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Subject: Re: [OMPI devel] Infinite Loop: ompi_free_list_wait
From: Jeff Squyres (jsquyres_at_[hidden])
Date: 2009-03-25 08:55:25

George -- correct me if I'm wrong -- we went through and audited ob1
and the relevant BTLs. The only places where WAIT remains are places
that are guaranteed to not be problematic.

So you shouldn't need to edit ob1 at all.

If you're working with Open MPI, you might want to investigate using
Mercurial. We use it fairly successfully internally for "branching"
from the main Open MPI development SVN. There's an official Open MPI
mercurial repository (it's a mirror of our SVN repository) that you
can clone and work from. This will make it relatively easy to keep up
with the Open MPI SVN trunk, if you're interested. See these pages
for some details:

As for upgrading your BTL from 1.2 to 1.3, I'm afraid I don't remember
the details of what changed. It shouldn't be *too* hard...?

On Mar 23, 2009, at 11:11 AM, Timothy Hayes wrote:

> That's a relief to know, although I'm still a bit concerned. I'm
> looking at the code for the OpenMPI 1.3 trunk and in the ob1
> component I can see the following sequence:
> mca_pml_ob1_recv_frag_callback_match -> append_frag_to_list ->
> __ompi_free_list_wait
> so I'm guessing unless the deadlock issue has been resolved for that
> function, it will still fail non deterministically. I'm quite eager
> to give it a try, but my component doesn't compile as is with the
> 1.3 source. Is it trivial to convert it?
> Or maybe you were suggesting that I go into the code of ob1 myself
> and manually change every _wait to _get?
> Kind regards
> Tim
> 2009/3/23 George Bosilca <bosilca_at_[hidden]>
> It is a known problem. When the freelist is empty going in the
> ompi_free_list_wait will block the process until at least one
> fragment became available. As a fragment can became available only
> when returned by the BTL, this can lead to deadlocks in some cases.
> The workaround is to ban the usage of the blocking _wait function,
> and replace it with the non-blocking version _get. The PML has all
> the required logic to deal with the cases where a fragment cannot be
> allocated. We changed most of the BTLs to use _get instead of _wait
> few months ago.
> Thanks,
> george.
> On Mar 23, 2009, at 11:58 , Timothy Hayes wrote:
> Hello,
> I'm working on an OpenMPI BTL component and am having a recurring
> problem, I was wondering if anyone could shed some light on it. I
> have a component that's quite straight forward, it uses a pair of
> lightweight sockets to take advantage of being in a virtualised
> environment (specifically Xen). My code is a bit messy and has lots
> of inefficiencies, but the logic seems sound enough. I've been able
> to execute a few simple programs successfully using the component,
> and they work most of the time.
> The problem I'm having is actually happening in higher layers,
> specifically in my asynchronous receive handler, when I call the
> callback function (cbfunc) that was set by the PML in the BTL
> initialisation phase. It seems to be getting stuck in an infinite
> loop at __ompi_free_list_wait(), in this function there is a
> condition variable which should get set eventually but just doesn't.
> I've stepped through it with GDB and I get a backtrace of something
> like this:
> mca_btl_xen_endpoint_recv_handler -> mca_btl_xen_endpoint_start_recv
> -> mca_pml_ob1_recv_frag_callback -> mca_pml_ob1_recv_frag_match ->
> __ompi_free_list_wait -> opal_condition_wait
> and from there it just loops. Although this is happening in higher
> levels, I haven't noticed something like this happening in any of
> the other BTL components so chances are there's something in my code
> that's causing this. I very much doubt that it's actually waiting
> for a list item to be returned since this infinite loop can occur
> non deterministically and sometimes even on the first receive
> callback.
> I'm really not too sure what else to include with this e-mail. I
> could send my source code (a bit nasty right now) if it would be
> helpful, but I'm hoping that someone might have noticed this problem
> before or something similar. Maybe I'm making a common mistake. Any
> advice would be really appreciated!
> I'm using OpenMPI 1.2.9 from the SVN tag repository.
> Kind regards
> Tim Hayes
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Jeff Squyres
Cisco Systems