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

On 05/10/2017 05:00 PM, Jason Wang wrote:

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:

I want to re-open the discussion left long time ago:
, 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
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.


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

Hi Jason,

I just posted a new discussion in another email. Please have a check if it's possible to solve the
issue from the existing implementation.


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