Re: [LAD] How is the TSC calibration accuracy on dual core 2 computers? (And what about HPET?)

I don't know about jitter, but certainly a few years ago, you  
sometimes got stalls - eg. under heavy DMA load. That may not be an  
issue with modern CPUs and chipsets. I think I posted some code that  
demonstrated it to the l-a-d list at the time, but good luck finding  
it :)

The TSC is only required to be monotonic, so you wont get any  
guarantees, just practical knowledge of whether you can get away with  
it or not.

- Steve

On 28 Apr 2009, at 13:46, Kjetil S. Matheussen wrote:

>
> I'm doing some benchmarking where I need about 0.1ms accuracy.
> I'm using an intel dual core 2 computer. This is for a paper,
> so I just need the numbers, and the code is not going to run
> on any other computer.
>
> I've looked at the HPET code in jack, but am unsure how accurate it  
> is,
> and whether there are any overhead using it?
>
> And I have also tried using tsc[1]. tsc seems to work perfectly,
> but I don't know how accurate it is on intel dual core machines?
>
> Testing the accuracy of tsc by bounding my thread to one processor
> using sched_setaffinity and using usleep(), and comparing
> with code which is forced to switch to read the tsc value from the
> other CPU, shows that the accuracy of tsc when reading and writing
> using two different CPUs is below 1ms since
> that's the accuracy of usleep(). So it looks promising, but
> I need at least 0.1ms accuracy...
>
> Anyone know how much jitter there might be for tsc?
> I've not found anything on google yet.
>
>
>
> [1]  __asm__ __volatile__("rdtsc" : "=A" (ret))
>
> 
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