* Christian Borntraeger <[email protected]> wrote:
> 1. Jan could finish his sched_clock implementation for s390 and we
> would get close to the precise numbers. This would also let CFS make
> better decisions. [...]
i think this is the best option and it should give us the same /proc
accuracy on s390 as before, plus improved scheduler precision. (and
improved tracing accuracy, etc. etc.) Note that for architectures that
already have sched_clock() at least as precise as the stime/utime stats
there's no problem - and that seems to include all architectures except
s390.
could you send that precise sched_clock() patch? It should be an order
of magnitude simpler than the high-precision stime/utime tracking you
already do, and it's needed for quality scheduling anyway.
> [...] Downside: its not as precise as before as we do some math on the
> numbers and it will burn cycles to compute numbers we already have
> (utime=sum*utime/stime).
i can see no real downside to it: if all of stime, utime and
sum_exec_clock are precise, then the numbers we present via /proc are
precise too:
sum_exec * utime / stime;
there should be no loss of precision on s390 because the
multiplication/division rounding is not accumulating - we keep the
precise sum_exec, utime and stime values untouched.
on x86 we dont really want to slow down every irq and syscall event with
precise stime/utime stats for 'top' to display. On s390 the
multiplication and division is indeed superfluous but it keeps the code
generic for arches where utime/stime is less precise and irq-sampled -
while the sum is always precise. It also animates architectures that
have an imprecise sched_clock() implementation to improve its accuracy.
Accessing the /proc files alone is many orders of magnitude more
expensive than this simple multiplication and division.
Ingo
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