linux-kernel - Re: [PATCH 1/8] Documentation, x86: Documentation for Intel Mem b/w allocation user interface

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Date:   Mon, 23 Jan 2017 10:57:26 -0800 (PST)
From:   Shivappa Vikas <vikas.shivappa@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>
cc:     Shivappa Vikas <vikas.shivappa@...el.com>,
        Vikas Shivappa <vikas.shivappa@...ux.intel.com>,
        linux-kernel@...r.kernel.org, x86@...nel.org, hpa@...or.com,
        mingo@...nel.org, peterz@...radead.org, ravi.v.shankar@...el.com,
        tony.luck@...el.com, fenghua.yu@...el.com, h.peter.anvin@...el.com
Subject: Re: [PATCH 1/8] Documentation, x86: Documentation for Intel Mem b/w
 allocation user interface



On Wed, 18 Jan 2017, Thomas Gleixner wrote:

> On Tue, 17 Jan 2017, Shivappa Vikas wrote:
>> On Mon, 16 Jan 2017, Thomas Gleixner wrote:
>>> This interface is really crap. The natural way to express it is:
>>>
>>>     Requested Bandwidth = X %
>>
>> I wanted to do it this way which did seem more intuitive but the issue is with
>> the non-linear scale which the hardware does not guarantee a particular
>> percentage for a particular value. Or we don't know the curve for delay value
>> vs. actual b/w throttled.
>>
>> ex: in non linear scale , the granularity is 2^n.
>> Max : 512
>>
>> Say a value of 256 is not guaranteed to have 50% or even follow a curve where
>> we can calculate the corresponding percentage.
>
> The question is whether this non linear scale thing is just a first
> implementation attempt and any sane hardware in the future will use the
> percentage value (which is an approximation as well).
>
> If that non-linear scale is not going to be prevalent, then we really can
> live with the fallout of a particular CPU type.
>
> If it's going to stay, then Intel should be able to provide simple tables
> which give us the required information for a particular CPU model.

By sample table - does this mean we can map a throttle value in non-linear 
scale to its percentage ?

Thanks,
Vikas

>
> Thanks,
>
> 	tglx
>