Mercurial > hg
view mercurial/help/revisions.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> |
---|---|
date | Sun, 27 Sep 2015 16:08:18 -0700 |
parents | 4edd179fefb8 |
children | bbb5cc55ab8b |
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Mercurial supports several ways to specify individual revisions. A plain integer is treated as a revision number. Negative integers are treated as sequential offsets from the tip, with -1 denoting the tip, -2 denoting the revision prior to the tip, and so forth. A 40-digit hexadecimal string is treated as a unique revision identifier. A hexadecimal string less than 40 characters long is treated as a unique revision identifier and is referred to as a short-form identifier. A short-form identifier is only valid if it is the prefix of exactly one full-length identifier. Any other string is treated as a bookmark, tag, or branch name. A bookmark is a movable pointer to a revision. A tag is a permanent name associated with a revision. A branch name denotes the tipmost open branch head of that branch - or if they are all closed, the tipmost closed head of the branch. Bookmark, tag, and branch names must not contain the ":" character. The reserved name "tip" always identifies the most recent revision. The reserved name "null" indicates the null revision. This is the revision of an empty repository, and the parent of revision 0. The reserved name "." indicates the working directory parent. If no working directory is checked out, it is equivalent to null. If an uncommitted merge is in progress, "." is the revision of the first parent.