Re: [Qemu-devel] Re: [kvm-devel] [PATCH 0/4] Rework alarm timer infrastrucure - take2

[Jamie Lokier <jamie@shareable.org>]

Avi Kivity wrote:
> >>>In this case the dyn-tick minimum res will be 1msec. I believe it should
> >>>work ok since this is the case without any dyn-tick.
> >>>      
> >>Actually minimum resolution depends on host HZ setting, but - yes -
> >>essentially you have the same behaviour of the "unix" timer, plus the
> >>overhead of reprogramming the timer. 
> >>    
> >
> >Is this significant?  At a high guest HZ, this is could be quite a lot
> >of additional syscalls right?
> >
> At HZ=1000, this adds a small multiple of 1000 syscalls, which is a 
> fairly small overhead.

Small, but maybe measurable.

That overhead could be removed if the dyn-tick code were to
predictively set the host timer into a repeating mode when guests do
actually require a regular tick.

I'm thinking when it detects it needed a tick a small number of times
in a row, with the same interval, it could set the host timer to
trigger repeatedly at that interval.  Then it wouldn't need to reprogram
if that stayed the same (except maybe to correct for drift?)

If a tick then _wasn't_ required for that interval (i.e. it was
required for less, more, or not at all), then it would have to
reprogram the host timer.  If it really mattered, it wouldn't have to
reprogram the host timer when the next required tick is further in the
future or not at all; it would simply be a redundant SIGALRM.  In
weird cases that's worthwhile, but I suspect it generally isn't.

-- Jamie