bundle2: avoid unbound read when seeking
Currently, seekableunbundlepart.seek() will perform a read() during
seek operations. This will allocate a buffer to hold the raw data
over the seek distance. This can lead to very large allocations
and cause performance to suffer.
We change the code to perform read(32768) in a loop to avoid
potentially large allocations.
`hg perfbundleread` on an uncompressed Firefox bundle reveals
a performance impact:
! bundle2 iterparts()
! wall 2.992605 comb 2.990000 user 2.260000 sys 0.730000 (best of 4)
! bundle2 iterparts() seekable
! wall 3.863810 comb 3.860000 user 3.000000 sys 0.860000 (best of 3)
! bundle2 part seek()
! wall 6.213387 comb 6.200000 user 3.350000 sys 2.850000 (best of 3)
! wall 3.820347 comb 3.810000 user 2.980000 sys 0.830000 (best of 3)
Since seekable bundle parts are (only) used by bundlerepo, this /may/
speed up initial loading of bundle-based repos. But any improvement
will likely only be noticed on very large bundles.
Differential Revision: https://phab.mercurial-scm.org/D1394
--- a/mercurial/bundle2.py Mon Nov 13 21:54:46 2017 -0800
+++ b/mercurial/bundle2.py Mon Nov 13 22:20:12 2017 -0800
@@ -1415,13 +1415,20 @@
newpos = self._pos + offset
elif whence == os.SEEK_END:
if not self.consumed:
- self.read()
+ # Can't use self.consume() here because it advances self._pos.
+ chunk = self.read(32768)
+ while chunk:
+ chunk = self.read(32768)
newpos = self._chunkindex[-1][0] - offset
else:
raise ValueError('Unknown whence value: %r' % (whence,))
if newpos > self._chunkindex[-1][0] and not self.consumed:
- self.read()
+ # Can't use self.consume() here because it advances self._pos.
+ chunk = self.read(32768)
+ while chunk:
+ chunk = self.read(32668)
+
if not 0 <= newpos <= self._chunkindex[-1][0]:
raise ValueError('Offset out of range')