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From Johan Corveleyn <jcor...@gmail.com>
Subject Re: Branching slow 1.8.11 https
Date Tue, 31 Mar 2015 12:13:28 GMT
On Tue, Mar 31, 2015 at 2:19 AM, Johan Corveleyn <jcorvel@gmail.com> wrote:
> On Sun, Mar 29, 2015 at 7:57 PM, Johan Corveleyn <jcorvel@gmail.com> wrote:
>> 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
>
> I think I've found a workaround: it seems the tree walk by mod_dav is
> avoided when the request has a header Depth with value 0. I've tried
> adding
>
>     <If "%{REQUEST_METHOD} == 'COPY'">
>         RequestHeader set Depth 0
>     </If>
>
> to the Location block of SVN, and the copy is fast again! And the good
> thing is: it's still a fully recursive copy :-) (otherwise it wouldn't
> be much of a workaround).
>
> 'svn copy' time for a very large tree (artificially generated with
> ~50000 folders and ~250000 files) is now down to 1,5 seconds (still
> three times slower than the same via file:/// or svn://, but good
> enough, and not O(sizeof(tree)) anymore).
>
> Is this workaround safe? Thoughts?
> It might even be something that can be exploited by our client, when
> 'svn copy'ing ... (though a "normal" server-side fix for this problem,
> within the normal workings of mod_dav, would of course be better
> still).

Seems this workaround is pretty OK for now (apparently the subversion
code on the server ignores the Depth:0 for COPY requests, so the copy
is handled like a normal recursive copy).

Bert suggested on irc to make the setting of the header also dependent
on the useragent string.

For completeness: I'm now no longer seeing the 1,5 seconds time for
copying over dav. Today it's more like 0,5 - 0,7 seconds, i.e. the
same as with file:// and svn://. Maybe something was slowing down my
network temporarily yesterday evening.

-- 
Johan

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