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<p class="MsoNormal"><span style="font-family:"Lucida Grande",sans-serif">The MPI implementation has to ensure that the probe will not return with message B first. It could use sequence numbers internally, for example.
<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Lucida Grande",sans-serif"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span style="font-size:12.0pt;color:black">From: </span></b><span style="font-size:12.0pt;color:black">"Hudson, Stephen Tobias P" <shudson@anl.gov><br>
<b>Date: </b>Monday, March 23, 2020 at 3:04 PM<br>
<b>To: </b>"Thakur, Rajeev" <thakur@anl.gov>, "Larson, Jeffrey M." <jmlarson@anl.gov>, "discuss@mpich.org" <discuss@mpich.org><br>
<b>Cc: </b>"Navarro, John-Luke Nicolas" <jnavarro@anl.gov><br>
<b>Subject: </b>Re: [mpich-discuss] In-order messages<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;color:black">Is it undefined where the matching happens?<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;color:black"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;color:black">So the question is:<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;color:black"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;color:black">Process 1 sends message A and then message B, both to process 0. message B travels faster through the network, and is received first by process 0. At this point an iprobe on process 0 occurs with
MPI_TAG_ANY. My thought is that it finds messageB (which happens to say - ALL FINISHED). A follow up MPI_recv will say all is finished. Is this possible?<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;color:black"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;color:black">I've assumed here that the probe is local and so the matching with message A has not yet happened.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;color:black"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-size:12.0pt;color:black">The given snippet tells me that a probe will return if there is a matching send, but in this case there is two matching sends.<o:p></o:p></span></p>
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<p class="MsoNormal"><b><span style="color:black">From:</span></b><span style="color:black"> Thakur, Rajeev <thakur@anl.gov><br>
<b>Sent:</b> Monday, March 23, 2020 2:41 PM<br>
<b>To:</b> Hudson, Stephen Tobias P <shudson@anl.gov>; Larson, Jeffrey M. <jmlarson@anl.gov>; discuss@mpich.org <discuss@mpich.org><br>
<b>Cc:</b> Navarro, John-Luke Nicolas <jnavarro@anl.gov><br>
<b>Subject:</b> Re: [mpich-discuss] In-order messages</span> <o:p></o:p></p>
<div>
<p class="MsoNormal"> <o:p></o:p></p>
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<p class="xmsonormal"><span style="font-family:"Lucida Grande",sans-serif">Pending means not already matched with another message.</span><o:p></o:p></p>
<p class="xmsonormal"><span style="font-family:"Lucida Grande",sans-serif"> </span><o:p></o:p></p>
<p class="xmsonormal"><span style="font-family:"Lucida Grande",sans-serif">This is what the standard says about probe:</span><o:p></o:p></p>
<p class="xmsonormal"><span style="font-family:"Lucida Grande",sans-serif"> </span><o:p></o:p></p>
<p class="xmsonormal"><span style="font-family:"Lucida Grande",sans-serif">“The MPI implementation of MPI_PROBE and MPI_IPROBE needs to guarantee progress: if a call to MPI_PROBE has been issued by a process, and a send that matches the probe has been initiated
by some process, then the call to MPI_PROBE will return, unless the message is received by another concurrent receive operation (that is executed by another thread at the probing process). Similarly, if a process busy waits with MPI_IPROBE and a matching message
has been issued, then the call to MPI_IPROBE will eventually return flag = true unless the message is received by another concurrent receive operation or matched by a concurrent matched probe.”</span><o:p></o:p></p>
<p class="xmsonormal"><span style="font-family:"Lucida Grande",sans-serif"> </span><o:p></o:p></p>
<p class="xmsonormal"><span style="font-family:"Lucida Grande",sans-serif">Eager protocol or any other protocol have to support the semantics specified by the standard.</span><o:p></o:p></p>
<p class="xmsonormal"><span style="font-family:"Lucida Grande",sans-serif"> </span><o:p></o:p></p>
<p class="xmsonormal"><span style="font-family:"Lucida Grande",sans-serif"> </span><o:p></o:p></p>
<p class="xmsonormal"><span style="font-family:"Lucida Grande",sans-serif"> </span><o:p></o:p></p>
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<p class="xmsonormal"><b><span style="font-size:12.0pt;color:black">From: </span>
</b><span style="font-size:12.0pt;color:black">"Hudson, Stephen Tobias P" <shudson@anl.gov><br>
<b>Date: </b>Monday, March 23, 2020 at 2:27 PM<br>
<b>To: </b>"Thakur, Rajeev" <thakur@anl.gov>, "Larson, Jeffrey M." <jmlarson@anl.gov>, "discuss@mpich.org" <discuss@mpich.org><br>
<b>Cc: </b>"Navarro, John-Luke Nicolas" <jnavarro@anl.gov><br>
<b>Subject: </b>Re: [mpich-discuss] In-order messages</span><o:p></o:p></p>
</div>
<div>
<p class="xmsonormal"> <o:p></o:p></p>
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<div>
<p class="xmsonormal" style="margin-bottom:12.0pt"><span style="font-size:12.0pt;color:black">The point where I am still a bit unclear is whether "pending" refers to something at the sender's end or in some buffer at the receiver's end. Perhaps an answer to
this will help me. Is mpi_probe/iprobe a local operation or does it actually communicate with the sender? Also, what effect does the 'eager' protocal have on this?</span><o:p></o:p></p>
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<p class="xmsonormal"><b><span style="color:black">From:</span></b><span style="color:black"> Thakur, Rajeev <thakur@anl.gov><br>
<b>Sent:</b> Thursday, March 12, 2020 6:30 PM<br>
<b>To:</b> Hudson, Stephen Tobias P <shudson@anl.gov>; Larson, Jeffrey M. <jmlarson@anl.gov>; discuss@mpich.org <discuss@mpich.org><br>
<b>Cc:</b> Navarro, John-Luke Nicolas <jnavarro@anl.gov><br>
<b>Subject:</b> Re: [mpich-discuss] In-order messages</span> <o:p></o:p></p>
<div>
<p class="xmsonormal"> <o:p></o:p></p>
</div>
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<div>
<p class="xxmsonormal"><span style="font-family:"Lucida Grande",sans-serif">The paragraph in Sec 3.5 (reproduced below) talks about the case when two sends match one receive and another case where two receives match one send. In the first case, the receive
can get data only from the first send. In the second case, only the first receive can get the data from the send. That is what is meant by non-overtaking.</span><o:p></o:p></p>
<p class="xxmsonormal"><span style="font-family:"Lucida Grande",sans-serif"> </span><o:p></o:p></p>
<p class="xxmsonormal"><span style="font-family:"Lucida Grande",sans-serif">“Messages are non-overtaking: If a sender sends two messages in succession to the same destination, and both match the same receive, then this operation cannot receive the second message
if the first one is still pending. If a receiver posts two receives in succession, and both match the same message, then the second receive operation cannot be satisfied by this message, if the first one is still pending. This requirement facilitates matching
of sends to receives.”</span><o:p></o:p></p>
<p class="xxmsonormal"><span style="font-family:"Lucida Grande",sans-serif"> </span><o:p></o:p></p>
<p class="xxmsonormal"><span style="font-family:"Lucida Grande",sans-serif"> </span><o:p></o:p></p>
<p class="xxmsonormal"><span style="font-family:"Lucida Grande",sans-serif"> </span><o:p></o:p></p>
<div style="border:none;border-top:solid #B5C4DF 1.0pt;padding:3.0pt 0in 0in 0in">
<p class="xxmsonormal"><b><span style="font-size:12.0pt;color:black">From: </span>
</b><span style="font-size:12.0pt;color:black">"Hudson, Stephen Tobias P" <shudson@anl.gov><br>
<b>Date: </b>Thursday, March 12, 2020 at 4:09 PM<br>
<b>To: </b>"Thakur, Rajeev" <thakur@anl.gov>, "Larson, Jeffrey M." <jmlarson@anl.gov>, "discuss@mpich.org" <discuss@mpich.org><br>
<b>Cc: </b>"Navarro, John-Luke Nicolas" <jnavarro@anl.gov><br>
<b>Subject: </b>Re: [mpich-discuss] In-order messages</span><o:p></o:p></p>
</div>
<div>
<p class="xxmsonormal"> <o:p></o:p></p>
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<div>
<p class="xxmsonormal"><span style="font-size:12.0pt;color:black">Paraphrasing the discussion so far, my understanding is:</span><o:p></o:p></p>
<div>
<p class="xxmsonormal"><span style="font-size:12.0pt;color:black"> </span><o:p></o:p></p>
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<div>
<p class="xxmsonormal"><span style="font-size:12.0pt;color:black">Note: I am only thinking of the case where both messages have the same tag and go to the same destination processor.</span><o:p></o:p></p>
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<div>
<p class="xxmsonormal"><span style="font-size:12.0pt;color:black"> </span><o:p></o:p></p>
</div>
<div>
<p class="xxmsonormal"><span style="font-size:12.0pt;color:black">Section 3.5 of the MPI standard mentions: "Messages are non-overtaking: "</span><o:p></o:p></p>
</div>
<div>
<p class="xxmsonormal"><span style="font-size:12.0pt;color:black"> </span><o:p></o:p></p>
</div>
<div>
<p class="xxmsonormal"><span style="font-size:12.0pt;color:black">I think this must mean that once the messages are in the receive buffer one cannot overtake the one infront. It is trying to make the point that your MPI_Recv cannot reach over message 1 and
say I want message 2 (the buffer is FIFO). But this is only in the receive buffer.</span><o:p></o:p></p>
</div>
<div>
<p class="xxmsonormal"><span style="font-size:12.0pt;color:black"> </span><o:p></o:p></p>
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<div>
<p class="xxmsonormal"><span style="font-size:12.0pt;color:black">However, the order the messages are received into the buffer (from the network) cannot be guaranteed to be the same as the order they were sent.</span><o:p></o:p></p>
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<div>
<p class="xxmsonormal"><span style="font-size:12.0pt;color:black"> </span><o:p></o:p></p>
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<p class="xxmsonormal"><span style="font-size:12.0pt;color:black">Is this correct?</span><o:p></o:p></p>
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<p class="xxmsonormal"><span style="font-size:12.0pt;color:black"> </span><o:p></o:p></p>
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<p class="xxmsonormal"><span style="font-size:12.0pt;color:black">Many Thanks,</span><o:p></o:p></p>
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<p class="xxmsonormal"><span style="font-size:12.0pt;color:black"> </span><o:p></o:p></p>
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<p class="xxmsonormal"><span style="font-size:12.0pt;color:black">Steve</span><o:p></o:p></p>
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<p class="xxmsonormal"><span style="font-size:12.0pt;color:black"> </span><o:p></o:p></p>
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<p class="xxmsonormal"><b><span style="color:black">From:</span></b><span style="color:black"> Thakur, Rajeev <thakur@anl.gov><br>
<b>Sent:</b> Thursday, March 12, 2020 1:18 PM<br>
<b>To:</b> Larson, Jeffrey M. <jmlarson@anl.gov>; discuss@mpich.org <discuss@mpich.org><br>
<b>Cc:</b> Navarro, John-Luke Nicolas <jnavarro@anl.gov>; Hudson, Stephen Tobias P <shudson@anl.gov><br>
<b>Subject:</b> Re: [mpich-discuss] In-order messages</span> <o:p></o:p></p>
<div>
<p class="xxmsonormal"> <o:p></o:p></p>
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<div>
<p class="xxxmsonormal"><span style="font-family:"Lucida Grande",sans-serif">Matching specifies which buffer the incoming message goes into. Two sends issued in order with the same tag and destination will match two receives issued in order with the same tag
or MPI_ANY_TAG at the destination. The two sends could take different paths through the network, one more congested than the other, and hence reach the destination out of order. But they will get placed in the intended buffers.</span><o:p></o:p></p>
<p class="xxxmsonormal"><span style="font-family:"Lucida Grande",sans-serif"> </span><o:p></o:p></p>
<p class="xxxmsonormal"><span style="font-family:"Lucida Grande",sans-serif">Rajeev</span><o:p></o:p></p>
<p class="xxxmsonormal"><span style="font-family:"Lucida Grande",sans-serif"> </span><o:p></o:p></p>
<p class="xxxmsonormal"><span style="font-family:"Lucida Grande",sans-serif"> </span><o:p></o:p></p>
<div style="border:none;border-top:solid #B5C4DF 1.0pt;padding:3.0pt 0in 0in 0in">
<p class="xxxmsonormal"><b><span style="font-size:12.0pt;color:black">From: </span>
</b><span style="font-size:12.0pt;color:black">"Larson, Jeffrey M." <jmlarson@anl.gov><br>
<b>Date: </b>Thursday, March 12, 2020 at 1:12 PM<br>
<b>To: </b>"Thakur, Rajeev" <thakur@anl.gov>, "discuss@mpich.org" <discuss@mpich.org><br>
<b>Cc: </b>"Navarro, John-Luke Nicolas" <jnavarro@anl.gov>, "Hudson, Stephen Tobias P" <shudson@anl.gov><br>
<b>Subject: </b>Re: [mpich-discuss] In-order messages</span><o:p></o:p></p>
</div>
<div>
<p class="xxxmsonormal"> <o:p></o:p></p>
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<div>
<p class="xxxmsonormal">On 3/12/20 1:04 PM, Thakur, Rajeev wrote:<o:p></o:p></p>
</div>
<blockquote style="margin-top:5.0pt;margin-bottom:5.0pt">
<p class="xxxmsonormal"><span style="font-family:"Times New Roman",serif">The standard specifies the order in which messages match, not complete. They can complete in any order.</span><o:p></o:p></p>
<p class="xxxmsonormal"><span style="font-family:"Times New Roman",serif"> </span><o:p></o:p></p>
<p class="xxxmsonormal"><span style="font-family:"Times New Roman",serif">Rajeev</span><o:p></o:p></p>
<p class="xxxmsonormal"><span style="font-family:"Times New Roman",serif"> </span><o:p></o:p></p>
<p class="xxxmsonormal"><span style="font-family:"Times New Roman",serif"> </span><o:p></o:p></p>
<div style="border:none;border-top:solid #B5C4DF 1.0pt;padding:3.0pt 0in 0in 0in">
<p class="xxxmsonormal"><b><span style="font-size:12.0pt;color:black">From: </span>
</b><span style="font-size:12.0pt;color:black">"Larson, Jeffrey M. via discuss" <a href="mailto:discuss@mpich.org">
<discuss@mpich.org></a><br>
<b>Reply-To: </b><a href="mailto:discuss@mpich.org">"discuss@mpich.org"</a> <a href="mailto:discuss@mpich.org">
<discuss@mpich.org></a><br>
<b>Date: </b>Thursday, March 12, 2020 at 12:46 PM<br>
<b>To: </b><a href="mailto:discuss@mpich.org">"discuss@mpich.org"</a> <a href="mailto:discuss@mpich.org">
<discuss@mpich.org></a><br>
<b>Cc: </b>"Larson, Jeffrey M." <a href="mailto:jmlarson@anl.gov"><jmlarson@anl.gov></a>, "Navarro, John-Luke Nicolas"
<a href="mailto:jnavarro@anl.gov"><jnavarro@anl.gov></a>, "Hudson, Stephen Tobias P"
<a href="mailto:shudson@anl.gov"><shudson@anl.gov></a><br>
<b>Subject: </b>[mpich-discuss] In-order messages</span><o:p></o:p></p>
</div>
<div>
<p class="xxxmsonormal"> <o:p></o:p></p>
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<div>
<p class="xxxmsonormal" style="background:white"><span style="font-size:12.0pt;color:black">Hello MPICH friends,</span><o:p></o:p></p>
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<p class="xxxmsonormal" style="background:white"><span style="font-size:12.0pt;color:black"> </span><o:p></o:p></p>
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<p class="xxxmsonormal" style="background:white"><span style="font-size:12.0pt;color:black">Consider the simple two-rank MPI scenario:</span><o:p></o:p></p>
</div>
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<ul style="margin-top:0in" type="disc">
<li class="xxxmsonormal" style="color:black;mso-list:l0 level1 lfo1;background:white">
<span style="font-size:12.0pt">Rank 1 is doing calculations and giving chunks of data to rank 0 using nonblocking sends and tag=0.</span><o:p></o:p></li><li class="xxxmsonormal" style="color:black;mso-list:l0 level1 lfo1;background:white">
<span style="font-size:12.0pt">When rank 1 is finished, it will send it's last data (or no data) with tag=1. </span><o:p></o:p></li><li class="xxxmsonormal" style="color:black;mso-list:l0 level1 lfo1;background:white">
<span style="font-size:12.0pt">Rank 0 is using probes to see when data is ready to be received. It receives with any tag, knowing when to stop receiving, or give new data when a tag=0 is received. </span><o:p></o:p></li></ul>
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<p class="xxxmsonormal" style="background:white"><span style="font-size:12.0pt;color:black">Is it possible that rank 0 receives a tag=1 message when there are outstanding tag=0 messages?</span><o:p></o:p></p>
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<p class="xxxmsonormal" style="background:white"><span style="font-size:12.0pt;color:black"> </span><o:p></o:p></p>
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<p class="xxxmsonormal" style="background:white"><span style="font-size:12.0pt;color:black">Looking at section 3.5 of the MPI standard lets me know that</span><o:p></o:p></p>
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<p class="xxxmsonormal" style="background:white"><span style="font-size:12.0pt;color:black"> </span><o:p></o:p></p>
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<p class="xxxmsonormal" style="background:white"><span style="font-size:12.0pt;color:black">"Messages are non-overtaking: If a sender sends two messages in succession to the same destination, and both match the same receive, then this operation cannot receive
the second message if the first one is still pending."</span><o:p></o:p></p>
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<p class="xxxmsonormal" style="background:white"><span style="font-size:12.0pt;color:black"> </span><o:p></o:p></p>
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<p class="xxxmsonormal" style="background:white"><span style="font-size:12.0pt;color:black">But I'm not sure if this applies to the above case. Is anytag "the same receive"? </span><o:p></o:p></p>
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<p class="xxxmsonormal" style="background:white"><span style="font-size:12.0pt;color:black">If rank 1 puts data in its buffer, doesn't the network have to be used to communicate that to the buffer of rank 0?</span><o:p></o:p></p>
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<p class="xxxmsonormal" style="background:white"><span style="font-size:12.0pt;color:black">While rank 1 is putting data into its buffer in order, is it possible that a tiny tag=1 message is registered in the rank 0 buffer before a massive tag=0 message? </span><o:p></o:p></p>
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<p class="xxxmsonormal" style="background:white"><span style="font-size:12.0pt;color:black"> </span><o:p></o:p></p>
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<p class="xxxmsonormal" style="background:white"><span style="font-size:12.0pt;color:black">Thank you for your help,</span><o:p></o:p></p>
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<p class="xxxmsonormal" style="background:white"><span style="font-size:12.0pt;color:black">Jeff</span><o:p></o:p></p>
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<p>Hi Rajeev,<o:p></o:p></p>
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What is the difference between matching and completing?<o:p></o:p></p>
<p>Does this mean that the quote from the standard that I gave means the messages between two fixed ranks could overtake each other? That is, they "match" but don't "complete" in the same order?<o:p></o:p></p>
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