Mercurial > hg
annotate mercurial/help/diffs.txt @ 26379:39d643252b9f
revlog: use existing file handle when reading during _addrevision
_addrevision() may need to read from revlogs as part of computing
deltas. Previously, we would flush existing file handles and open
a new, short-lived file handle to perform the reading.
If we have an existing file handle, it seems logical to reuse it
for reading instead of opening a new file handle. This patch
makes that the new behavior.
After this patch, revlog files are only reopened when adding
revisions if the revlog is switched from inline to non-inline.
On Linux when unbundling a bundle of the mozilla-central repo, this
patch has the following impact on system call counts:
Call Before After Delta
write 827,639 673,390 -154,249
open 700,103 684,089 -16,014
read 74,489 74,489 0
fstat 493,924 461,896 -32,028
close 249,131 233,117 -16,014
stat 242,001 242,001 0
lstat 18,676 18,676 0
lseek 20,268 20,268 0
ioctl 14,652 13,173 -1,479
TOTAL 3,180,758 2,930,679 -250,079
It's worth noting that many of the open() calls fail due to missing
files. That's why there are many more open() calls than close().
Despite the significant system call reduction, this change does not
seem to have a significant performance impact on Linux.
On Windows 10 (not a VM, on a SSD), this patch appears to reduce
unbundle time for mozilla-central from ~960s to ~920s. This isn't
as significant as I was hoping. But a decrease it is nonetheless.
Still, Windows unbundle performance is still >2x slower than Linux.
Despite the lack of significant gains, fewer system calls is fewer
system calls. If nothing else, this will narrow the focus of potential
areas to optimize in the future.
author | Gregory Szorc <gregory.szorc@gmail.com> |
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date | Sun, 27 Sep 2015 16:08:18 -0700 |
parents | ebfc46929f3e |
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1 Mercurial's default format for showing changes between two versions of |
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2 a file is compatible with the unified format of GNU diff, which can be |
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3 used by GNU patch and many other standard tools. |
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4 |
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5 While this standard format is often enough, it does not encode the |
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6 following information: |
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7 |
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8 - executable status and other permission bits |
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9 - copy or rename information |
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10 - changes in binary files |
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11 - creation or deletion of empty files |
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12 |
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13 Mercurial also supports the extended diff format from the git VCS |
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14 which addresses these limitations. The git diff format is not produced |
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15 by default because a few widespread tools still do not understand this |
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16 format. |
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17 |
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18 This means that when generating diffs from a Mercurial repository |
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19 (e.g. with :hg:`export`), you should be careful about things like file |
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20 copies and renames or other things mentioned above, because when |
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21 applying a standard diff to a different repository, this extra |
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22 information is lost. Mercurial's internal operations (like push and |
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23 pull) are not affected by this, because they use an internal binary |
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24 format for communicating changes. |
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25 |
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26 To make Mercurial produce the git extended diff format, use the --git |
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27 option available for many commands, or set 'git = True' in the [diff] |
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28 section of your configuration file. You do not need to set this option |
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29 when importing diffs in this format or using them in the mq extension. |