Re: [PATCH] tsc-disable_B9

[Pavel Machek <pavel@ucw.cz>]

Hi!
> > >> And following your argument that these apps have been silenty broken
> > >> since 1999, if there's no broken app out there, nobody will ever get the
> > >> instruction fault. If there's any app broken out there we probably like
> > > 
> > > No. rdtsc is still usefull if you are clever and statistically filter
> > > out. Also rdtsc provides you number of cycles, so if you want to know
> > > how many cycles mov %eax,%ebx takes, you can do that even on
> > > speedstep. Anything that correlates rdtsc to real time is broken, however.
> > 
> > It's not correlating it to real time that's the problem. It's getting resceduled
> > inbetween calls that hurts. Take your example.
> > 
> > rdtsc
> > mov %eax,%ebx
> > 			<- get rescheduled here
> > rdtsc
> > 
> > Broken. May even take negative "time".
> > 
> > M.
> 
> The CPU counters are synchronized on SMP machines. How can you
> ever get negative time? Even GHz machines take several months
> to wrap the count.

This thread was about numa machines that do not keep tsc synchronized.