Re: [Xen-devel] [PATCH 0/2] x86: improve remote CPU wakeup

[Tim Deegan <tim@xxxxxxx>]

At 10:36 +0100 on 11 Sep (1410428168), Jan Beulich wrote:
> Mass wakeups (via vlapic_ipi()) can take enormous amounts of time,
> especially when many of the remote pCPU-s are in deep C-states. For
> 64-vCPU Windows Server 2012 R2 guests on Ivybridge hardware,
> accumulated times of over 2ms were observed (average 1.1ms).
> Considering that Windows broadcasts IPIs from its timer interrupt,
> which at least at certain times can run at 1kHz, it is clear that this
> can't result in good guest behavior. In fact, on said hardware guests
> with significantly beyond 40 vCPU-s simply hung when e.g. ServerManager
> gets started.
> 
> The two patches bring down the average to 250us on said hardware.
> 
> 1: x86: suppress event check IPI to MWAITing CPUs
> 2: x86/HVM: batch vCPU wakeups
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

Reviewed-by: Tim Deegan <tim@xxxxxxx>

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