[PATCH 2/2] gta02-drm-defconfig: enable FCSE and PREEMPT

[Gennady Kupava]

Hi, Martix,

В Чтв, 02/09/2010 в 01:09 +0200, Martix пишет:
> Hi,
> 
> I have problem with missing option for ar when running lmbench on SHR.
> I can try it with Qt Moko, but I'm leaving for two weeks and I don't
> know whether I'll have enough time to do it before I return.
> 
> Yes, X11 performance is mostly about Glamo, but X11 is also protocol
> which consumes CPU and can be affected by overall system latency, when
> running multiple applications. I am looking around at benchmark, which
> could compare real application use cases.

Would be also good to measure 

> 
> My only case is low latency mobile phone/PDA.

Question is what is your definition of 'low latency'. I think that
PREEMPT should be enabled only in case of:
1. _problems_ with latency. Do we have any?
2. overall feeling. do you really can notice defference between kernels
with and without PREEMPT while working with you favorite distribution?

> 
> May I ask what is your reason for keeping PREEMPT disabled? I thought
> the reason was too expensive (time consuming) context switching.
> PREEMPT causes more frequent context switches, which was expensive
> without FCSE, but situation has changed. Now with FCSE we need less
> time for context switch and we don't need flush cache at every context
> switch. So, we can use low latency advantages.

Keeping PREEMPT on complicates kernel, influencing code complexity and
thus affecting speed without any benefit. Preemption effectively makes
your kernel SMP kernel. While on modern high speed PCs this complication
is negligible, on our embedded device which is much slower good idea to
take this into account. Also, we have 200Hz timer, which provide far
more than enought context switches. Also, using FCSE patch at it current
form DO NOT fixes problem for _all_ applications, only fixing it for
small subset - for limited amount of applications not exceeding some
limited memory size. So, switching to and from your 'big' main thing
like browser or almost anything with big gui still costs like before.

> 
> I would like to see also benchmark numbers instead of feeling or user
> experience. I found interbench. [1] I'll use it to provide some basic
> result, but it would be nice to have proper EFL/FSO/D-Bus benchmark
> for testing and comparing our Neo FreeRunner use cases on multiple SW
> configurations.
> 
> [1] http://users.on.net/~ckolivas/interbench/

Thinking a bit more on topic, i realized that main thing really is
existace of problem or noticeable difference in everyday use. if not, no
reason to enable it.

Regards, 

Gennady