Re: Strange problem with PREEMPT_RT

[Punit Agrawal <punitagrawal@gmail.com>]

Pierre FICHEUX <pierre.ficheux@smile.fr> writes:

> Hi,
>
> I have a strange problem on a PREEMPT_RT system.
>
> I have a process with 2 threads,
>
> - 1 TR thread (10 ms period) which places 350 KB blocks in a fifo (1
> block every 10 ms).
> - 1 non-TR thread (SCHED_OTHER) which reads the block in the fifo and
> writes it to the disk
>
> If I run this on a powerful machine (HP Z4-i9, 14 cores, NVME disk,
> CentOS 7 with 3.10 PREEMPT_RT kernel, yes that's ooold), the max
> jitter WITH hackbench remains around 20 to 30 µs while the max jitter
> WITHOUT hackbench goes up to 350 µs!
>
> -> hackbench -p -g 20 -l 10000000
>
> Running the program with  taskset 01 doesn't change anything
> If I don't write the data to disk it doesn't change anything either.
>
> The important jitters appear rather at the beginning (but sometimes also later).
>
> Any ideas ?

Is power management (cpuidle, cpufreq) enabled on the system?

One possible explanation -

The load from the 10ms task, isn't high enough to keep the system at
high-frequencies or prevent it from going into deeper sleep states. Both
of these can impact latencies.

With hackbench, the system is sufficiently busy to avoid the going into
idle.

>
>
> thanks by advance
>
> --
> Pierre