[openib-general] some questions: sdp/perftest/hyperthreading

[Grant Grundler]

On Tue, Sep 20, 2005 at 08:28:11PM +0200, umaxx wrote:
...
> 2. Is it better to test performance with disabled Hyperthreading?

In general, yes.

> perftest/rdma_lat is using get_cycles(). get_cycles() uses internal
> the rdtscll() function. Correct me if I am wrong, but this could be a 
> problem with a hyperthreading enabled cpu? Because it's not clear which 
> cpu(-core) is used by rdtsll() at which time? The time stamp counters 
> between processors are not synchronized, and therefore the timing 
> measured will be incorrect.

I don't know if this is a problem in practice. It potentually could be.

> Maybe something like cpuid() is missing - to set/get the wanted?

I suggest using taskset(1) to bind a test program to a logical CPU
if it really is a problem. The tests have fairly short run times
and it's not worth making them more complex to deal with this.

Someone else might take a shot at answering the other two questions.

hth,
grant