[Barrelfish-users] Intel SCC latency measurements for MPB operations

[Haas, Werner]

Konstantin,

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.

Please also note that the bypass mode should _not_ 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.

Best regards,
Werner


From: Konstantin Zertsekel [mailto:zertsekel at gmail.com]
Sent: Tuesday, March 01, 2011 4:32 PM
To: barrelfish-users at lists.inf.ethz.ch
Cc: Dan Tsafrir; Konstantin Zertsekel; Roei; Ido Shamay; Avi Mendelson; Prof. Assaf Schuster
Subject: [Barrelfish-users] Intel SCC latency measurements for MPB operations

Hi all,
I am engaged in the project with Intel SCC chip where communication latency is important factor.
Now, according to RCCE inter-core Ping-Pong test, the minimum latency is 5 microseconds (see this graph<https://picasaweb.google.com/lh/photo/DP8vsfafDDHqcf_SZGzZMg?feat=directlink>: https://picasaweb.google.com/lh/photo/DP8vsfafDDHqcf_SZGzZMg?feat=directlink).
But according to latency table for various memory accesses by Intel (see this table<https://picasaweb.google.com/lh/photo/j-m4PkXxumRCoCQy3jmAuQ?feat=directlink>: https://picasaweb.google.com/lh/photo/j-m4PkXxumRCoCQy3jmAuQ?feat=directlink), the minimum latency (when accessing the local MPB with bypass) is measured in ~100 clocks, not microseconds.
What is the reason for such a wide gap in RCCE implementation and hardware latency table?
I guess all kinds of memory access latency measurements is a must-know stuff for porting Barrelfish to SCC...
Thanks, KostaZ.
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