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Subject: Re: [OMPI devel] trac ticket 1944 and pending sends
From: George Bosilca (bosilca_at_[hidden])
Date: 2009-06-24 02:46:28


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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