Re: [RFC, PATCH 24/24] i386 Vmi no idle hz

[Dan Hecht <dhecht@vmware.com>]

Pavel Machek wrote:
> Hi!
> 
>> When a VCPU enters its idle loop, it disables its periodic
>> alarm and sets up a one shot alarm for the next time event.
>> That way, it does not become ready to run just to service
>> the periodic alarm interrupt. Instead, it can remain halted
>> until there is some real work pending for it.  This allows
>> the hypervisor to use the physical resources more
>> effectively since idle VCPUs will have lower overhead.
> 
> Does this NO_IDLE_HZ work only on VMI-enabled runs or globally? We are
> trying to get NO_IDLE_HZ working to save some power on notebooks; how
> is it related to this?
> 

The NO_IDLE_HZ implementation provided here is enabled when the 
VMI-Timer device is used as the timer interrupt source.  The VMI-Timer 
device is only present on paravirtual hardware.

However, the hooks introduced into the kernel (stop_hz_timer, 
restart_hz_timer) with these patches will be approximately the same 
(perhaps a subset) for implementing NO_IDLE_HZ on systems that use other 
interrupt sources.

> Can you use NO_IDLE_HZ patches that are already floating around?
> 

Certainly we plan to merge our NO_IDLE_HZ implementation with those 
patches once they are stabilized in the kernel.  Presently, they seem to 
be too fast of a moving target to be worth merging at this point, 
though.  Additionally, the VMI-Timer NO_IDLE_HZ implementation does not 
need all the machinery provided with the NO_IDLE_HZ patches that are 
floating around because the VMI-Timer code does not track time by 
counting interrupts.  This leads to a fairly simplistic NO_IDLE_HZ 
implementation.

Dan