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Re: svn diff optimization to make blame faster?

From: Johan Corveleyn <jcorvel_at_gmail.com>
Date: Thu, 19 Aug 2010 13:54:30 +0200

On Wed, Aug 18, 2010 at 3:44 PM, Johan Corveleyn <jcorvel_at_gmail.com> wrote:
> On Wed, Aug 18, 2010 at 12:49 PM, Stefan Sperling <stsp_at_elego.de> wrote:
>> Can you show a profiler run that illustrates where the client is
>> spending most of its time during diff? That would probably help with
>> getting opinions from people, because it saves them from spending time
>> doing this research themselves.
>> You've already hinted at svn_diff__get_tokens() in another mail, but
>> a real profiler run would show more candidates.
> Sorry, I'm still very much a beginner in C (I've been programming for
> 10 years, but mainly in Java). Especially the tooling is a steep part
> of the learning curve :-), so I don't know (how to use) a profiler
> yet. Any suggestions? I'm on Windows (XP), using VCE 2008, and used
> cygwin to compare with GNU diff.

I googled around a bit for C profilers on Windows. I found several
which seem useful:
- very sleepy (http://www.codersnotes.com/sleepy/sleepy)
- Shiny (http://sourceforge.net/projects/shinyprofiler/)
- AMD CodeAnalyst (http://developer.amd.com/cpu/CodeAnalyst/Pages/default.aspx)
- AQTime - not free, but has a trial of 30 days

Before I dive in and try them out: any preference/favorites from the
windows devs on this list? Or other suggestions?

Further, some additional context to the "manual-profiling numbers": see below.

> For the time being, I've helped myself using "apr_time_now()" from
> apr_time.h and printf statements. Not terribly accurate and probably
> somewhat overheadish, but it gives me some hints about the
> bottlenecks.
> FWIW, below is the output of a recent run with my instrumented trunk
> build. I've instrumented svn_diff_diff in diff.c and
> svn_diff__get_tokens in token.c. I checked out bigfile.xml from a
> repository, and edited a single line of it in my working copy. The two
> statements "Starting diff" and "Diff finished" are the first and last
> statements inside the svn_diff_diff function. These are numbers from
> the second run (any following runs show approximately the same
> numbers).
> $ time svn diff bigfile.xml
> Starting diff ... (entered svn_diff_diff in diff.c)
>  -> calling svn_diff__get_tokens for svn_diff_datasource_original
>      == svn_diff__get_tokens datasource_open: 0 usec
>      == svn_diff__get_tokens while loop: 265625 usec
>         calls to datasource_get_next_token: 62500 usec
>                svn_diff__tree_insert_token: 171875 usec
>                                       rest: 15625 usec
>  <- done: 281250 usec
>  -> calling svn_diff__get_tokens for svn_diff_datasource_modified
>      == svn_diff__get_tokens datasource_open: 234375 usec
>      == svn_diff__get_tokens while loop: 312500 usec
>         calls to datasource_get_next_token: 62500 usec
>                svn_diff__tree_insert_token: 171875 usec
>                                       rest: 46875 usec
>  <- done: 562500 usec
>  -> calling svn_diff__lcs
>  <- done: 0 usec
>  -> calling svn_diff__diff
>  <- done: 0 usec
> Diff finished in 875000 usec (875 millis)
> Index: bigfile.xml
> ===================================================================
> [snip]
> real    0m1.266s
> user    0m0.015s
> sys     0m0.031s

For comparison: a debug build from trunk, with only one
instrumentation spot (at start and end of svn_diff_diff):
$ time svn diff bigfile.xml
Starting diff ... (entered svn_diff_diff in diff.c)
Diff finished in 703125 usec (703 millis)
real 0m1.109s
user 0m0.015s
sys 0m0.031s

So the instrumentation in the critical loop probably costs me around
150-200 ms.

A release build will also probably be faster, but I have no time to
make one and test it now. If I time my 1.6.5 build from tigris.org,
that's still a lot faster:
$ time svn diff bigfile.xml
real 0m0.468s
user 0m0.015s
sys 0m0.031s

Maybe that can be totally attributed to the build (release vs. debug),
or maybe 1.6.5 was faster at svn diff than trunk is?

> Some observations:
> - svn_diff__get_tokens takes most of the time
> - for some reason, in the case of datasource_modified, the call to
> datasource_open takes a long time (234 ms). In case of
> datasource_original, it's instantaneous. Maybe because of translation,
> ... of the pristine file? But I'd think that would be the original??
> - apart from that, most of the time goes to the while loop in
> svn_diff__get_tokens.
> - Inside the while loop, most time is spent on calls to
> svn_diff__tree_insert_token (which compares tokens (=lines) to insert
> them into some tree structure). Calls to datasource_get_next_token
> also take some time.
> I'm not too sure of the accuracy of those last numbers with my simple
> profiling method, because it's the accumulated time of calls inside a
> while loop (with 61000 iterations).
> For completeness, the same diff with /usr/bin/diff (under cygwin), of
> the edited bigfile.xml vs. the original, as of the second diff run:
> $ ls -l settings.xml
> -rwxr-xr-x+ 1 User None 1447693 2010-08-17 14:20 bigfile.xml
> $ time diff bigfile.xml bigfile_orig.xml
> [snip]
> real    0m0.078s
> user    0m0.046s
> sys     0m0.046s
> Cheers,
> --
> Johan

Received on 2010-08-19 13:55:25 CEST

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