linux-kernel - Re: Real-time scheduling policies and hyper-threading

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Message-ID: <20140424202443.GE26782@laptop.programming.kicks-ass.net>
Date:	Thu, 24 Apr 2014 22:24:43 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Kirill Tkhai <tkhai@...dex.ru>
Cc:	Roman Gushchin <klamm@...dex-team.ru>,
	LKML <linux-kernel@...r.kernel.org>,
	"mingo@...hat.com" <mingo@...hat.com>,
	Artyom Tarasenko <atar4qemu@...il.com>
Subject: Re: Real-time scheduling policies and hyper-threading

On Fri, Apr 25, 2014 at 12:16:52AM +0400, Kirill Tkhai wrote:
> 24.04.2014, 22:59, "Peter Zijlstra" <peterz@...radead.org>:
> 
> [snip]
> 
> >>  Does anyone use rt-scheduler for runtime-like cpu-bound tasks?
> >
> > So in general cpu bound tasks in the RT classes (FIFO/RR/DEADLINE) are
> > bad and can make the system go funny.
> >
> > For general system health it is important that various system tasks
> > (kthreads usually) can run. Many of these kthreads run at !rt prios, and
> > by having cpu bound tasks in rt prios they don't get to run.
> 
> One more word to this. I had such expirience on 2.6.33 kernel with RT patch
> and weak hardware (sparc32).
> 
> Networking was actively used and application did not use any IO operations.
> 
> User needs to set all RT priorities by himself. It's necessary to set RT
> priorities at least for softirqs and rcus. RT bandwidth must be switched
> off.
> 
> The most giving optimization, which I receive, was after rejection from NAPI
> for network adapters and splitting interrupt handler on hard and threadparts.
> In this case game with binding for everything strongly improves the picture
> for single problem.

Sure, it can be made to work, but you really need to know what you're
doing and you get to keep all pieces when it comes apart :-)
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