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Re: [Qemu-devel] virtio-net: configurable TX queue size


From: Wang, Wei W
Subject: Re: [Qemu-devel] virtio-net: configurable TX queue size
Date: Sun, 7 May 2017 04:39:53 +0000

On 05/06/2017 04:37 AM, Michael S. Tsirkin wrote:
> On Fri, May 05, 2017 at 10:27:13AM +0800, Jason Wang wrote:
> >
> >
> > On 2017年05月04日 18:58, Wang, Wei W wrote:
> > > Hi,
> > >
> > > I want to re-open the discussion left long time ago:
> > > https://lists.gnu.org/archive/html/qemu-devel/2015-11/msg06194.html
> > > , and discuss the possibility of changing the hardcoded (256) TX
> > > queue size to be configurable between 256 and 1024.
> >
> > Yes, I think we probably need this.
> >
> > >
> > > The reason to propose this request is that a severe issue of packet
> > > drops in TX direction was observed with the existing hardcoded 256
> > > queue size, which causes performance issues for packet drop
> > > sensitive guest applications that cannot use indirect descriptor
> > > tables. The issue goes away with 1K queue size.
> >
> > Do we need even more, what if we find 1K is even not sufficient in the
> > future? Modern nics has size up to ~8192.
> >
> > >
> > > The concern mentioned in the previous discussion (please check the
> > > link
> > > above) is that the number of chained descriptors would exceed
> > > UIO_MAXIOV (1024) supported by the Linux.
> >
> > We could try to address this limitation but probably need a new
> > feature bit to allow more than UIO_MAXIOV sgs.
> 
> I'd say we should split the queue size and the sg size.
> 

I think we can just split the iov size in the virtio-net backend,
that is, split the large iov[] into multiple iov[1024] to send to writev.

Best,
Wei



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