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From Branko ─îibej <br...@wandisco.com>
Subject Re: Branching slow 1.8.11 https
Date Tue, 31 Mar 2015 13:20:16 GMT
On 31.03.2015 14:43, Mark Phippard wrote:
>> On Mar 31, 2015, at 8:13 AM, Johan Corveleyn <jcorvel@gmail.com> wrote:
>>
>>> 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
> Are we going to change the client to send this header? This seems like a very significant
regression in our primary "promises" to allow it to wait for a mod_dav fix that might never
even happen.

The problem is that there are other Subversion DAV server
implementations out there that could break if we tried to do a
non-recursive copy of a directory (whatever that means).

-- Brane


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