linux-kernel - Re: [PATCH v5] KVM: halt-polling: poll for the upcoming fire timers

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Date:	Thu, 26 May 2016 12:30:20 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org
Cc:	Wanpeng Li <wanpeng.li@...mail.com>,
	Radim Krčmář <rkrcmar@...hat.com>,
	David Matlack <dmatlack@...gle.com>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	Yang Zhang <yang.zhang.wz@...il.com>
Subject: Re: [PATCH v5] KVM: halt-polling: poll for the upcoming fire timers



On 26/05/2016 12:26, Paolo Bonzini wrote:
> As discussed on IRC, I would like to understand why the adaptive
> adjustment of halt_poll_ns is failing.  It seems like you have so few
> halts that you don't get halt_poll_ns>0.  Yet, when the VM halts, it's
> very close to the timer tick---often enough for this patch to have an
> effect.
> 
> Please send a trace of halt_poll_ns_grow and halt_poll_ns_shrink
> tracepoints, so that we can find out more about this.

And 30 seconds after I wrote this email, you told me on IRC that the
guest had HZ=1000 and the module parameter was set to 1 ms in order to
_really_ benefit from the patch.  So basically you could obtain the same
effect with idle=poll in the guest.

This explains why your reported results were not so great (as David noted).

Thanks,

Paolo