linux-kernel - Re: [ANNOUNCE] Native Linux KVM tool v2

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Date:	Thu, 16 Jun 2011 10:09:01 +0100
From:	Stefan Hajnoczi <stefanha@...il.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Pekka Enberg <penberg@...nel.org>, Alexander Graf <agraf@...e.de>,
	Anthony Liguori <anthony@...emonkey.ws>,
	Prasad Joshi <prasadjoshi124@...il.com>,
	Avi Kivity <avi@...hat.com>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Sasha Levin <levinsasha928@...il.com>,
	Cyrill Gorcunov <gorcunov@...il.com>,
	Asias He <asias.hejun@...il.com>,
	Jens Axboe <jaxboe@...ionio.com>
Subject: Re: [ANNOUNCE] Native Linux KVM tool v2

On Thu, Jun 16, 2011 at 8:24 AM, Ingo Molnar <mingo@...e.hu> wrote:
>  - executing AIO in the vcpu thread eats up precious vcpu execution
>   time: combined QCOW2 throughput would be limited by a single
>   core's performance, and any time spent on QCOW2 processing would
>   not be spent running the guest CPU. (In such a model we certainly
>   couldnt do more intelligent, CPU-intense storage solutions like on
>   the fly compress/decompress of QCOW2 data.)

This has been a problem in qemu-kvm.  io_submit(2) steals time from
the guest (I think it was around 20us on the system I measured last
year).

Add the fact that the guest kernel might be holding a spinlock and it
becomes a scalability problem for SMP guests.

Anything that takes noticable CPU time should be done outside the vcpu thread.

Stefan
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