Re: X15 alpha release: as fast as TUX but in user space (fwd)

[Andi Kleen]

On Sat, Apr 28, 2001 at 05:52:42PM +0200, Ingo Molnar wrote:
> 
> On Sat, 28 Apr 2001, Andi Kleen wrote:
> 
> > You can also just use the cycle counter directly in most modern CPUs.
> > It can be read with a single instruction. In fact modern glibc will do
> > it for you when you use clock_gettime(CLOCK_PROCESS_CPUTIME_ID, ...)
> 
> well, it's not reliable while using things like APM, so i'd not recommend
> to depend on it too much.

*If* you use APM on your server boxes. Not likely even when it doesn't have more than one CPU
and it can be checked at runtime.

I guess glibc could also regularly (every 10 calls or so) call regular gettimeofday
to recheck synchronization; at least for a web server that potential inaccuracy would
be acceptable ("best effort") and the cost of the system call is 1/10.

In x86-64 there are special vsyscalls btw to solve this problem that export
a lockless kernel gettimeofday()

-Andi