Re: [dpdk-dev] [PATCH] testpmd: add nanosleep in main loop

[Bruce Richardson <bruce.richardson@intel.com>]

On Fri, Nov 10, 2017 at 11:42:56AM +0100, Daniel Bristot de Oliveira wrote:
> 
> 
> On 11/10/2017 11:14 AM, Ananyev, Konstantin wrote:
> > Agree with Adrian here - the patch doesn't fix the problem in any case,
> 
> I would agree with you if it were possible to assume one can fully
> isolate a CPU on Linux... but it is not...
> 
> This:
> https://lwn.net/Articles/659490/
> 
> is still an open issue, and the reason why it is an open issue is the
> kernel threads that need to run on every CPU, mainly when using the
> PREEMPT_RT, which turns almost everything on threads.
> 
> > while introducing an unnecessary slowdown in testpmd iofwd mode.
> > Please think up some other approach.
> 
> The other approach is to increase the priority of all other threads that
> run on the isolate CPU. But that is not a good idea at all, as the other
> threads might preempt the busy-loop thread at the worst possible moment.
> 
> Using the knowledge of the thread about when it is the best time to give
> a chance for other threads to run would be a smarter decision.
> 
I don't like having this in the main loop either, and to echo others I
wouldn't have thought that testpmd was actually used as anything other
than a testing app. Also, I would have thought that running it at
realtime priority wouldn't be a good idea, because of exactly this
problem.

On the specifics of the solution, would using sched_yield() rather than
nanosleep not be more suitable, given that the reason for this sleep is
just to give the CPU to other threads?

/Bruce