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Subject: Re: [OMPI devel] trac ticket 1944 and pending sends
From: Brian Barrett (brbarret_at_[hidden])
Date: 2009-06-24 08:50:30


Or go to what I proposed and USE A LINKED LIST! (as I said before,
not an original idea, but one I think has merit) Then you don't have
to size the fifo, because there isn't a fifo. Limit the number of
send fragments any one proc can allocate and the only place memory can
grow without bound is the OB1 unexpected list. Then use SEND_COMPLETE
instead of SEND_NORMAL in the collectives without barrier semantics
(bcast, reduce, gather, scatter) and you effectively limit how far
ahead any one proc can get to something that we can handle, with no
performance hit.

Brian

On Jun 24, 2009, at 12:46 AM, George Bosilca wrote:

> In other words, as long as a queue is peer based (peer not peers),
> the management of the pending send list was doing what it was
> supposed to, and there was no possibility of deadlock. With the new
> code, as a third party can fill up a remote queue, getting a
> fragment back [as you stated] became a poor indicator for retry.
>
> I don't see how the proposed solution will solve the issue without a
> significant overhead. As we only call the MCA_BTL_SM_FIFO_WRITE once
> before the fragment get into the pending list, reordering the
> fragments will not solve the issue. When the peers is overloaded,
> the fragments will end-up in the pending list, and there is nothing
> to get it out of there except a message from the peer. In some
> cases, such a message might never be delivered, simply because the
> peer doesn't have any data to send us.
>
> The other solution is to always check all pending lists. While this
> might work, it will certainly add undesirable overhead to the send
> path.
>
> You last patch was doing the right thing. Globally decreasing the
> size of the memory used by the MPI library is _the right_ way to go.
> Unfortunately, your patch only address this at the level of the
> shared memory file. Now, instead of using less memory we use even
> more because we have to store that data somewhere ... in the
> fragments returned by the btl_sm_alloc function. These fragments are
> allocated on demand and by default there is no limit to the number
> of such fragments.
>
> Here is a simple fix for both problems. Enforce a reasonable limit
> on the number of fragments in the BTL free list (1K should be more
> than enough), and make sure the fifo has a size equal to p *
> number_of_allowed_fragments_in_the_free_list, where p is the number
> of local processes. While this solution will certainly increase
> again the size of the mapped file, it will do it by a small margin
> compared with what is happening today in the code. This is without
> talking about the fact that it will solve the deadlock problem, by
> removing the inability to return a fragment. In addition, the PML is
> capable of handing such situations, so we're getting back to a
> deadlock free sm BTL.
>
> george.
>
>
> On Jun 23, 2009, at 11:04 , Eugene Loh wrote:
>
>> The sm BTL used to have two mechanisms for dealing with congested
>> FIFOs. One was to grow the FIFOs. Another was to queue pending
>> sends locally (on the sender's side). I think the grow-FIFO
>> mechanism was typically invoked and the pending-send mechanism used
>> only under extreme circumstances (no more memory).
>>
>> With the sm makeover of 1.3.2, we dropped the ability to grow
>> FIFOs. The code added complexity and there seemed to be no need to
>> have two mechanisms to deal with congested FIFOs. In ticket 1944,
>> however, we see that repeated collectives can produce hangs, and
>> this seems to be due to the pending-send code not adequately
>> dealing with congested FIFOs.
>>
>> Today, when a process tries to write to a remote FIFO and fails, it
>> queues the write as a pending send. The only condition under which
>> it retries pending sends is when it gets a fragment back from a
>> remote process.
>>
>> I think the logic must have been that the FIFO got congested
>> because we issued too many sends. Getting a fragment back
>> indicates that the remote process has made progress digesting those
>> sends. In ticket 1944, we see that a FIFO can also get congested
>> from too many returning fragments. Further, with shared FIFOs, a
>> FIFO could become congested due to the activity of a third-party
>> process.
>>
>> In sum, getting a fragment back from a remote process is a poor
>> indicator that it's time to retry pending sends.
>>
>> Maybe the real way to know when to retry pending sends is just to
>> check if there's room on the FIFO.
>>
>> So, I'll try modifying MCA_BTL_SM_FIFO_WRITE. It'll start by
>> checking if there are pending sends. If so, it'll retry them
>> before performing the requested write. This should also help
>> preserve ordering a little better. I'm guessing this will not hurt
>> our message latency in any meaningful way, but I'll check this out.
>>
>> Meanwhile, I wanted to check in with y'all for any guidance you
>> might have.
>> _______________________________________________
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>> devel_at_[hidden]
>> http://www.open-mpi.org/mailman/listinfo.cgi/devel
>
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