On Fri, May 05, 2017 at 05:20:07PM +0800, Jason Wang wrote: > > > On 2017年05月05日 13:53, Wei Wang wrote: > > On 05/05/2017 10:27 AM, 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. > > > > That's great, thanks. > > > > > > > > > > > > > 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. > > > > Yes. Probably, we can also set the RX queue size to 8192 (currently it's > > 1K) as well. > > > > > > > > > > > > > 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 think we should first discuss whether it would be an issue below. > > > > > > > > > > > > > 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. > > > > I think the number of page frags are filled into the same number of > > descriptors > > in the virtio-net driver (e.g. use 10 descriptors for 10 page frags). On > > the other > > side, the virtio-net backend uses the same number of iov for the > > descriptors. > > > > Since the number of page frags is limited to 18, I think there wouldn't > > be more > > than 18 iovs to be passed to writev, right? > > Looks not, see skb_copy_datagram_from_iter(). > > Thanks
Besides, guests don't all use linux drivers. Some use e.g. dpdk pmd ones. It's best not to make assumptions outside the spec - if you need to make an assumption, it's best to add a flag to specify it. > > > > Best, > > Wei > > > > --------------------------------------------------------------------- > > To unsubscribe, e-mail: virtio-dev-unsubscr...@lists.oasis-open.org > > For additional commands, e-mail: virtio-dev-h...@lists.oasis-open.org > >
