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From Johan Corveleyn <jcor...@gmail.com>
Subject Re: Branching slow 1.8.11 https
Date Sun, 29 Mar 2015 17:57:34 GMT
On Sat, Mar 28, 2015 at 5:09 PM, Bert Huijben <bert@qqmail.nl> wrote:
>
>
>> -----Original Message-----
>> From: Johan Corveleyn [mailto:jcorvel@gmail.com]
>> Sent: vrijdag 27 maart 2015 22:03
>> To: users@subversion.apache.org
>> Subject: Branching slow 1.8.11 https
>>
>> Does the following ring a bell for someone?
>>
>> Recently upgraded our server (on Solaris 10 SPARC) from 1.5.4 to
>> 1.8.11 (CollabNet package). Some time after that, we discovered that
>> branching was very slow. I'm talking about pure server-side branching
>> ('svn copy $URL/trunk $URL/branches/br1'). I'm testing with a 1.8.11
>> client (tried both from same machine as the server, and from another
>> machine on the LAN (100 Mbit)).
>>
>> - Branching trunk (containing many directories and files): 6-8 minutes
>> - Branching a subfolder of trunk: 20-30 seconds (still very slow)
>> - Branching a single file is fast (< 0.5s or so).
>>
>> So it seems the performance degrades depending on the depth or size of the
>> tree.
>>
>> Now, it gets more interesting:
>> - The resulting rev file on the server is always very small (as it
>> should be, it contains only a lightweight 'copy' of the trunk node).
>> - Our repos is currently served via https (Apache 2.2.29).
>> - Branching with file:/// urls is fast (branching trunk takes 0.6s).
>> - When starting an svnserve instance serving the same repository, and
>> branching with svn:// urls, it's fast as well (also 0.6s).
>> - We reproduced it on a copy of the production repo.
>> - Experimenting with the test copy, we found that
>> $repos/dav/activities.d contains ~2000 files. When we clear that
>> directory, the branching times go down by more than half (~2 minutes
>> for trunk, ~10s for subdir of trunk --- i.e. still slow, but it
>> definitely has an impact).
>> - With a 1.7 client connecting with neon, the problem is the same.
>> - During the 'svn copy', an httpd child consumes a lot of cpu (around
>> half a core).
>> - There is no authz configured for this repo (SVNPathAuthz off).
>> - Backend is still in 1.5 format (we have not run svnadmin upgrade
>> yet, a dump+load is planned in a couple of weeks).
>>
>> So it seems clearly mod_dav_svn related (and not for instance related
>> to the FSFS backend).
>>
>> I don't think we have anything special in our httpd config:
>> [[[
>>    <Location /test_svn>
>>       SVNInMemoryCacheSize 131072
>>       SVNCacheFullTexts on
>>       SVNCacheTextDeltas on
>>       SSLRequireSSL
>>       AuthName "TEST Subversion Repository"
>>       AuthType Basic
>>       AuthBasicProvider ldap
>>       AuthBasicAuthoritative off
>>       AuthLDAPURL "ldap://redacted:389"
>>       AuthLDAPBindDN "redacted"
>>       AuthLDAPBindPassword redacted
>>       Require ldap-group redacted
>>       DAV svn
>>       SVNPath /path/to/test_repos
>>       SVNPathAuthz off
>>    </Location>
>> ]]]
>>
>> Any ideas?
>> Why the cpu usage by the server, what's it doing?
>> What is the dav/activities.d directory for? How come it contains so
>> many files? Is it ok to purge the old files from that directory?
>
> Httpd's mod_dav was updated in some recent version to do a full lock traversal on copies
and moves. I think we already applied some optimizations, but the real fix would be that mod_dav
shouldn't do this work (which our repos layer already does).
>
> I'm not sure which release we applied the first set of optimizations.
>

Thanks for refreshing my memory.

So the problem is known as issue #4531 (server-side copy (over dav)
uses too much memory) [1]. The memory usage issue has been fixed in
SVN 1.8.11 and 1.7.19 (see CHANGES), but a performance problem remains
(copy is no longer O(1), but depends on the size of the tree being
copied). That's a direct violation of one of Subversion's "old selling
points" vs. CVS: that branching / tagging is O(1). Branching / tagging
taking several minutes brings back "fond memories" from CVS' days.

As Philip pointed out in his last comment on #4531 [2]: "This issue is
related to a change in mod_dav in 2.2.25 to fix PR54610 which
added a walk over the copy source looking for lock tokens." (also
released in 2.4.5; so both httpd 2.2.25+ and 2.4.5+ are affected --
older httpd's won't have this problem I guess).

Again quoting Philip: "Apache knows in advance that the walk is
redundant in cases such as Subversion's URL-to-URL copy but Subversion
cannot avoid the read access. We should attempt to fix mod_dav to
avoid the walk where possible."

So my hope rests with Philip and others who might have the necessary
knowledge to fix this in mod_dav. It's really not acceptable that
branching / tagging (or I'm guessing also: moving a large tree with a
server-side move) takes several minutes.

[1] http://subversion.tigris.org/issues/show_bug.cgi?id=4531
[2] http://subversion.tigris.org/issues/show_bug.cgi?id=4531#desc12

-- 
Johan

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