linux-kernel - Re: kvm & dyntick

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Date:	Fri, 12 Jan 2007 19:04:31 +0200
From:	Avi Kivity <avi@...ranet.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	kvm-devel <kvm-devel@...ts.sourceforge.net>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: kvm & dyntick

Ingo Molnar wrote:
> * Ingo Molnar <mingo@...e.hu> wrote:
>
>   
>>> 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)
>>     
>
> using the dynticks code from the -rt kernel makes the overhead of an 
> idle guest go down by a factor of 10-15:
>
>   PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
>  2556 mingo     15   0  598m 159m 157m R  1.5  8.0   0:26.20 qemu
>
>   

As usual, great news.

> ( for this to work on my system i have added a 'hyper' clocksource 
>   hypercall API for KVM guests to use - this is needed instead of the 
>   running-to-slowly TSC. )
>   

What's the problem with the TSC?  The only issue I'm aware of is that 
the tsc might go backwards if the vcpu is migrated to another host cpu 
(easily fixed).

A pv clocksource makes sense in any case.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.

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