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Re: [Qemu-devel] is it possible to use a disk with multiple iothreads ?

From: Stefan Hajnoczi
Subject: Re: [Qemu-devel] is it possible to use a disk with multiple iothreads ?
Date: Fri, 24 Oct 2014 10:04:06 +0100
User-agent: Mutt/1.5.23 (2014-03-12)

On Thu, Oct 23, 2014 at 08:26:17PM +0200, Alexandre DERUMIER wrote:
> I was reading this interesting presentation, 
> http://vmsplice.net/~stefan/stefanha-kvm-forum-2014.pdf 
> and I have a specific question.
> I'm currently evaluate ceph/rbd storage performance through qemu,
> and the current bottleneck seem to be the cpu usage of the iothread.
> (rbd procotol cpu usage is really huge).
> Currently It's around 10000iops on 1core of recent xeons.
> So, I had tried with virtio-scsi multiqueue, but it doesn't help. (Not sure, 
> but I think that all queues are mapped on 1 iothread ?)
> I had tried with with virtio-blk and iothread/dataplane, it's help a little , 
> but not too much too.
> So, my question is : Is it possible to use multiple iothreads with 1 disk ?
> (something like 1 queue - 1 iothread?)

virtio-blk and virtio-scsi emulation only runs in 1 thread at a time.
It is currently not possible to achieve true multiqueue from guest,
through QEMU, and down to the host.

This is what the final slides in my presentation were about.

Regarding Ceph, do you know why it burns a lot of CPU?

CCed Josh in case he has ideas for reducing Ceph library CPU


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