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
https://bz.mercurial-scm.org/522
In the merge below, the file "foo" has the same contents in both
parents, but if we look at the file-level history, we'll notice that
the version in p1 is an ancestor of the version in p2. This test makes
sure that we'll use the version from p2 in the manifest of the merge
revision.
$ hg init
$ echo foo > foo
$ hg ci -qAm 'add foo'
$ echo bar >> foo
$ hg ci -m 'change foo'
$ hg backout -r tip -m 'backout changed foo'
reverting foo
changeset 2:4d9e78aaceee backs out changeset 1:b515023e500e
$ hg up -C 0
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ touch bar
$ hg ci -qAm 'add bar'
$ hg merge --debug
searching for copies back to rev 1
unmatched files in local:
bar
resolving manifests
branchmerge: True, force: False, partial: False
ancestor: bbd179dfa0a7, local: 71766447bdbb+, remote: 4d9e78aaceee
foo: remote is newer -> g
getting foo
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
(branch merge, don't forget to commit)
$ hg debugstate | grep foo
m 0 -2 unset foo
$ hg st -A foo
M foo
$ hg ci -m 'merge'
$ hg manifest --debug | grep foo
c6fc755d7e68f49f880599da29f15add41f42f5a 644 foo
$ hg debugindex foo
rev offset length ..... linkrev nodeid p1 p2 (re)
0 0 5 ..... 0 2ed2a3912a0b 000000000000 000000000000 (re)
1 5 9 ..... 1 6f4310b00b9a 2ed2a3912a0b 000000000000 (re)
2 14 5 ..... 2 c6fc755d7e68 6f4310b00b9a 000000000000 (re)