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

["Michael S. Tsirkin" <mst@redhat.com>]

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.

> > 
> >  From the code,  I think the number of the chained descriptors is limited to
> > MAX_SKB_FRAGS + 2 (~18), which is much less than UIO_MAXIOV.
> 
> This is the limitation of #page frags for skb, not the iov limitation.
> 
> Thanks
> 
> > Please point out if I missed anything. Thanks.
> > 
> > Best,
> > Wei
> > 
> >