[uml-devel] Kernel 2.6.13-rc2 host kernel configuration

[uml-devel] Kernel 2.6.13-rc2 host kernel configuration

[Anthony Brock]

I'm about to build a 2.6.13-rc2 kernel for testing as a host kernel.
I've patched it with the '-mm1' patches and the 'skas-2.6.12-v8.2'
patch. The later applied with a minor tweak to 'mm/Makefile'.

However, I'm now trying to build a configuration for the newer kernel.
Unfortunately, I'm not certain how to best answer some of the questions
for a machine whose primary purpose is to host UML instances.
Specifically, any idea concerning the "Timer frequency" option? Here is
the help provided during configuration:


Timer frequency
  1. 100 HZ (HZ_100) (NEW)
> 2. 250 HZ (HZ_250) (NEW)
  3. 1000 HZ (HZ_1000) (NEW)
choice[1-3?]: ?

Allows the configuration of the timer frequency. It is customary
to have the timer interrupt run at 1000 HZ but 100 HZ may be more
beneficial for servers and NUMA systems that do not need to have
a fast response for user interaction and that may experience bus
contention and cacheline bounces as a result of timer interrupts.
Note that the timer interrupt occurs on each processor in an SMP
environment leading to NR_CPUS * HZ number of timer interrupts
per second.


Are timer interrupts used by a UML instance? If so, should I select
"1000 HZ"? Or would "100 HZ" free up system resources and allow the
instances to run with less contention?

Thanks!

Tony



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Re: [uml-devel] Kernel 2.6.13-rc2 host kernel configuration

[Blaisorblade]

On Tuesday 12 July 2005 02:25, Anthony Brock wrote:
> I'm about to build a 2.6.13-rc2 kernel for testing as a host kernel.
> I've patched it with the '-mm1' patches and the 'skas-2.6.12-v8.2'
> patch. The later applied with a minor tweak to 'mm/Makefile'.
>
> However, I'm now trying to build a configuration for the newer kernel.
> Unfortunately, I'm not certain how to best answer some of the questions
> for a machine whose primary purpose is to host UML instances.
> Specifically, any idea concerning the "Timer frequency" option? Here is
> the help provided during configuration:
>
> Timer frequency
>   1. 100 HZ (HZ_100) (NEW)
>
> > 2. 250 HZ (HZ_250) (NEW)
>
>   3. 1000 HZ (HZ_1000) (NEW)
> choice[1-3?]: ?

>
>
> Are timer interrupts used by a UML instance? If so, should I select
> "1000 HZ"? Or would "100 HZ" free up system resources and allow the
> instances to run with less contention?
The question is difficult to answer, and somebody should run some benchmarks. 
I think I'll have the time to answer properly ASAP.

However, what I see is that UML uses heavily signals, and increasing HZ speeds 
up signal delivery.

So, using 1000 (which is what previous kernels used) is IMHO the best option.

Don't know if the downsides of HZ=1000 would have bigger impact, but I still 
think that's the best choice.
-- 
Inform me of my mistakes, so I can keep imitating Homer Simpson's "Doh!".
Paolo Giarrusso, aka Blaisorblade (Skype ID "PaoloGiarrusso", ICQ 215621894)
http://www.user-mode-linux.org/~blaisorblade

		
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