Re: [Perfctr-devel] Re: Enabling RDPMC in user space by default

Lee Revell
2005-11-29T19:05:50Z

On Tue, 2005-11-29 at 10:29 -0800, John Reiser wrote:
> Andi Kleen wrote:
> > I think it's also a useful convention - RDTSC is becomming more and more
> > useless and you cannot expect user applications who just want to
> > measure some cycles to rely on ever changing instable or non existing
> > performance counter APIs.
>
> Users are even more unhappy with ever-changing ABIs -- such as the
> kernel taking away RDTSC.
>
> RDTSC+perfctr [Pettersson] still is the fastest way for user-mode code
> to count something that is highly correlated with both "billable"
> CPU time and "code quality" for a fixed task. With a little care
> RDTSC is close enough to monotonic that I find it very useful.
> Please don't take away user-mode RDTSC.
>

The kernel didn't take this away, the hardware vendors did. For years
although it was risky on paper it was perfectly usable as a cheap high
res timer. I agree that it's unfortunate.

Lee