# Re: Unexpected Merge Behaviour

From: Neels Hofmeyr <neels_at_elego.de>
Date: Tue, 09 Sep 2008 03:05:50 +0200

"
Hey Daniel,

it is best to checkout subtrees of a large repository. For example
svn checkout http://my.server/my_repos/some/sub/dir
will checkout only dir', speeding up your subsequent svn calls.

As you have observed, running svn in a subdir of a large checkout has the
same effect. That's because svn does only operate on subdirectories, not on
parents (usually). But I humbly prefer separate working copies.

To tell you more, I'd also need the actual svn commands to reproduce the
problem and the complete output seen (try to copy-and-paste).
"

...This is the answer users@ should have given. About whether svn could be
made faster in this case, it seems that dev@ doesn't know what to say. Let
me try to reply from a developer's point of view.

Version control is extremely complex and so is subversion. Mostly, if you
want to know reasons for something you have to dig them up from the logs. It

Furthermore, subversion's behaviour does, eventually, give the right
results. So there's no pressing reason to abandon the current developments
and dive into this one...

You are welcome to investigate this issue yourself or have someone do it for
you. dev@ will be happy to assist!

Have you searched the archives for this issue? Otherwise this might qualify
for a new feature request in the issue tracker (which must have been voted
on before submitting). http://subversion.tigris.org/issue-tracker.html

~Neels

Daniel Becroft wrote:
> Hi all,
>
> Apologies for posting on the developer list. I have posted this on the
> users@ originally, but did not get any explanation. I thought I would go
> to the 'source' (no pun intended), to try and get an explanation.
>
> =============================================
>
> I have only recently started playing around with SVN's merge
> capabilities, and have found something strange (SVN 1.5.2 on Windows
> XP).
>
> I have checked-out a fresh, unmodified copy of a branch in the
> repository.
>
> The structure is similar to this:
>
> \alpha
> \A
> \B
> \C.txt
> \beta
> ...
> \gamma
> ...
>
> I am attempting to cherry-pick a single revision from trunk and apply it
> to said working copy. The revision (r1234) only modifies a single file
> (C.txt above), but when I run the following command:
>
> svn merge -c 1234 svn://localhost/repo/trunk .
>
> the svn process appears to be reading EVERY file in the working copy,
> including those located under the \beta and \gamma directories. As a
> result, the merge takes approximately 3 minutes to complete. (I used
> sysinternal's FileMonitor utility to determine this). When the working
> copy contains upwards of 12,000 files, this can extremely slow.
>
> This performance does not sound so bad, but when using the 'multiple
> revisions' option for the -C command, like so:
>
> svn merge -C 1234,1236,1239,1255 svn://localhost/repo/trunk .
>
> it takes approximate 3 minutes PER REVISION - it seems to do a loop
> through the revisions and do the same thing for each.
>
> However, I have found that if I structure the merge command such that
> the merge is being done on the subdirectory, then the problem is
> minimized (all files under alpha/A/B are read, but not /beta or /gamma).
>
> svn merge -c 1234 svn://localhost/repo/trunk/alpha/A/B ./alpha/A/B
>
> Using either version of the merge command, the only files/directories
> that are modified are the parent directory, and C.txt.
>
> The only explanation I got from users@ was that this might be the
> 'elliding' functionality.
>
> Is this expected behavior for svn merge? Why is it reading completely
> unrelated files when attempting a merge, rather than looking at the
> changes being applied first? Is this the 'elliding' feature, and if so,
> why is it doing this?
>
> Cheers,
> Daniel B.
>

--
Neels Hofmeyr -- elego Software Solutions GmbH
Gustav-Meyer-Allee 25 / Gebäude 12, 13355 Berlin, Germany
phone: +49 30 23458696  mobile: +49 177 2345869  fax: +49 30 23458695
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