Re: [Qemu-devel] question about VM kernel parameter idle=<poll/mwait/halt/nomwait>

[Gleb Natapov]

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.