* Christian Borntraeger <[email protected]> wrote:
> Another question:
>
> nanosecond resolution seems not ideal for 64bit values, at least if an
> architecture has to do calculations. For example our cpu timer is
> signed 64bit and bit 51 (63=LSB) steps by one each microsecond. To
> create a nanosecond based timer we need: nsecs= clock*125/512 or nsecs
> = clock/512*125. The first variant overflows in a time frame that is
> still reasonable to be seen in practice (about 2 years if I made no
> errors), the second variant introduces a stepping rate of 125ns. Of
> course we could use nsec = (((((((clock/8)*5)/4)*5)/4)*5)/4), to have
> a long overflow period and a 1.25ns stepping rate but this looks quite
> ugly. Are you going to stick with nanosecond resolution? If yes, do
> you think a stepping rate of 125ns is ok? Any chance to change the
> scheduler resolution to microseconds? ;-)
there are noticeable accounting artifacts on fast boxes that do
sub-microsecond scheduling, so getting the best sched_clock() resolution
is certainly handy. (Also, nanoseconds gives us some rounding-error
room.) But 0.125 usecs should still be fine.
the 2 years overflow is not an issue: you could solve that by only using
the first 55 bits of the clock. This means the clock would wrap around
every 1.14 years - the effects of that are that the "dont let time go
backwards" code in the scheduler will ignore a very small interval
(which happens at the wraparound) and will continue with the
wrapped-around clock from that point on. The rq->clock itself is a true,
monotonic 64-bit clock that wraps every 584.9 years.
[ and even after 584.9 years it should have no serious failure mode, as
the timestamps are used in a relative manner. The only, minimal effect
is on tasks that sleep for more than 584 years - which could get a few
millisecs less sleeper fairness share. I am not overly worried about
getting bugreports about that in my lifetime though =B-) (unless
someone gets serious about bio-cryogenics R&D, real soon.) ]
Ingo
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