linux-kernel - Re: [PATCH] sched-rt: Reduce excessive task push rate by not pushing tasks with equal priority as the current task

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Message-ID: <20150122235048.5246a611@grimm.local.home>
Date:	Thu, 22 Jan 2015 23:50:48 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Mike Galbraith <umgwanakikbuti@...il.com>
Cc:	Tim Chen <tim.c.chen@...ux.intel.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Andi Kleen <andi@...stfloor.org>, Ingo Molnar <mingo@...e.hu>,
	Shawn Bohrer <sbohrer@...advisors.com>,
	Suruchi Kadu <suruchi.a.kadu@...el.com>,
	Doug Nelson <doug.nelson@...el.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched-rt: Reduce excessive task push rate by not
 pushing tasks with equal priority as the current task

On Fri, 23 Jan 2015 05:29:30 +0100
Mike Galbraith <umgwanakikbuti@...il.com> wrote:

> On Thu, 2015-01-22 at 10:53 -0800, Tim Chen wrote: 
> > Commit 3be209a8 tries to migrate task of equal priority as the
> > running one to other cpus to balance load and eliminate any idle
> > cpus.  However, for system that is fully busy and running workload
> > of a few priorities, we found this change to cause tasks getting
> > pushed around without improving cpu utilization. On a fully loaded
> > system running a well known OLTP benchmark, it causes 70% more run
> > queue locking in the push task path without improving cpu
> > utilization and make throughput degrade by 1.5%. We observe much
> > higher rq lock contention due to excessive lockings of target run
> > queues on task wakeup.
> 
> Pushing tasks of equal priority is about getting rt tasks to a CPU
> they can utilize NOW.  Trying to improve throughput and whatnot is
> all well and good, but sacrificing the most sacred rt cow on the
> planet to improve some benchmark number is a very bad idea :)
> 

What Mike said.

With RT tasks the #1 importance is running as soon as they should run.
Throughput is #2. We never sacrifice #1 to improve #2.

Non RT tasks have much better throughput algorithms than RT tasks. But
they are much less likely to react to an event consistently as an RT
task will.

Never give a task an RT priority if your main objective is to have it
"run faster". If you do, you have no clue about what RT is used for.

-- Steve
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