Re: Intel P6 vs P7 system call performance

[H. Peter Anvin]

dean gaudet wrote:
> On Mon, 16 Dec 2002, Linus Torvalds wrote:
> 
>>It's not as good as a pure user-mode solution using tsc could be, but
>>we've seen the kinds of complexities that has with multi-CPU systems, and
>>they are so painful that I suspect the sysenter approach is a lot more
>>palatable even if it doesn't allow for the absolute best theoretical
>>numbers.
> 
> don't many of the multi-CPU problems with tsc go away because you've got a
> per-cpu physical page for the vsyscall?
> 
> i.e. per-cpu tsc epoch and scaling can be set on that page.
> 
> the only trouble i know of is what happens when an interrupt occurs and
> the task is rescheduled on another cpu... in theory you could test %eip
> against 0xfffffxxx and "rollback" (or complete) any incomplete
> gettimeofday call prior to saving a task's state.  but i bet that test is
> undesirable on all interrupt paths.
> 

Exactly.  This is a real problem.

	-hpa