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From: Marcelo Tosatti <mtosatti@redhat.com>
To: Bruce Richardson <bruce.richardson@intel.com>
Cc: Daniel Bristot de Oliveira <bristot@redhat.com>,
	"Ananyev, Konstantin" <konstantin.ananyev@intel.com>,
	Adrien Mazarguil <adrien.mazarguil@6wind.com>,
	"dev@dpdk.org" <dev@dpdk.org>,
	Luiz Capitulino <lcapitulino@redhat.com>
Subject: Re: [dpdk-dev] [PATCH] testpmd: add nanosleep in main loop
Date: Sat, 11 Nov 2017 01:54:14 -0200	[thread overview]
Message-ID: <20171111035414.GC23577@amt.cnet> (raw)
In-Reply-To: <20171110111451.GA9164@bricha3-MOBL3.ger.corp.intel.com>

On Fri, Nov 10, 2017 at 11:14:51AM +0000, Bruce Richardson wrote:
> 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.

Bruce,

Its just as an example for application developers, in the official DPDK
repository. And its disabled by default so it does not affect the
performance numbers.

That said, you have a problem integrating the patch?

> 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

Yes but unfortunately "sched_yield()" does not return in a timely
fashion, we would need a new "sched_yield_time(X us)" with a 
guaranteed return from the call in a maximum of X us.

(Note the values of nanosleep_frequency=100Hz, nanosleep_length=10us, 
does increase the latency values a bit but still maintains acceptable
latency). The arguments about performance must be considered
against this results.




Yes Bruce sched_yield() would be mkore

  parent reply	other threads:[~2017-11-11 16:14 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-11-10  6:02 Marcelo Tosatti
2017-11-10  9:12 ` Adrien Mazarguil
2017-11-10 10:13   ` Daniel Bristot de Oliveira
2017-11-10 10:14   ` Ananyev, Konstantin
2017-11-10 10:42     ` Daniel Bristot de Oliveira
2017-11-10 11:14       ` Bruce Richardson
2017-11-10 13:51         ` Luiz Capitulino
2017-11-11  3:59           ` Marcelo Tosatti
2017-11-11  4:01             ` Marcelo Tosatti
2017-11-11  3:54         ` Marcelo Tosatti [this message]
2017-11-11  3:49     ` Marcelo Tosatti
2017-11-12 23:14       ` Ananyev, Konstantin
2017-11-13 18:01         ` Marcelo Tosatti
2017-11-11  3:45   ` Marcelo Tosatti

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