Re: Strange behavior of pthread_setaffinity_np

[Sujit K M <sjt.kar@gmail.com>]

All these are best guesses.

On Mon, Apr 19, 2010 at 3:15 PM, Primiano Tucci <p.tucci@gmail.com> wrote:
> Hi all,
> I am an Italian researcher and I am working on a Real Time scheduling
> infrastructure. I am currently using Linux Kernel 2.6.29.6-rt24-smp
> (PREEMPT-RT Patch) running on a Intel Q9550 CPU.
> I am experiencing strange behaviors with the pthread_setaffinity_np API.
>
> This is my scenario, I have 4 Real Time Threads (SCHED_FIFO)
> distributed as follows:
>
> T0 : CPU 0, Priority 2 (HIGH)
> T1 : CPU 1, Priority 2 (HIGH)
> T3 : CPU 0, Priority 1 (LOW)
> T4 : CPU 1, Priority 1 (LOW)

Could you check with the manual whether the following documentation
specifies your Processor.
http://www.intel.com/design/core2quad/documentation.htm

The reason I am asking is that what ever you are stating above in
terms of thread affinity would not
even qualify as an Core2duo.

>
> So T0 and T1 are actually the "big bosses" on CPUs #0 and #1, T3 and
> T4, instead, never execute (let's assume that each thread is a simple
> busy wait that never sleeps/yields)
> Now, at a certain point, from T0 code, I want to migrate T4 from CPU
> #1 to #0, keeping its low priority.
> Therefore I perform a pthread_setaffinity_np from T0 changing T4 mask
> from CPU #1 to #0.

This approach is not at all correct as the thread affinity should be
closer to the core than the processor.
If this is supported.

>
> In this scenario it happens that T3 (that should never execute since
> there is T0 with higher priority currently running on the same CPU #0)
> "emerge" and executes for a bit.
> It seems that the pthread_setaffinity_np syscall is somehow
> "suspensive" for the time needed to migrate T4 and let the scheduler
> to execute T3 for that bunch of time.

I think what is happening is that Once you have scheduled the code on
processor basis, It tends to ignore
core logic, but depends more on the processor logic.

>
> Is this behavior expected (I did not find any documentation about
> this)? How can avoid it?

I think you will have to set the affinity to core level than at processor level.

>
> Thanks in advance,
> Primiano
>
> --
>  Primiano Tucci
>  http://www.primianotucci.com
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-- 
-- Sujit K M

blog(http://kmsujit.blogspot.com/)
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