On Mon, 16 Dec 2002, Linus Torvalds wrote:
>
> On my P4 machine, a "getppid()" is 641 cycles with sysenter/sysexit, and
> something like 1761 cycles with the old "int 0x80/iret" approach. That's a
> noticeable improvement, but I have to say that I'm a bit disappointed in
> the P4 still, it shouldn't be even that much.
On a slightly more real system call (gettimeofday - which actually matters
in real life) the difference is still visible, but less so - because the
system call itself takes more of the time, and the kernel entry overhead
is thus not as clear.
For gettimeofday(), the results on my P4 are:
sysenter: 1280.425844 cycles
int/iret: 2415.698224 cycles
1135.272380 cycles diff
factor 1.886637
ie sysenter makes that system call almost twice as fast.
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.
Linus
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This archive was generated by hypermail 2b29 : Mon Dec 23 2002 - 22:00:16 EST