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

[Jason Wang <jasowang@redhat.com>]

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.

>  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