Re: Why was the timehands_count sysctl added?

On Sat, 2021-10-09 at 14:54 -0600, Warner Losh wrote:
> On Sat, Oct 9, 2021, 1:56 PM Konstantin Belousov
> <kostikbel@gmail.com>
> wrote:
> 
> > On Sat, Oct 09, 2021 at 09:41:28PM +0200, Sebastian Huber wrote:
> > > Hello,
> > > 
> > > I synchronize currently the port of the FreeBSD timecounters to
> > > RTEMS and
> > > came across a change in 2019 which I do not understand. Some time
> > > ago the
> > > timehands were reduced from 10 to two:
> > > 
> > > https://reviews.freebsd.org/D7302
> > > 
> > > In 2019 this changed again to be up to 16:
> > > 
> > > https://reviews.freebsd.org/D21563
> > This review did not changed it back to 16, the default value is
> > still 2.
> > It allows to bump the number of used timehands, but normally
> > systems run
> > with only 2.
> > 
> > > 
> > > The corresponding sysctl is not documented:
> > > 
> > > https://www.freebsd.org/cgi/man.cgi?query=timecounters&sektion=4
> > > 
> > > Does someone know why this sysctl to select the count of
> > > timehands was
> > > added? Is this a performance optimization for some systems?
> > 
> > To allow for experimentation, and to satisfy some requests where
> > people
> > wanted to have more that 2 timehands.
> > 
> 
> When would someone want that? What's the use case?
> 
> Warner
> 
> > 

One known usecase is a single-cpu (non-SMP) system that uses a PPS
signal.  With only two timehands, a pps pulse that arrives while the
system is in an idle-sleep (wait-for-interrupt) state will switch
timehands too many times between the wakeup and the capture and trying
to use the capture data, so that the th generation count never matches,
and in effect you never capture a PPS pulse.  I found that on a
BeagleBone system.  It takes a minimum of 3 timehands for it to work
right.

-- Ian