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Re: Random serf checkout failures

From: Lieven Govaerts <lgo_at_mobsol.be>
Date: Mon, 5 Nov 2012 22:21:04 +0100

Hi Mike

On Mon, Nov 5, 2012 at 9:39 PM, C. Michael Pilato <cmpilato_at_collab.net> wrote:
> On 11/05/2012 01:33 PM, C. Michael Pilato wrote:
>> On 11/05/2012 11:49 AM, Lieven Govaerts wrote:
>>> My current test with svn trunk & apache+mod_dav_svn 1.6.12 does not
>>> reproduce this issue.
>>>
>>> What I am seeing is that svn is using a lot of memory, up to 1GB.
>>> Currently looking at the root cause there.
>>>
>>> First clue: many directories are not closed until the call to
>>> close_all_dirs in libsvn_ra_serf/update.c:finish_report. This means
>>> that for the most part of the checkout, the associated info object is
>>> kept in memory.
>>
>> My debugging indicates that when close_all_dirs() is called, there are a
>> slew of unclosed directories which each have a non-zero ref_count. Save for
>> the root directory of the edit (/tags), fetch_props == TRUE,
>> propfind_handler->done == TRUE, and tag_closed == TRUE for all open child
>> directories. So I'm looking around at how file references are managed to
>> see if anything has leaked. I might have inadvertently botched something
>> when adding the pull-contents-from-the-wc-cache logic.
>
> So ... if I'm reading the ra_serf code correctly (and let me tell you,
> there's a good chance I'm not), the only time we opportunistically close
> directory batons is when we're processing the current list of completed file
> fetches. See the code which follows this comment:
>
> /* If we have a valid directory and
> * we have no open items in this dir and
> * we've closed the directory tag (no more children can be added)
> * and either:
> * we know we won't be fetching props or
> * we've already completed the propfind
> * then, we know it's time for us to close this directory.
> */
>
> That code is all encompassed within an outer loop that looks like so:
>
> /* prune our fetches list if they are done. */
> done_list = report->done_fetches;
> while (done_list)
> {
>
> And all that is nested within yet another loop that runs until the REPORT
> response is all parse and all auxiliary GETs and PROPFINDs are finished.
>
> So, back to the opportunistic directory closure: *if* we've finished any
> GETs since the last time we checked, and if ${STUFF_IN_THAT_BIG_COMMENT},
> then we try to close the parent directory of the file we fetched, plus any
> other close-able parent directories thereof.
>
> But what about when there are no file content fetches to manage?
>
> Using a trunk client and 1.7.x server, I just tried to export a tree with
> 15k directories (10 wide at the top, 15 or so below that, and 100 or so
> below those) and no files. 153 Mb of memory peak usage, and not a single
> directory closure until the whole tree had been processed/PROPFINDed/etc.
>
> Unless I'm mistaken, we will delay until the very end the closure of every
> directory (and by refcount-management-extension, every parent thereof) for
> which we don't execute at least one GET for an immediately child thereof.

With Philip's test scenario, you can see this clearly as the 3 empty
directories in svn trunk are only closed at the very end:
..
A /tmp/tags/52/subversion/bindings/swig/proxy
A /tmp/tags/51/subversion/tests/cmdline/import_tests_data/import_tree/DIR1.noo
A /tmp/tags/51/subversion/tests/cmdline/import_tests_data/import_tree/DIR6/DIR7/DIR8.noo
A /tmp/tags/51/subversion/bindings/swig/proxy
A /tmp/tags/50/subversion/tests/cmdline/import_tests_data/import_tree/DIR1.noo
A /tmp/tags/50/subversion/tests/cmdline/import_tests_data/import_tree/DIR6/DIR7/DIR8.noo
A /tmp/tags/50/subversion/bindings/swig/proxy
Checked out revision 151.

> I expect this situation is not new to trunk. (In fact, repeating the above
> export with a 1.7 client, I see a peak memory using of 458 Mb!) Memory
> usage improvements in trunk help matters. But then, the feature which
> causes ra_serf to use local file contents where it could avoid hitting the
> network will, I expect, cause still more instances of directories for which
> no immediate file children's contents will be fetched.
>
> Philip, Lieven, somebody -- care to double-check my thinking here? I'm only
> 80% confident in my findings at the moment.
>

I haven't checked the code for all details yet, but I can confirm from
looking at finish_report that directories are only closed when at
least all files in it have been processed, and empty directories
aren't even considered in the outer close_dir loop.
Your analysis confirms what I see with additional logging.

Do you plan on patching this issue? If yes I can continue looking at
Philip's original issue in this thread - assuming it is not caused by
the memory leak.

Lieven
Received on 2012-11-05 22:21:57 CET

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