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Date: Thu, 16 Jun 2011 10:09:01 +0100
Message-ID: <BANLkTi=j1EqMP7gwKKUYOCv4OET1N8uJrQ@mail.gmail.com>
Subject: Re: [ANNOUNCE] Native Linux KVM tool v2
From: Stefan Hajnoczi <stefanha@gmail.com>
To: Ingo Molnar <mingo@elte.hu>
Cc: Pekka Enberg <penberg@kernel.org>, Alexander Graf <agraf@suse.de>,
        Anthony Liguori <anthony@codemonkey.ws>,
        Prasad Joshi <prasadjoshi124@gmail.com>, Avi Kivity <avi@redhat.com>,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Sasha Levin <levinsasha928@gmail.com>,
        Cyrill Gorcunov <gorcunov@gmail.com>, Asias He <asias.hejun@gmail.com>,
        Jens Axboe <jaxboe@fusionio.com>
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On Thu, Jun 16, 2011 at 8:24 AM, Ingo Molnar <mingo@elte.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