freebsd 10 kqueue timer regression

[Ian Lepore]

On Thu, 2014-10-02 at 16:00 -0400, John Baldwin wrote:
> On Thursday, October 02, 2014 3:53:28 pm Ian Lepore wrote:
> > On Thu, 2014-10-02 at 12:47 -0700, Adrian Chadd wrote:
> > > I'm confused; it's doing 50 loops of a 20msec timer, right? So that's 
> 1000ms.
> > 
> > Yes, so the entire loop should take 1000ms maybe + 1ms.  Instead it
> > takes 1070.  When I run it on an armv6 system running -current it takes
> > 1050.  When I run it on my 8.4 desktop (pre-eventtimers) it takes 1013.
> > 
> > -- Ian
> 
> What if you set kern.eventtimer.periodic=1?
> 

Some interesting results...

           HZ   100   500    1000
---------------------------------
periodic=0     1050  1050    1080
periodic=1     1110  1012    1049


The 1080 number was +/- 3ms, all the other numbers were +/- 1ms (except
for one outlier of 24363 at 100Hz non-periodic which I'm going to
pretend didn't happen).

The 1050 numbers are probably each 20ms sleep actually taking 21ms, but
the old tvtohz code with -1 adjustments from the old email thread isn't
in play anymore.  I don't know how to account for the other numbers at
all.  There's all kinds of stuff I don't understand in the new code
involving tick thresholds and such.

-- Ian