[PATCH 3 of 3] revlog: optimize _chunkraw when startrev==endrev

Gregory Szorc gregory.szorc at gmail.com
Tue Nov 1 21:25:47 EDT 2016


# HG changeset patch
# User Gregory Szorc <gregory.szorc at gmail.com>
# Date 1477244433 25200
#      Sun Oct 23 10:40:33 2016 -0700
# Node ID 4c96a177e4ff6a63156ec8cf49483a78e4437756
# Parent  0c41c0cb9b1ef7df40a30672927229ac195b1c92
revlog: optimize _chunkraw when startrev==endrev

In many cases, _chunkraw() is called with startrev==endrev. When
this is true, we can avoid an extra index lookup and some other
minor operations.

On the mozilla-unified repo, `hg perfrevlogchunks -c` says this
has the following impact:

! read w/ reused fd
! wall 0.371846 comb 0.370000 user 0.350000 sys 0.020000 (best of 27)
! wall 0.337930 comb 0.330000 user 0.300000 sys 0.030000 (best of 30)

! read batch w/ reused fd
! wall 0.014952 comb 0.020000 user 0.000000 sys 0.020000 (best of 197)
! wall 0.014866 comb 0.010000 user 0.000000 sys 0.010000 (best of 196)

So, we've gone from ~25x slower than batch to ~22.5% slower.

At this point, there's probably not much else we can do except
implement an optimized function in the index itself, including in C.

diff --git a/mercurial/revlog.py b/mercurial/revlog.py
--- a/mercurial/revlog.py
+++ b/mercurial/revlog.py
@@ -1113,9 +1113,12 @@ class revlog(object):
         # (functions are expensive).
         index = self.index
         istart = index[startrev]
-        iend = index[endrev]
         start = int(istart[0] >> 16)
-        end = int(iend[0] >> 16) + iend[1]
+        if startrev == endrev:
+            end = start + istart[1]
+        else:
+            iend = index[endrev]
+            end = int(iend[0] >> 16) + iend[1]
 
         if self._inline:
             start += (startrev + 1) * self._io.size


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