Re: mmx support with smp

Lawrence (
Thu, 10 Jul 1997 14:41:11 +1000

There is a 52-clock cycles when switching FPU and MMX mode.

I used procinfo to monitor a busy system and found that normally there
is under 1,000 context switches per second. So, 52,000 clock cycles are
used for that. For a 200Mhz MMX CPU, 52,000 / 200,000,000 = 0.026%.
Mmmm....seems not that much :)


> But isn't there some other things going on insode the CPU re: FPU/MMX use?
> If you context switch a task off a processor the is using MMX, then load
> any other process expecting FPU setup...
> As I remember, there is a price (a heavy one) for switching back and forth
> between MMX and FPU usage. The kernel may not be able to tell what mode
> the application has put the CPU in. We shall see, I'm sure.
> (Doesn't the Tyan Tomcat III support two 166-MMX processors? All I remember
> is the power warnings -- MMX processors draw more power than the non-MMX
> cousins which causes problems for the Tomcat's as they were designed before
> there was such a thing.)
> --Ricky