Re: some questions using rdtsc in user space

["Alexandre P. Nunes" <alex@PolesApart.wox.org>]

Chris Friesen wrote:

>[snip]
>
>>If using rdtsc is a good way, someone knows how do I do some sort of
>>loop, converting the rdtsc difference (is it in cpu clocks, right?) to
>>nano/microseconds, and if there could be bad behaviour from this (I
>>believe there could be some SMP issues, but for now this is irrelavant
>>for us).
>>    
>>
>
>You'd be more portable looping on gettimeofday(), which returns back seconds and
>microseconds.  The disadvantage is that if you change the system time while your
>program is running you can get screwed.
>

I guess ntpd would be the evil on this issue, because at least in some 
machines I have here, it makes minor (sometimes not "that" minor) 
corrections every half hour or so...

>
>If you want to loop on rdtsc (which as you say is often clockspeed, but that is
>not necessarily exactly what is advertised), then the first thing you need to do
>is to figure out how fast you're going.  What I usually do is grab a timestamp,
>sleep for 10 seconds using select(), and grab another timestamp.  Figure out the
>difference in timestamps, divide by 10, and you get ticks/sec.  If it's close to
>your advertised clock rate, then round it up to the clock rate for ease of
>calculation.  As an example, my 550MHz P3 gives 548629000 ticks/sec using this
>method, so for your purposes I'd round it up to 550000000.
>
>Once you have this information, just divide by 5 million to get the number of
>ticks in 200ns.
>
>  
>
Ok, that's exactly where I was trying to get to, I'll do some 
experiments, thank you!

Alexandre