Messages in this thread | | | Date | Fri, 02 Aug 2002 16:05:59 -0300 | From | "Alexandre P. Nunes" <> | Subject | Re: some questions using rdtsc in user space |
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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
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