Hi all,
I have noted a minor anomaly with jitter when more than one periodic RT task
is scheduled and running.
If I schedule one periodic RT task to execute every 50msec, I can place
heavy loads on the system and observe average jitter in the 2 to 5usec range
with rare peak jitter in the 17.6usec range (very impressive and well within
spec).
However...
If I schedule a second periodic RT task at 50msec (but with a start time
5msec behind the first RT task), and at equal priority, it's average and
peak jitter figures are slightly worse. Average jitter in the range of 5 to
12usec and rare peak jitter in the 30usec range. During all of this the
first periodic RT task (the first one on the task list) continues to show 2
to 5usec average and 17.6 peak jitter. I have tried this on two different
motherboards, one with a P2-350 and one with a K6-330 (overclocked). Both
give similar results (the K6-330 is only marginally slower). I am measuring
jitter by sampling rdtsc at the start of each iteration.
Maybe this has already been addressed and discussed, and I have missed it.
If so I apologize for rehashing "old news".
I guess this isn't really a world-shattering anomaly. Just a bit curious.
Anybody have any ideas?
Thanks
Todd Geaheart
P.S. Just for reference: { This 50msec task is a modular exec, which is
scheduling roughly 70 smaller real-time modules. A relatively large RT task
- it occupies over 100 pages in memory as reported by lsmod. It requires
roughly 1msec per pass to execute.
My definition of a "heavy load" consists of 5 other terminals simultaneously
logged on, 4 of which are performing continuous compiles using gcc, with the
fifth performing repeated greps on the entire hard drive. The first
available terminal (for a total of six simultaneously running) is running
our real-time debugger which is showing as many as 72 different variable
calculations, module cycle times, as well as minimum, maximum and average
jitter all updated at a 20hz rate. }
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