Patchwork [2,of,2] revlog: remove unnecessary cache validation in _chunks

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Submitter Gregory Szorc
Date Jan. 6, 2016, 3:52 a.m.
Message ID <617f06db4d30f9ce1d6b.1452052333@ubuntu-vm-main>
Download mbox | patch
Permalink /patch/12555/
State Accepted
Delegated to: Martin von Zweigbergk
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Comments

Gregory Szorc - Jan. 6, 2016, 3:52 a.m.
# HG changeset patch
# User Gregory Szorc <gregory.szorc@gmail.com>
# Date 1448243855 28800
#      Sun Nov 22 17:57:35 2015 -0800
# Node ID 617f06db4d30f9ce1d6b429fa65de176e4dd7485
# Parent  2506af2e83054d590b9b12912c91074f5f58a274
revlog: remove unnecessary cache validation in _chunks

Previously, we likely called _chunkraw() multiple times in order to
ensure it didn't change out from under us. I'm pretty certain this code
had its origins in the days where we attempted to have thread safety of
localrepository and thus revlog instances.

revlog instances are already not thread safe for writing. And, as of
Mercurial 3.6, hgweb uses a separate localrepository instance per
request, so there should only be a single thread reading a revlog at
a time. We more or less decided that attempting to make classes like
revlog thread safe is a lost cause.

So, this patch removes thread safety from _chunks. As a result, we make
one less call into _chunkraw() when the initial read isn't serviced
by the cache. This translates to savings of 4 function calls overall
and possibly prevents the creation of an additional buffer view into the
cache. I doubt this translates into any real world performance wins
because decompression will almost certainly dwarf time spent in
_chunks(). But it does make the code simpler, so it is an improvement.

Patch

diff --git a/mercurial/revlog.py b/mercurial/revlog.py
--- a/mercurial/revlog.py
+++ b/mercurial/revlog.py
@@ -1117,25 +1117,18 @@  class revlog(object):
         length = self.length
         inline = self._inline
         iosize = self._io.size
         buffer = util.buffer
 
         l = []
         ladd = l.append
 
-        # preload the cache
         try:
-            while True:
-                # ensure that the cache doesn't change out from under us
-                _cache = self._chunkcache
-                self._chunkraw(revs[0], revs[-1], df=df)[1]
-                if _cache == self._chunkcache:
-                    break
-            offset, data = _cache
+            offset, data = self._chunkraw(revs[0], revs[-1], df=df)
         except OverflowError:
             # issue4215 - we can't cache a run of chunks greater than
             # 2G on Windows
             return [self._chunk(rev, df=df) for rev in revs]
 
         for rev in revs:
             chunkstart = start(rev)
             if inline: