[Myrinet] weird latency

Hi all,

I am measuring the latency between two hosts with Myrinet adapters and
have some curious results(*).  I am using GM 1.5.1 on 33MHz 32bit
machines and am getting 65-70 us round-trip for 1000 byte packets using
a ping utility I wrote(**).  There are >70 us outliers, which I expect,
but I also see <65 us outliers, clustered together in predictable
intervals (~13 ms).  What could be causing these?  I am running Linux
2.4.18 and have scheduled the process with the soft real-time scheduler.
I am measuring times by reading the cycle counter available on Pentium
and newer processors.

Also, I am working on a project that requires using Myrinet from
RTLinux; is has anyone done something similar?  I have read a few
papers(***) written by Anthony Skjellum and his group, although their
code may not be available.

(*)   http://gaul.org/images/myri.png
(**)  http://gaul.org/files/myri-ping.c
(***) "A Synchronized Real-Time Linux Based Myrinet Cluster for
      Deterministic High Performance Computer and MPI/RT" by Apte,
      Chakravarthi, Padmanabhan, and Skjellum

Thank you for you time,

-- 
Andrew Gaul
[email protected]