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

["Michael S. Tsirkin" <mst@redhat.com>]

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
> > 
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