linux-kernel - Re: [RFC v2 PATCH 0/8] CFS Hard limits

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Date:	Tue, 13 Oct 2009 16:45:15 +0400
From:	Pavel Emelyanov <xemul@...nvz.org>
To:	Dhaval Giani <dhaval@...ux.vnet.ibm.com>
CC:	Herbert Poetzl <herbert@...hfloor.at>, vatsa@...ibm.com,
	Bharata B Rao <bharata@...ux.vnet.ibm.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org,
	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
	Gautham R Shenoy <ego@...ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Avi Kivity <avi@...hat.com>,
	Chris Friesen <cfriesen@...tel.com>,
	Paul Menage <menage@...gle.com>,
	Mike Waychison <mikew@...gle.com>
Subject: Re: [RFC v2 PATCH 0/8] CFS Hard limits - v2

Dhaval Giani wrote:
> On Tue, Oct 13, 2009 at 04:19:41PM +0400, Pavel Emelyanov wrote:
>>> as I already stated, it seems perfectly fine for me 
>> You're not the only one interested in it, sorry. Besides, I
>> got your point in "I'm find with it". Now get mine which is
>> about "I am not".
>>
>>> can be trivially mapped to the two values, by chosing a
>>> fixed multiplicative base (let's say '1s' to simplify :) 
>>>
>>>   with 50%, you get 1s/0.5s
>>>   with 20%, you get 1s/0.2s
>>>   with  5%, you get 1s/0.05s
>>>
>>> well, you get the idea :)
>> No I don't.
>> Is 1s/0.5s worse or better than 2s/1s?
>> How should I make a choice?
> 
> I would say it depends on your requirement. How fast do you want to
> respond back to the user? Wiht lower bandwidth, you would want to have
> shorter periods so that the user would not get the impression that he
> has to "wait" to get CPU time. But having a very short period is not a
> good thing, since there are other considerations (such as the overhead of
> hard limits).

That's it - long period is bad for one reason, short period is bad for 
some other one and neither of them is clearly described unlike the 
limit itself.

In other words there are two numbers we're essentially playing with:
* the limit (int percents, Hz, whatever)
* and this abstract "badness"

Can't we give the user one of them for "must be configured" usage, put
the other one in some "good for most users" position and let the user
move it later on demand?

Yet again - weights in CFQ CPU-sched, ionoce in CFQ-iosched, bandwidth
in tc (traffic shaping), etc. are all clean for end-user. Plus there are
other fine tunes, that user should not configure by default, but which
change the default behavior. I propose to create simple and clean 
interface for limits as well. If you think that virtual cpu power is 
not good, ok. Let's ask user for a percentage and give him yet another
option to control this "badness" or "responsiveness".

> thanks,

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