Realtime linux kernel on multicore processor

Realtime linux kernel on multicore processor

[Neel Mehta]

Hi. I have 2 computers with ubuntu:
1. Core 2 Duo - Non realtime linux kernel
2. Quad core - Realtime linux kernel

Both of them are using ubuntu. I am running a process that generates 2
threads, both of which are continuously processing something.
1. On core 2 duo (normal kernel): both the cores are running above 90% load.
2. on quad core(realtime kernel): only one core runs above 90%, and
the other cores are not at such a high load. The core which is on high
load keeps on alternating among the 4 cores.

So, my question is why doesnt 2 cores run on high load on quad core?
Is there a problem in multiprocessing in realtime linux kernel? Can
someone guide me where the source of problem could be?

Thanks and Regards,
Neel Mehta
Third year Undergraduate student,
Electrical Engineering,
IIT Bombay

Re: Realtime linux kernel on multicore processor

[Nivedita Singhvi]

Neel Mehta wrote:
> Hi. I have 2 computers with ubuntu:
> 1. Core 2 Duo - Non realtime linux kernel
> 2. Quad core - Realtime linux kernel
> 
> Both of them are using ubuntu. I am running a process that generates 2
> threads, both of which are continuously processing something.
> 1. On core 2 duo (normal kernel): both the cores are running above 90% load.
> 2. on quad core(realtime kernel): only one core runs above 90%, and
> the other cores are not at such a high load. The core which is on high
> load keeps on alternating among the 4 cores.
> 
> So, my question is why doesnt 2 cores run on high load on quad core?
> Is there a problem in multiprocessing in realtime linux kernel? Can
> someone guide me where the source of problem could be?

I'll start with the obvious - if the same amount of work is
being done, but now on a larger number of cores, the load on
each individual core could reduce (since you don't say what
else is contributing to the load on the system, etc).

As an example of this class of behavior, look at your irq distribution.
The rotation of the load sounds a lot like it's being driven by
interrupt round-robining, for instance.

thanks,
Nivedita

Re: Realtime linux kernel on multicore processor

[Neel Mehta]

Since, Ubuntu is being run on the kernel, the standard processes of
Ubuntu are running in the background.

Now, I was just checking out RT_PREEMPT yesterday, and I came across
the kernel 2.6.31-9-rt (It is an RT_PREEMPT kernel) in the ubuntu
repositories (for ubuntu 9.10). I installed the kernel, and booted
into ubuntu using this kernel. Now, I see the results as I had
expected - 2 out of the 4 cores running on full load. What could be
the problem in my configuration as I am unable to see the same results
with the kernel that I configured?

Thanks and Regards,
Neel Mehta
Third year Undergraduate student,
Electrical Engineering,
IIT Bombay


On Thu, Jun 17, 2010 at 10:16 PM, Nivedita Singhvi <niv@us.ibm.com> wrote:
>
> Neel Mehta wrote:
>>
>> Hi. I have 2 computers with ubuntu:
>> 1. Core 2 Duo - Non realtime linux kernel
>> 2. Quad core - Realtime linux kernel
>>
>> Both of them are using ubuntu. I am running a process that generates 2
>> threads, both of which are continuously processing something.
>> 1. On core 2 duo (normal kernel): both the cores are running above 90% load.
>> 2. on quad core(realtime kernel): only one core runs above 90%, and
>> the other cores are not at such a high load. The core which is on high
>> load keeps on alternating among the 4 cores.
>>
>> So, my question is why doesnt 2 cores run on high load on quad core?
>> Is there a problem in multiprocessing in realtime linux kernel? Can
>> someone guide me where the source of problem could be?
>
> I'll start with the obvious - if the same amount of work is
> being done, but now on a larger number of cores, the load on
> each individual core could reduce (since you don't say what
> else is contributing to the load on the system, etc).
>
> As an example of this class of behavior, look at your irq distribution.
> The rotation of the load sounds a lot like it's being driven by
> interrupt round-robining, for instance.
>
> thanks,
> Nivedita

Re: Realtime linux kernel on multicore processor

[Alessio Igor Bogani]

Hi,

2010/6/23 Neel Mehta <neelmehta89@gmail.com>:
[...]
> the kernel 2.6.31-9-rt (It is an RT_PREEMPT kernel) in the ubuntu
> repositories (for ubuntu 9.10). I installed the kernel, and booted
[...]

I suggest you to update your Ubuntu installation or try with a more
recent kernel.
In Lucid 10.04 there is 2.6.31-11-rt and (at the moment) it is in
-proposed archive: https://launchpad.net/ubuntu/+source/linux-rt
Otherwise you can install the latest kernel at my PPA:
https://launchpad.net/~abogani/+archive/ppa

Hope this helps.

Ciao,
Alessio