ARM cortex A9 performance issue

[Dave Martin]

On Mon, Jul 11, 2011 at 01:56:35PM +0530, rd bairva wrote:
> Attaching the new code.
> 
> On Fri, Jul 8, 2011 at 7:03 PM, Dave Martin <dave.martin at linaro.org> wrote:
> > On Fri, Jul 08, 2011 at 05:08:38PM +0530, rd bairva wrote:
> >> Now i have modified a source a little bit. Now I am doing a pingpong
> >> using msgsnd and msgrcv. using this I am getting 40000req/sec and 55%
> >> CPU usage.
> >> In another version I have taken the same lock in both the processes to
> >> ensure same thread is unlocking the mutex. But CPU usage is 100%.
> >> Shouldn't be the behaviour 50%.
> >
> > Possibly.  Do you see that behaviour on all platforms, or just A9?
> On single processor it is always 100% on ARM as well as X86. but on
> X86, cpu is >50% free.

For me, on x86:

With WITH_MSG, the test consumes about 56-57% on two CPUs.
Without WITH_MSG, the test consumes about 59-60% on two CPUs.

(as reported by running top during the test)

For me, this is as expected: there is some latency involved in signalling
between CPUs, so there should be a bit less than 50% of useful work per
CPU.  However, there is overhead in both process, and this can parallelise
against the signalling latency, giving an overall load of a bit more than
50% per CPU.  The total is about the same on each CPU, since both processes
are doing essentially the same thing.

I quickly hacked up a completely different implementation using pthreads
condition variables to signal between threads and got similar results (though
with a bit more CPU load).

Someone borrowed by pandaboard, but I'll try on there when I get a chance...

Cheers
---Dave

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