Re: Intel P6 vs P7 system call performance

[Linus Torvalds]

On Thu, 19 Dec 2002, Linus Torvalds wrote:
> 
> So it's between 0-4 cycles on machines that take 200 - 1000 cycles for
> just the system call overhead.

Side note: I'd expect indirect calls - and especially the predictable 
ones, like this - to maintain competitive behaviour on CPU's. Even the P4, 
which usually has really bad worst-case behaviour for more complex 
instructions (just look at the 2000 cycles for a regular int80/iret and 
shudder) does a indirect call without huge problems.

That's because indirect calls are actually very common, and to some degree 
getting _more_ so with the proliferation of OO languages (and I'm 
discounting the "indirect call" that is just a return statement - they've 
obviously always been common, but a return stack means that CPU 
optimizations for "ret" instructions are different from "real" indirect 
calls).

So I don't worry about the indirection per se. I'd worry a lot more about
some of the tricks people mentioned (ie the "pushl $0xfffff000 ; ret"  
approach probably sucks quite badly on some CPU's, simply because it does
bad things to return stacks on modern CPU's - not necessarily visible in 
microbenchmarks, but..).

			Linus