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