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From Evgeny Kotkov <evgeny.kot...@visualsvn.com>
Subject [RFC/PATCH] Handling PROPFIND in mod_dav_svn (was: Re: Issue with browsing a SVN 1.9.2, schema 7, packed, repository)
Date Wed, 30 Dec 2015 15:49:08 GMT
Evgeny Kotkov <evgeny.kotkov@visualsvn.com> writes:

> Stefan Fuhrmann <stefan2@apache.org> writes:
>
>> Thanks, it would be nice if could do something within Subversion because
>> httpd and apr patches may take a while to trickle down into releases.
>> Meanwhile, I posted a patch for mod_dav on the httpd dev list.
>
> I took a look at what's happening, and I think the root cause of the problem
> is that during a PROPFIND walk, we use the request pool for allocations that
> happen per every walked entry.  This occurs when mod_dav_svn/repos.c:
> do_walk() calls dav_propfind_walker() with dav_walk_resource.pool pointing
> to the request pool.

The problem with unbounded memory usage during PROPFINDs in mod_dav is old
and quite fundamental.  Its first acknowledgment dates back to year 2003 [1],
and the corresponding Bugzilla issue has been open since 2009 [2, 3].

Subversion's mod_dav_svn module is built around mod_dav, and is affected as
well.  The propfind walker makes different FS calls that allocate a lot of
memory — especially if a cache miss happens.  A typical walk can cross the
mod_dav - mod_dav_svn boundary multiple times.  Since mod_dav was written
long ago and doesn't follow the same pool usage practices as Subversion,
some of the allocations happen in long-living pools, such as the request
pool.

So, the memory usage for a PROPFIND of depth 1 (svn ls -v) is unbounded.
Although the actual memory consumption can be mitigated by the cache hits,
since a cache hit either reduces or eliminates additional allocations, a real
server can easily consume gigabytes of RAM, when the data is not and possibly
can't fit in the cache.  The problem is serious, and can lead to various out
of memory conditions, such as constant swapping or server crashes.

I'm aware of two possible ways of solving the problem:

 (1) Fix mod_dav, adjust mod_dav_svn accordingly

 (2) Reimplement PROPFIND in mod_dav_svn

Doing (1) is hard, since it requires revving mod_dav's API and special care
for other modules using it.  As far as I know, those modules don't have tests
and not breaking their behavior could be a challenging task.

I believe that currently we don't have the necessary resources (aka committers)
to do that.  Given that the problem exists since 2003, I also doubt that it's
going to solve itself in the nearby future.  And moreover, even if somehow
managed to do that, the fix would probably only be available in the newer
2.6.x httpd releases.

With that in mind, I propose we do (2).

Although I'm not entirely happy with reimplementing a part of the protocol, I
don't think that there are other realistic approaches to this serious problem.
It wouldn't be the first time we're doing something like this, since we already
handle POST ourselves, as well as provide the custom Subversion's HTTPv2
protocol implementation.

Apart from solving the memory usage issue, having the PROPFIND implementation
in mod_dav_svn would allow us to improve a couple of other aspects of how we
handle these requests, e.g.:

 - Shrink down the PROPFIND response size by avoiding duplicate xmlns:
   namespace declarations that currently exist.

 - Optimize how we retrieve properties from the FS layer, since we would
   no longer be doing mod_dav - mod_dav_svn - mod_dav transitions, and
   would have more flexibility.

I attached a proof-of-concept patch that does (2).  This patch solves the
memory usage problem in my experiments and passes HTTPv2 / HTTPv1 tests.

Thoughts?

[1] https://mail-archives.apache.org/mod_mbox/httpd-dev/200305.mbox/%3C86smqzgcuf.fsf@kepler.ch.collab.net%3E
[2] https://bz.apache.org/bugzilla/show_bug.cgi?id=52123
[3] https://bz.apache.org/bugzilla/show_bug.cgi?id=48130
[4] https://mail-archives.apache.org/mod_mbox/subversion-users/201512.mbox/%3C1CEE115D02633942A40D49D447DCF46E432F3A32@SD01CFMM0202.OMEGA.DCE-EIR.NET%3E


Regards,
Evgeny Kotkov

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