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Date: Thu, 21 Nov 2013 09:22:00 +0200
From: Gleb Natapov <gleb@redhat.com>
Message-ID: <20131121072200.GE2108@redhat.com>
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Subject: Re: [Qemu-devel] question about VM kernel parameter
	idle=<poll/mwait/halt/nomwait>
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To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: "Huangweidong (C)" <weidong.huang@huawei.com>, KVM <kvm@vger.kernel.org>, "Zhanghaoyu (A)" <haoyu.zhang@huawei.com>, Luonengjun <luonengjun@huawei.com>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>, Paolo Bonzini <pbonzini@redhat.com>

On Thu, Nov 21, 2013 at 09:01:39AM +0200, Michael S. Tsirkin wrote:
> On Thu, Nov 21, 2013 at 03:45:28AM +0000, Zhanghaoyu (A) wrote:
> > Hi, all
> > 
> > What's the difference of the linux guest kernel parameter idle=<poll/mwait/halt/nomwait>, especially in performance?
> > 
> > Taking the performance into account, which one is best?
> > 
> > In my opinion, if the number of all VMs' vcpus is far more than that of pcpus, e.g. SPECVirt test, idle=halt is better for server's total throughput,
> > otherwise, e.g. in some CT scenario, the number of total vcpus is not greater than that of pcpus, idle=poll is better for server's total throughput,
> > because of less latency and VMEXIT.
> 
> Makes sense overall.
> 
> > linux-3.9 and above, idle=mwait is not recommended.
> > 
> > Thanks,
> > Zhang Haoyu
> 
> Does it actually have effect? I didn't think it would.
mwait instruction is never exposed to a guest. With idle=mwait is will
likely fall back to halt silently.

--
			Gleb.