linux-kernel - Re: The 10ms averager in fair.c

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Date:	Sun, 30 Sep 2012 21:18:57 +0200
From:	Uwaysi Bin Kareem <uwaysi.bin.kareem@...adoxuncreated.com>
To:	linux-kernel@...r.kernel.org
Subject: Re: The 10ms averager in fair.c

Just to illustrate, you have a filter that lasts 10ms, and a cpu process  
that lasts 100uS

Original spike

5 |
4 |
3 |
2 |
1 |
0 |
0ms_______________________10ms
Filtered spike

5
4
3
2
1   .....................
0..                      ..
0ms________________________10ms

Not only is the filtered spike, much lower, but it lasts long beyond the  
100uS spike. (10ms). Why would that be used in something that should  
represent cpu-usage?

Peace Be With You.


On Sun, 30 Sep 2012 13:44:14 +0200, Uwaysi Bin Kareem  
<uwaysi.bin.kareem@...adoxuncreated.com> wrote:

> Hiya. I just had an initial look at fair.c
>
> There seems to be a 10ms averager in there?
>
> You are aware that that means you work on delayed values?
>
> Isn`t that counterintuitive to the principle of sharing?
>
> That means short bursts of cpu-use will be filtered out, and given less  
> cpu time.
> Starting applications won`t have their cpu-usage before 5ms, which is  
> quite a bit on modern machines. Well if you use a linearphase filter, I  
> don`t know what kind of averager you use. The best would ofcourse be to  
> use a minimalphase gaussian averager. Which might be overkill. Atleast a  
> one-pole iir, buf = buf + (-buf + in) * cut)); One pole IIRs also have a  
> better frequency response.
>
> When you are working with low-latencies, wouldn`t it be better if such  
> things are tuned for target latency. I think few care about latency  
> after 0.2ms. So say the filter should be set to 0.4ms max.
>
> Why would you want to filter cpu-usage also really?
>
> Peace Be With You.
>
> (please CC me.)
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