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Re: [Qemu-devel] snabbswitch integration with QEMU for userspace etherne

From: Michael S. Tsirkin
Subject: Re: [Qemu-devel] snabbswitch integration with QEMU for userspace ethernet I/O
Date: Tue, 28 May 2013 18:44:55 +0300

On Tue, May 28, 2013 at 05:35:55PM +0200, Julian Stecklina wrote:
> On 05/28/2013 03:56 PM, Michael S. Tsirkin wrote:
> > and in fact that was how vhost worked originally.
> > There were some issues to be fixed before it worked
> > without issues, but we do plan to go back to that I think.
> Do you know why they abandoned this execution model?
> Julian

Yes I do.  Two main issues:
1. Originally vhost used a shared workqueue for everything.
It turned out that an access to userspace memory might sometimes
block e.g. if it hit swap. When this happened the whole workqueue
got blocked, and no guest could make progress.
2. workqueue was sticking to one CPU too aggressively
E.g. there could be 10 free CPUs on the box, workqueue
was still using the same one which queued the work
even if that one was very busy.

This all got sorted out in core workqueue code,
so we should go back and try using the regular workqueue.


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