static user probe performance

[Frank Ch. Eigler]

>> run with stap
>> [...]
>> elapsed time
>>       volatile semaphore 1858.71
>>       no volatile no semaphore 1865.68
>>       volatile no semaphore 1873.07
>>       no volatile semaphore 1878.09
>
> So for this particular microbenchmark, systemtap probing imposes a 30x
> slowdown, yikes.  How many probe hits does this represent?

>From scox's later numbers, that's 192 million hits, for about 9
us/hit, which seems rather high.  (In this microbenchmark, it seems
that the instrumentation is taking an impressive 98% of the cpu time,
which ought to have triggered the overload protection.  I assume there
was an -DSTP_NO_OVERLOAD in effect.)

- FChE