Re: rt20 scheduling latency testcase and failure data

Sébastien Dugué
2006-05-15T11:11:31Z

On Fri, 2006-05-12 at 19:24 -0700, Darren Hart wrote:
> I have been noticing unexpected intermittant large latencies. I wrote the
> attached test case to try and capture some information on them. The librt.h
> file contains convenience functions I use for writing other tests as well, so
> much of it is irrelevant, but the test case itself is pretty clear I believe.
>
> The test case emulates a periodic thread that wakes up on time%PERIOD=0, so
> rather than sleeping the same amount of time each round, it checks now
> against the start of its next period and sleeps for that length of time.
> Every so often it will miss it's period, I've captured that data and included
> a few of the interesting bits below. The results are from a run with a
> period of 5ms, although I have seen them with periods as high as 17ms. The
> system was under heavy network load for some of the time, but not all.
>

Hi Darren,

FWIW:

I've been running you test program on my box under a stress-kernel
load and did not observe any failure as you describe, not even a max
latency overshooting the 100 us limit (max latencies in the 60~70 us).

I even went to decrease PERIOD to 1 ms and still no failure.

I'm running rt20 with the futex priority based wakeup patch on
a dual 2.8 GHz HT Xeon box. All hardirq and softirq threads are at their
default priority.

How do you generate the network load you mention? Maybe I could try at
least with the same load you're using.

Sébastien.