Re: Intel P6 vs P7 system call performance

dean gaudet
2002-12-17T06:35:34Z

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.

-dean