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

From: Arjan van de Ven <arjan@linux.intel.com>
To: Jacob Pan <jacob.jun.pan@linux.intel.com>,
	Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
	LKML <linux-kernel@vger.kernel.org>, Paul Turner <pjt@google.com>,
	Len Brown <len.brown@intel.com>,
	Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>,
	Tim Chen <tim.c.chen@linux.intel.com>,
	Andi Kleen <andi.kleen@intel.com>,
	Rafael Wysocki <rafael.j.wysocki@intel.com>
Subject: Re: [RFC PATCH 3/3] sched: introduce synchronized idle injection
Date: Thu, 5 Nov 2015 08:06:16 -0800	[thread overview]
Message-ID: <563B7E78.1010209@linux.intel.com> (raw)
In-Reply-To: <20151105073242.0a67f011@icelake>

On 11/5/2015 7:32 AM, Jacob Pan wrote:
> On Thu, 5 Nov 2015 15:33:32 +0100
> Peter Zijlstra <peterz@infradead.org> wrote:
>
>> 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.
>>>
> Just to clarify, the low frequency here is not necessarily the minimum
> frequency. It is usually the Pe (max efficiency).

to translate that from Intelese to English:
The system already is at the lowest frequency that's relatively efficient. To go even lower in instant power
consumption (e.g. heat) by even a little bit, a LOT of frequency needs to be sacrificed.

Idle injection sucks. But it's more efficient (at the point that it would get used) than any other methods,
so it also sucks less than those other methods for the same amount of reduction in heat generation.
It only gets used if the system HAS to reduce the heat generation, either because it's a mobile device with
little cooling capacity, or because the airconditioning in your big datacenter is currently
not able to keep up.