linux-kernel - Re: [PATCH] kvm: irqchip: Break up high order allocations of kvm_irq_routing

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Date:	Mon, 11 May 2015 13:25:22 +0200
From:	Joerg Roedel <jroedel@...e.de>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	Joerg Roedel <joro@...tes.org>, Gleb Natapov <gleb@...nel.org>,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	Christian Borntraeger <borntraeger@...ibm.com>
Subject: Re: [PATCH] kvm: irqchip: Break up high order allocations of
 kvm_irq_routing_table

Hi Paolo,

On Fri, May 08, 2015 at 06:26:13PM +0200, Paolo Bonzini wrote:
> It probably doesn't matter much indeed, but can you time the difference?
>  kvm_set_irq_routing is not too frequent, but happens enough often that
> we had to use a separate SRCU instance just to speed it up (see commit
> 719d93cd5f5, kvm/irqchip: Speed up KVM_SET_GSI_ROUTING, 2014-01-16).

The results vary a lot, but what I can say for sure is that the
kvm_set_irq_routing function takes at least twice as long (~10.000 vs
~22.000 cycles) as before on my AMD Kaveri machine (maximum was between
3-4 times as long).

On the other side this function is only called 2 times at boot in my
test, so I couldn't detect a noticable effect on the overall boot time
of the guest (37 disks were attached).


	Joerg

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