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Date: Fri, 5 May 2017 23:36:33 +0300
From: "Michael S. Tsirkin" <mst@redhat.com>
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Subject: Re: [Qemu-devel] virtio-net: configurable TX queue size
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To: Jason Wang <jasowang@redhat.com>
Cc: "Wang, Wei W" <wei.w.wang@intel.com>, Stefan Hajnoczi <stefanha@gmail.com>, =?iso-8859-1?Q?Marc-Andr=E9?= Lureau <marcandre.lureau@gmail.com>, "pbonzini@redhat.com" <pbonzini@redhat.com>, "virtio-dev@lists.oasis-open.org" <virtio-dev@lists.oasis-open.org>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>, Jan Scheurich <jan.scheurich@ericsson.com>

On Fri, May 05, 2017 at 10:27:13AM +0800, Jason Wang wrote:
>=20
>=20
> On 2017=E5=B9=B405=E6=9C=8804=E6=97=A5 18:58, Wang, Wei W wrote:
> > Hi,
> >=20
> > 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  que=
ue
> > size to be configurable between 256 and 1024.
>=20
> Yes, I think we probably need this.
>=20
> >=20
> > The reason to propose this request is that a severe issue of packet d=
rops 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 go=
es away
> > with 1K queue size.
>=20
> Do we need even more, what if we find 1K is even not sufficient in the
> future? Modern nics has size up to ~8192.
>=20
> >=20
> > The concern mentioned in the previous discussion (please check the li=
nk
> > above) is that the number of chained descriptors would exceed
> > UIO_MAXIOV (1024) supported by the Linux.
>=20
> 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.

> >=20
> >  From the code,  I think the number of the chained descriptors is lim=
ited to
> > MAX_SKB_FRAGS + 2 (~18), which is much less than UIO_MAXIOV.
>=20
> This is the limitation of #page frags for skb, not the iov limitation.
>=20
> Thanks
>=20
> > Please point out if I missed anything. Thanks.
> >=20
> > Best,
> > Wei
> >=20
> >=20