Re: [ANNOUNCE] Native Linux KVM tool v2

From: Stefan Hajnoczi
Date: Thu Jun 16 2011 - 05:09:09 EST

On Thu, Jun 16, 2011 at 8:24 AM, Ingo Molnar <mingo@xxxxxxx> 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

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.

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