Large interruptions for EAL thread running on isol core

[Carsten Andrich]

On 24.06.22 17:01, Stephen Hemminger wrote:
> On Thu, 23 Jun 2022 21:03:49 +0200
> Carsten Andrich <carsten.andrich at tu-ilmenau.de> wrote:
>
>>   2. Use real-time priority (SCHED_FIFO w/ priority 99) for the DPDK
>>      threads and
>>      echo -1 > /proc/sys/kernel/sched_rt_runtime_us
>>      to disable the runtime limit. With the runtime limit in place, the
>>      SCHED_FIFO performance will be significantly worse than SCHED_OTHER.
> This can cause major issues if application is normal DPDK application (never does system calls).
> If an interrupt or other event happens on your isolated CPU, the work that it would
> do in soft irq is never performed. FIFO has higher priority than kernel threads.
> This can lead to mystery lockups from other applications (reads not completing, network timeouts, etc).

Thanks for pointing that out. Do you know of any official kernel 
documentation that could shed some light on that? I haven't had any 
serious issues like the ones you list, but maybe I've been lucky. My 
DPDK applications typically run on fairly minimal systems used 
exclusively for DPDK tasks, which require minimal latency/jitter. Minor 
side-effects from using SCHED_FIFO are tolerable in my case, if it 
improves performance.


> In any semi-recent kernel using SCHED_NO_HZ_FULL will keep clock ticks from happening
> on the isolated cores.
On 24.06.22 18:42, Stephen Hemminger wrote:
> Sorry, wrong option.
>
> Full docs here:
> https://docs.kernel.org/timers/no_hz.html

Thanks. nohz_full is already in the list of kernel cmdline options I use.

Best regards,
Carsten