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<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>The original question was about relatively small messages –
only 500 bytes each<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>You can often get better throughput if you send say two smaller
messages rather than one large one.<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>This is since the interconnect can generate multiple RDMA
requests that can proceed concurrently.<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>This old paper from 2003 illustrates this<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><a
href="http://www.docstoc.com/docs/5579957/Quadrics-QsNetII-A-network-for-Supercomputing-Applications">http://www.docstoc.com/docs/5579957/Quadrics-QsNetII-A-network-for-Supercomputing-Applications</a><o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Page 25 shows a graph where 1,2,4 and 8 concurrent RDMA are
issued concurrently. For large messages (>256KB) there is no significant difference
in the achieved total bandwidth – it is limited by the PCIe/PCI-X
interface or the interconnect fabric itself.<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>But at smaller messages sizes there are measurable differences –
eg. two 1K messages show higher total bandwidth than a single 2K message.<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Daniel<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>p.s. did you really mean to compare three 500bytes transfers
with a single 2000byte transfer, rather than the same total message size in
both cases?<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>pps. Case A is really a broadcast – interconnects that implement
broadcast in hardware are bound to do A faster than B<o:p></o:p></span></p>
<p class=MsoNormal><span style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<div style='border:none;border-top:solid #B5C4DF 1.0pt;padding:3.0pt 0cm 0cm 0cm'>
<p class=MsoNormal><b><span lang=EN-US style='font-size:10.0pt;font-family:
"Tahoma","sans-serif"'>From:</span></b><span lang=EN-US style='font-size:10.0pt;
font-family:"Tahoma","sans-serif"'> beowulf-bounces@beowulf.org
[mailto:beowulf-bounces@beowulf.org] <b>On Behalf Of </b>Bruno Coutinho<br>
<b>Sent:</b> 23 May 2009 16:44<br>
<b>To:</b> tribur@vision.ee.ethz.ch<br>
<b>Cc:</b> beowulf@beowulf.org<br>
<b>Subject:</b> Re: [Beowulf] MPI - time for packing, unpacking, creating a
message...<o:p></o:p></span></p>
</div>
<p class=MsoNormal><o:p> </o:p></p>
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<p class=MsoNormal>If you are using Gigabit Ethernet with jumbo frames (9000
bytes for example): <br>
A will send 3 packets with 4000 bytes and <br>
B will send one of 9000 bytes and one of 7000 bytes.<br>
<br>
For the cpu B is better, because will generate one system call and A will
generate three and<br>
as many high speed interconnects today need large packets to fully utilize
their bandwidth, I think that B should be faster.<br>
But the only way to be sure is testing.<br>
<br>
<br>
2009/5/18 <<a href="mailto:tribur@vision.ee.ethz.ch">tribur@vision.ee.ethz.ch</a>><o:p></o:p></p>
<p class=MsoNormal>Hi all,<br>
<br>
is there anyone who can tell me if A) or B) is probably faster?<br>
<br>
A)<br>
process 0 sends 3x500 elements, e.g. doubles, to 3 different processors using
something like<br>
if(rank==0){<br>
MPI_Send(sendbuf, 500, MPI_DOUBLE, 1, 1, MPI_COMM_WORLD);<br>
MPI_Send(sendbuf, 500, MPI_DOUBLE, 2, 2, MPI_COMM_WORLD);<br>
MPI_Send(sendbuf, 500, MPI_DOUBLE, 3, 3, MPI_COMM_WORLD);<br>
}<br>
else<br>
MPI_Recv(recvbuf, 500, MPI_DOUBLE, 0, rank, MPI_COMM_WORLD, status);<br>
<br>
<br>
B)<br>
process 0 sends 2000 elements to process 1 using<br>
if(rank==0)<br>
MPI_Send(sendbuf, 2000, MPI_DOUBLE, 1, 1, MPI_COMM_WORLD);<br>
else<br>
MPI_Recv(recvbuf, 2000, MPI_DOUBLE, 0, rank, MPI_COMM_WORLD, status);<br>
<br>
<br>
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