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

[Jason Wang <jasowang@redhat.com>]

On 2017年05月07日 12:39, Wang, Wei W wrote:
> 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

Maybe, but let's first clarify the limitation and new ability in the 
spec. Want to send a patch?

Thanks