linux-kernel - Re: kvm & dyntick

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Message-ID: <20070112062006.GA32714@elte.hu>
Date:	Fri, 12 Jan 2007 07:20:07 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Avi Kivity <avi@...ranet.com>
Cc:	kvm-devel <kvm-devel@...ts.sourceforge.net>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: kvm & dyntick

* Avi Kivity <avi@...ranet.com> wrote:

> It occurs to me that kvm could benefit greatly from dyntick:
> 
> dyntick-enabled host:
> - generate virtual interrupts at whatever HZ the guest programs its 
> timers, be it 100, 250, 1000 or whatever
> - avoid expensive vmexits due to useless timer interrupts
> 
> dyntick-enabled guest:
> - reduce the load on the host when the guest is idling
>   (currently an idle guest consumes a few percent cpu)

yeah. KVM under -rt already works with dynticks enabled on both the host 
and the guest. (but it's more optimal to use a dedicated hypercall to 
set the next guest-interrupt)

> What are the current plans wrt dyntick?  Is it planned for 2.6.21?

yeah, we hope to have it in v2.6.21.

note that s390 (and more recently Xen too) uses a next_timer_interrupt() 
based method to stop the guest tick - which works in terms of reducing 
guest load, but it doesnt stop the host-side interrupt. The highest 
quality approach is to have dynticks on both the host and the guest, and 
this also gives high-resolution timers and a modernized 
time/timer-events subsystem for both the host and the guest.

	Ingo
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