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Subject: Re: [OMPI devel] mca_btl_sm_sendi question
From: Eugene Loh (Eugene.Loh_at_[hidden])
Date: 2009-02-25 10:36:12


George Bosilca wrote:

> On Feb 24, 2009, at 18:08 , Eugene Loh wrote:
>
>> (Probably this message only for George, but I'll toss it out to the
>> alias/archive.)
>
Actually, maybe Rich should weigh in here, too. This relates to the
overflow mechanism in MCA_BTL_SM_FIFO_WRITE.

>> I have a question about the sm sendi() function. What should happen
>> if the sendi() function attempts to write to the FIFO, but the FIFO
>> is full?
>
> The write should not be queued except in the case where the whole
> data referred by the convertor is copied out of the user memory.

And this is indeed the case. The data-convertor copy completed
successfully.

> If the FIFO is full, the best will be to allocate the descriptor and
> give it back to the PML.

Why? The data has been copied out of the user's buffer. The pointer to
that data has been queued for sending. (It hasn't been queued in the
FIFO, which is full, but it has been queued in the pending-send list.)

The FIFO has an overflow mechanism. Actually, prior to my recent
putbacks, it had two overflow mechanisms. One was to grow the FIFO, and
the other was to use the pending-send queue. While adding support for
multiple senders per FIFO and at Rich's suggestion, I pulled out the
ability to grow the FIFO. (Some number of folks didn't even believe
that the FIFO-grow stuff even existed or was enabled or worked
properly.) That still leaves the pending sends. So, the "out of
resource" return code from the FIFO write is kind of spurious. The FIFO
write is returning that code even though it has accepted the write and
queued it up.

>> Currently, it appears that the sendi() function returns an error
>> code to the PML, which assumes that the sendi() tried to send the
>> message but failed and so just tried to allocate a descriptor.
>
> Yes, this is the expected behavior.
>
>> But is that what should happen? The condition of the FIFO being
>> full is a little misleading since the write is still queued for
>> further progress -- not in the FIFO itself but in the pending-send
>> queue. This distinction should perhaps not matter to the upper
>> layers. The upper layers should still view the send as "completed"
>> (buffered by the MPI implementation to be progressed later). I
>> would think that the sendi() function should return a SUCCESS code.
>
> If the write is queued then this is more or less a bug. We will
> nicely cope with this case, because we have this sequence number and
> we will drop a message duplicate, but we will end-up sending the same
> message twice. The problem is that I don't know which of the copies
> will be used on the receiver side, I guess the first one reaching the
> receiver.

Arrgh! When the primary mechanism (FIFO) starts getting congested, we
start pumping duplicate messages into the system?

The proper fix (IMHO) is to have the sendi function return a SUCCESS
code once it's written the message and the pointer to the message. And,
once it's written those two things, it seems to me to be a bug to return
any other code.

>> Relevent source code is
>>
>> PML, line 496
>> https://svn.open-mpi.org/source/xref/ompi_1.3/ompi/mca/pml/ob1/pml_ob1_sendreq.c
>> #496
>>
>> BTL, line 785
>> https://svn.open-mpi.org/source/xref/ompi_1.3/ompi/mca/btl/sm/
>> btl_sm.c#785
>>
>> FIFO write, line 18
>> https://svn.open-mpi.org/opengrok/xref/ompi_1.3/ompi/mca/btl/sm/btl_sm_fifo.h
>> #18
>