$ echo abcd > bugfix
$ hg status
M bugfix
$ hg log --template "{rev} {desc} {files}\n" -r5:
5 fix 2 bugfix file1
6 x bugfix file1
7 y file1
$ cd ..