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

[John Reiser]

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.

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