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