> On 5 May 2017, at 12:20, Jason Wang <jasow...@redhat.com> 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? > This limitation assumption is incorrect for Windows. We saw cases (and strangely enough with small packets) when Windows returns scatter gather list with 32 descriptors or more.
Best regards, Yan. > Looks not, see skb_copy_datagram_from_iter(). > > Thanks > >> >> Best, >> Wei >> >> --------------------------------------------------------------------- >> To unsubscribe, e-mail: virtio-dev-unsubscr...@lists.oasis-open.org >> <mailto:virtio-dev-unsubscr...@lists.oasis-open.org> >> For additional commands, e-mail: virtio-dev-h...@lists.oasis-open.org >> <mailto:virtio-dev-h...@lists.oasis-open.org> >> > > > --------------------------------------------------------------------- > To unsubscribe, e-mail: virtio-dev-unsubscr...@lists.oasis-open.org > <mailto:virtio-dev-unsubscr...@lists.oasis-open.org> > For additional commands, e-mail: virtio-dev-h...@lists.oasis-open.org > <mailto:virtio-dev-h...@lists.oasis-open.org>
