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<p>Thank you both. I am accustomed to having to have buffers pre-posted for all unexpected (untagged) messages so I was pleasantly surprised that someone buffered the tagged message for me. I will explore this functionality more.</p>
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<p>JB<br>
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<div id="x_divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" color="#000000" style="font-size:11pt"><b>From:</b> Hefty, Sean <sean.hefty@intel.com><br>
<b>Sent:</b> 16 March 2020 17:53:12<br>
<b>To:</b> Chris Dolan; Biddiscombe, John A.<br>
<b>Cc:</b> libfabric-users@lists.openfabrics.org<br>
<b>Subject:</b> RE: [libfabric-users] Tagged message question</font>
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<div class="PlainText">> Who provided the memory that the received message went into between being<br>
> received and my posting the recv buffer? Did libfabric supply some memory buffer 'on<br>
> the fly' that the tagged message went into?<br>
<br>
Adding to Chris' comments, these are known as 'unexpected messages'. They are a common MPI problem and nearly impossible to avoid, even for well-designed apps.<br>
<br>
Most commonly available hardware lack the ability to perform tag matching. As a result, rxm buffers all receives before deciding how to process it. If you're using RDM endpoints, there's really no additional cost for using tagged transfers, versus untagged.<br>
<br>
If the transfer is large, it won't complete immediately and the buffering will occur on the send side. Only the tagged information will be transferred. Once the receiver posts a receive with the correct tag, the data will be retrieved.<br>
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- Sean<br>
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