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Re: Branching slow 1.8.11 https

From: Branko ─îibej <brane_at_wandisco.com>
Date: Tue, 31 Mar 2015 15:20:16 +0200

On 31.03.2015 14:43, Mark Phippard wrote:
>> On Mar 31, 2015, at 8:13 AM, Johan Corveleyn <jcorvel_at_gmail.com> wrote:
>>> On Tue, Mar 31, 2015 at 2:19 AM, Johan Corveleyn <jcorvel_at_gmail.com> wrote:
>>>> On Sun, Mar 29, 2015 at 7:57 PM, Johan Corveleyn <jcorvel_at_gmail.com> wrote:
>>>>> On Sat, Mar 28, 2015 at 5:09 PM, Bert Huijben <bert_at_qqmail.nl> wrote:
>>>>>> -----Original Message-----
>>>>>> From: Johan Corveleyn [mailto:jcorvel_at_gmail.com]
>>>>>> Sent: vrijdag 27 maart 2015 22:03
>>>>>> To: users_at_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
Received on 2015-03-31 15:22:18 CEST

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