On Mon, 27 May 2002, Keith Whitwell wrote:
> Linus Torvalds wrote:
> >
> > Much heavier. For some yet unexplained reason, a P4 takes about 1us to do
> > a simple system call. That's on a 1.8GHz system, so it basically implies
> > that a P4 takes 1800 cycles to do a "int 0x80 + iret", which is just
> > ludicrous. A 1.2Gz athlon does the same in 0.2us, ie around 250 cycles
> > (the 200+ cycles also matches a pentium reasonably well, so it's really
> > the P4 that stands out here).
>
> This is remarkable. I thought things were getting better, not worse.
In general, they are. I suspect the P4 system call slowness is just
another artifact of some first-generation issues - the same way the P4
tends to be limited when it comes to shifts etc. It will get fixed
eventually. And running at 3GHz+ makes some CPU cycles seem cheap if you
can make up for them elsewhere.
However, you should put all of this into perspective: those 1800 cycles
are just about the same time it takes to do one _single_ read from an ISA
device. It's roughly the time it takes for one cacheline to be DMA'd over
PCI.
Linus
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