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Re: [Qemu-devel] virtio-net: configurable TX queue size
From: |
Michael S. Tsirkin |
Subject: |
Re: [Qemu-devel] virtio-net: configurable TX queue size |
Date: |
Fri, 5 May 2017 23:36:33 +0300 |
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
> >
> >
- [Qemu-devel] virtio-net: configurable TX queue size, Wang, Wei W, 2017/05/04
- Re: [Qemu-devel] virtio-net: configurable TX queue size,
Michael S. Tsirkin <=
- Re: [Qemu-devel] [virtio-dev] RE: virtio-net: configurable TX queue size, Wei Wang, 2017/05/10
- Re: [Qemu-devel] [virtio-dev] RE: virtio-net: configurable TX queue size, Michael S. Tsirkin, 2017/05/10
- Re: [Qemu-devel] [virtio-dev] RE: virtio-net: configurable TX queue size, Wei Wang, 2017/05/11