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<p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Konstantin,<o:p></o:p></span></p>
<p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>I work at Intel Labs so let me try answering the RCCE-related
part: The latency table reflects the numbers from looking at the actual
hardware, i.e. without taking software operation into account. The RCCE
round-trip times, however, were measured by running an actual application, i.e.
they rather reflect the efficiency of one particular communication algorithm
than hardware properties. I do not know the precise number but there are actually
several MPB accesses involved in passing data via RCCE. <o:p></o:p></span></p>
<p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Please also note that the bypass mode should _<i>not</i>_ be
used as we have a hardware bug which can lead to reading incorrect data.
Unfortunately this greatly reduces the benefit of using the on-die SRAM vs.
off-die DDR3.<o:p></o:p></span></p>
<p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'><o:p> </o:p></span></p>
<p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Best regards,<o:p></o:p></span></p>
<p class=MsoNormal><span lang=EN-US style='font-size:11.0pt;font-family:"Calibri","sans-serif";
color:#1F497D'>Werner<o:p></o:p></span></p>
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color:#1F497D'><o:p> </o:p></span></p>
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<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"'> Konstantin Zertsekel
[mailto:zertsekel@gmail.com] <br>
<b>Sent:</b> Tuesday, March 01, 2011 4:32 PM<br>
<b>To:</b> barrelfish-users@lists.inf.ethz.ch<br>
<b>Cc:</b> Dan Tsafrir; Konstantin Zertsekel; Roei; Ido Shamay; Avi Mendelson;
Prof. Assaf Schuster<br>
<b>Subject:</b> [Barrelfish-users] Intel SCC latency measurements for MPB
operations<o:p></o:p></span></p>
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<p class=MsoNormal>Hi all,<br>
I am engaged in the project with Intel SCC chip where communication latency is
important factor.<br>
Now, according to RCCE inter-core Ping-Pong test, the minimum latency is 5
microseconds (see <a
href="https://picasaweb.google.com/lh/photo/DP8vsfafDDHqcf_SZGzZMg?feat=directlink">this
graph</a>: <a
href="https://picasaweb.google.com/lh/photo/DP8vsfafDDHqcf_SZGzZMg?feat=directlink">https://picasaweb.google.com/lh/photo/DP8vsfafDDHqcf_SZGzZMg?feat=directlink</a>).<br>
But according to latency table for various memory accesses by Intel (see <a
href="https://picasaweb.google.com/lh/photo/j-m4PkXxumRCoCQy3jmAuQ?feat=directlink">this
table</a>: <a
href="https://picasaweb.google.com/lh/photo/j-m4PkXxumRCoCQy3jmAuQ?feat=directlink">https://picasaweb.google.com/lh/photo/j-m4PkXxumRCoCQy3jmAuQ?feat=directlink</a>),
the minimum latency (when accessing the local MPB with bypass) is measured in
~100 clocks, not microseconds.<br>
What is the reason for such a wide gap in RCCE implementation and hardware
latency table?<br>
I guess all kinds of memory access latency measurements is a must-know stuff
for porting Barrelfish to SCC...<br>
Thanks, KostaZ.<o:p></o:p></p>
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