HZ value change groundings (Was: jitterless touchscreen input in .34)

Gennady Kupava gb at bsdmn.com
Sun Oct 24 20:10:00 CEST 2010


В Вск, 24/10/2010 в 19:34 +0200, Radek Polak пишет:
> You wrote: > В Чтв, 21/10/2010 в 22:04 +0200, Radek Polak пишет:
> 
> > And now, as this conceptual hack put me so high to hall of fame, i
> > consider i can cast 3 simple wishes for qtmoko ;) :
> > 
> > 1. i wish compiled-in guid partition table type into kernel;
> 
> I can try if i find correct config options. If not i'll ask :)

CONFIG_EFI_PARTITION, thanks :)

> 
> > 2. i wish xfs module to kernel (as ubifs is comressed fs, it should not
> > eat too much space);
> 
> As module or compiled in?

Module is ok. It's a bit too large to compile in for everyone due to our
128Mb of ram size.

> 
> > 3. i wish 100HZ patch included to qtmoko
> > (http://www.bsdmn.com/openmoko/kernel/0001-Use-100-as-HZ-value-on-S3C24XX.p
> > atch)
> 
> Oki, i can try, is there any description for the patch? E.g. What does it do 
> and how can i test it?

It does simple thing - change HZ value of linux kernel from 200HZ to
100HZ. Main reason is performance.

My rationale is following:
Basically this mean that timer interrupts amount will be reduced from
200 per second to 100 per second. This allows:

Pros:
1. Stay more in IDLE state. (less power consumption)
2. If several tasks are active, do 2 times less context switches (and
our context switches are expensive). Do not think it will be 'slow'.
This is not applied to all tasks - but only to CPU bound ones.
3. Less interrupts is good on it's own.

Cons:
1. Worse granilarity of delays and sheduled works. I think it's
impossible
2. May reveal some bugs, as any new non-trivial system-wide change.
Basically it works here, but i didn't test everything.

Other arguments:
I checked other boards config files and it turned out that only few of
them (10% or so) using 200 as HZ value.

I did lmbenching and found that difference is few percent, but still
noticeable.

I can't 'feel' that something changed.

So, this patch is just sets new value for HZ and changes s3c timer
interrupt frequency.

Big thanks to Vasily 'anarsoul' Khoruzhick for patch written on my
request.

Regards, 
Gennady




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