Python uses a C long (32 bits on Windows 64) rather than an ssize_t in
read(), and thus has a 2G size limit. Work around this by falling back
to reading one chunk at a time on overflow. This approximately doubles
our headroom until we run back into the size limit on single reads.
Moves the code that actually writes to a file to a separate function in
revlog.py. This allows extensions to intercept and use the data being written to
disk. For example, an extension might want to replicate these writes elsewhere.
When cloning the Mercurial repo on /dev/shm with --pull, I see about a 0.3% perf change.
It goes from 28.2 to 28.3 seconds.
Running perfmoonwalk on the Mercurial repo (with almost 20,000 changesets) on
Mac OS X with an SSD, before this change:
$ hg --config format.chunkcachesize=1024 perfmoonwalk
! wall 2.022021 comb 2.030000 user 1.970000 sys 0.060000 (best of 5)
(16,154 cache hits, 3,840 misses.)
$ hg --config format.chunkcachesize=4096 perfmoonwalk
! wall 1.901006 comb 1.900000 user 1.880000 sys 0.020000 (best of 6)
(19,003 hits, 991 misses.)
$ hg --config format.chunkcachesize=16384 perfmoonwalk
! wall 1.802775 comb 1.800000 user 1.800000 sys 0.000000 (best of 6)
(19,746 hits, 248 misses.)
$ hg --config format.chunkcachesize=32768 perfmoonwalk
! wall 1.818545 comb 1.810000 user 1.810000 sys 0.000000 (best of 6)
(19,870 hits, 124 misses.)
$ hg --config format.chunkcachesize=65536 perfmoonwalk
! wall 1.801350 comb 1.810000 user 1.800000 sys 0.010000 (best of 6)
(19,932 hits, 62 misses.)
$ hg --config format.chunkcachesize=131072 perfmoonwalk
! wall 1.805879 comb 1.820000 user 1.810000 sys 0.010000 (best of 6)
(19,963 hits, 31 misses.)
We may want to change the default size in the future based on testing and
user feedback.
When reading a revlog chunk, instead of reading up to 64 KB ahead of the
request offset and caching that, this change caches a fixed window before
and after the requested data that falls on 64 KB boundaries. This increases
cache hits when reading revlogs backwards.
Running perfmoonwalk on the Mercurial repo (with almost 20,000 changesets) on
Mac OS X with an SSD, before this change:
$ hg perfmoonwalk
! wall 2.307994 comb 2.310000 user 2.120000 sys 0.190000 (best of 5)
(Each run has 10,668 cache hits and 9,304 misses.)
After this change:
$ hg perfmoonwalk
! wall 1.814117 comb 1.810000 user 1.810000 sys 0.000000 (best of 6)
(19,931 cache hits, 62 misses.)
On a busy NFS share, before this change:
$ hg perfmoonwalk
! wall 17.000034 comb 4.100000 user 3.270000 sys 0.830000 (best of 3)
After:
$ hg perfmoonwalk
! wall 1.746115 comb 1.670000 user 1.660000 sys 0.010000 (best of 5)
The previous revlog strip computation would walk every rev in the revlog, from
the bottom to the top. Since we're usually stripping only the top few revs of
the revlog, this was needlessly expensive on large repos.
The new algorithm walks the exact number of revs that will be stripped, thus
making the operation not dependent on the number of revs in the repo.
This makes amend on a large repo go from 8.7 seconds to 6 seconds.
When computing the commonmissing, it greedily computes the entire set
immediately. On a large repo where the majority of history is irrelevant, this
causes a significant slow down.
Replacing it with a lazy set makes amend go from 11 seconds to 8.7 seconds.
Previously basecache was incorrectly initialized before adding the first
revision from a changegroup. Basecache value influences when full revisions are
stored in revlog (when using generaldelta). As a result it was possible to
generate a generaldelta-revlog that could be bigger by arbitrary factor than its
non-generaldelta equivalent.
In case we don't have a cached text already, add the base rev to the list
passed to _chunks. In the cached case this also avoids unnecessarily preloading
the chunk for the cached rev.
We do this in a somewhat hacky way, relying on the fact that our sole caller
preloads the cache right before calling us. An upcoming patch will make this
more sensible.
For a 20 MB manifest with a delta chain of > 40k, perfmanifest goes from 0.49
seconds to 0.46.
Previously the length of data preloaded did not account for the interleaved io
contents. This meant that we'd sometimes have cache misses in _chunks despite
the preloading.
Having a correctly filled out cache will become essential in an upcoming patch.
This moves _chunkraw into the loop. Doing that improves revlog decompression --
in particular, manifest decompression -- significantly. For a 20 MB manifest
which is the result of a > 40k delta chain, hg perfmanifest improves from 0.55
seconds to 0.49 seconds.
This change will allow revlog subclasses that override 'checkhash' method
to use custom strategy of computing nodeids without overriding 'addrevision'
method. In particular this change is necessary to implement manifest
compression.
Extract method that decides whether nodeid is correct for paricular revision
text and parent nodes. Having this method extracted will allow revlog
subclasses to implement custom way of computing nodes. In particular this
change is necessary to implement manifest compression.
When we deployed the latest crew mercurial to our users, a few of them
had issues where a filelog would have an entry with a -1 linkrev. This
caused operations like rebase and amend to create a bundle containing the
entire repository, which took a long time.
I don't know what the issue is, but adding this check should prevent repos
from getting in this state, and should help us pinpoint the issue next time
it happens.
The performance of both the old and new Python ancestor algorithms
depends on the number of revs they need to traverse. Although the
new algorithm performs far better than the old when revs are
numerically and topologically close, both algorithms become slow
under other circumstances, taking up to 1.8 seconds to give answers
in a Linux kernel repo.
This C implementation of the new algorithm is a fairly straightforward
transliteration. The only corner case of interest is that it raises
an OverflowError if the number of GCA candidates found during the
first pass is greater than 24, to avoid the dual perils of fixnum
overflow and trying to allocate too much memory. (If this exception
is raised, the Python implementation is used instead.)
Performance numbers are good: in a Linux kernel repo, time for "hg
debugancestors" on two distant revs (24bf01de7537 and c2a8808f5943)
is as follows:
Old Python: 0.36 sec
New Python: 0.42 sec
New C: 0.02 sec
For a case where the new algorithm should perform well:
Old Python: 1.84 sec
New Python: 0.07 sec
New C: measures as zero when using --time
(This commit includes a paranoid cross-check to ensure that the
Python and C implementations give identical answers. The above
performance numbers were measured with that check disabled.)
Previously, we chose a rev based on numeric ordering, which could
cause "the same merge" in topologically identical but numerically
different repos to choose different merge bases.
We now choose the lexically least node; this is stable across
different revlog orderings.
Instead of walking all the way to the root of the DAG, we generate
a set of candidate GCA revs, then figure out which ones will win
the race to the root (usually without needing to traverse all the
way to the root).
In the common case of nodes that are close to each other in both
revision number and topology, this is usually a big win: it makes
"hg --time debugancestors" up to 9 times faster than the more general
ancestor function when measured on heads of the linux-2.6 hg repo.
Victory is not assured, however. The older function can still win
by a large margin if one node is much closer to the root than the
other, or by a much smaller amount if one is an ancestor of the
other.
For now, we've also got a small paranoid harness function that calls
both ancestor functions on every input and ensures that they give
equivalent answers.
Even without the checker function, the old ancestor function needs
to stay alive for the time being, as its generality is used by
context.filectx.merge.
This is in preparation for an upcoming refactoring. This also fixes a bug in
incancestors, where if an element of revs was an ancestor of another it would
be generated twice.
We often need to perform rev iteration in reverse order. This
changeset makes it possible to do so, in order to avoid costly reverse
or reversed() calls later.
This also speeds up other commands that use findmissing, like
incoming and merge --preview. With a large linear repository (>400000
commits) and with one incoming changeset, incoming is sped up from
around 4-4.5 seconds to under 3.
When commiting to a repo with lots of history (>400000 changesets)
the filteredrevs check (added with 373606589de5) in changelog.py
takes a bit of time even if the filteredrevs set is empty. Skipping
the check in that case shaves 0.36 seconds off a 2.14 second commit.
A 17% gain.
Make the pure python implementation of headrevs available to derived classes. It
is important because filtering logic applied by `revlog` derived class won't
have effect on `index`. We want to be able to bypass this C call to implement
our own.
This prepares changelog level filtering. We can't assume that any revision can
be heads because filtered revisions need to be excluded.
New algorithm:
- All revisions now start as "non heads",
- every revision we iterate over is made candidate head,
- parents of iterated revisions are definitely not head.
Filtered revisions are never iterated over and never considered as candidate
head.
This prepares changelog level filtering. We need the algorithms used in revlog to
work on a subset of revisions. To achieve this, the use of explicit range of
revision is banned. `range` and `xrange` calls are replaced by a `revlog.irevs`
method. Filtered super class can then overwrite the `irevs` method to filter out
revision.
The decision whether or not to store a full snapshot instead of a delta is done
based on the distance value calculated in _addrevision.builddelta(rev).
This calculation traditionally used the fact of deltas only using the previous
revision as base. Generaldelta mechanism is changing this, yet the calculation
still assumes that current-offset minus chainbase-offset equals chain-length.
This appears to be wrong.
This patch corrects the calculation by means of using the chainlength function
if Generaldelta is used.
This allows an extension to optionally use a new compression type based
on the options applied by the repo to the revlog's opener.
(decompress doesn't need the same treatment, as it can be replaced using
extensions.wrapfunction, and can figure out which compression algorithm
is in use based on the first byte of the compressed payload.)
