linux-kernel - Re: [RFC PATCH 3/3] sched: introduce synchronized idle injection

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Message-ID: <20151105143332.GQ17308@twins.programming.kicks-ass.net>
Date:	Thu, 5 Nov 2015 15:33:32 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Arjan van de Ven <arjan@...ux.intel.com>
Cc:	Jacob Pan <jacob.jun.pan@...ux.intel.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Paul Turner <pjt@...gle.com>, Len Brown <len.brown@...el.com>,
	Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Andi Kleen <andi.kleen@...el.com>,
	Rafael Wysocki <rafael.j.wysocki@...el.com>
Subject: Re: [RFC PATCH 3/3] sched: introduce synchronized idle injection

On Thu, Nov 05, 2015 at 06:22:58AM -0800, Arjan van de Ven wrote:
> On 11/5/2015 2:09 AM, Peter Zijlstra wrote:
> 
> >I can see such a scheme having a fairly big impact on latency, esp. with
> >forced idleness such as this. That's not going to be popular for many
> >workloads.
> 
> idle injection is a last ditch effort in thermal management, before
> this gets used the hardware already has clamped you to a low frequency,
> reduced memory speeds, probably dimmed your screen etc etc.
> 
> at this point there are 3 choices
> 1) Shut off the device
> 2) do uncoordinated idle injection for 40% of the time
> 3) do coordinated idle injection for 5% of the time
> 
> as much as force injecting idle in a synchronized way sucks, the alternatives are worse.

OK, it wasn't put that way. I figured it was a way to use less power on
any workload with idle time on.

That said; what kind of devices are we talking about here; mobile with
pittyful heat dissipation? Surely a well designed server or desktop
class system should never get into this situation in the first place.
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