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Subject: Re: [OMPI devel] [OMPI svn-full] svn:open-mpi r23936
From: Jeff Squyres (jsquyres_at_[hidden])
Date: 2010-10-26 06:20:27

On Oct 25, 2010, at 9:29 PM, George Bosilca wrote:

> 1. Not all processes deadlock in btl_sm_add_procs. The process that setup the shared memory area, is going forward, and block later in a barrier.

Yes, I'm seeing the same thing (I didn't include all details like this in my post, sorry). I was running with -np 2 on a local machine and saw vpid=0 get stuck in opal_progress (because the first time through, seg_inited < n_local_procs). vpid=1 increments seg_inited and therefore doesn't enter the loop that calls opal_progress(), and therefore continues on.

> 2. All other processes, loop around the opal_progress, until they got a message from all other processes. The variable used for counting is somehow updated correctly, but we still call opal_progress. I couldn't figure out is we loop more that we should, or if opal_progress doesn't return. However, both of these possibilities look very unlikely to me: the loop in the sm_add_procs is pretty straightforward, and I couldn't find any loops in opal_progress. I wonder if some of the messages get lost on the exchange.

I had this problem, too, until I tried to use padb to get stack traces. I noticed that when I ran padb, my blocked process un-blocked itself and continued. After more digging, I determined that my blocked process was, in fact, blocked in poll() with an infinite timeout. padb (or any signal at all) caused it to unblock and therefore continue.

> 3. If I unblock the situation by hand, everything goes back to normal. NetPIPE runs to completion but the performances are __really__ bad. On my test machine I get around 2000Mbs, when the expected value is at least 10 times more. Similar finding on the latency side, we're now at 1.65 micro-sec up from the usual 0.35 we had before.

It's a feature!

Jeff Squyres
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