Re: [PATCH net-next V2 3/3] tap: XDP support

[Jason Wang]

On 2017å08æ15æ 00:01, Michael S. Tsirkin wrote:
> On Sat, Aug 12, 2017 at 10:48:49AM +0800, Jason Wang wrote:
> > On 2017å08æ12æ 07:12, Jakub Kicinski wrote:
> > > On Fri, 11 Aug 2017 19:41:18 +0800, Jason Wang wrote:
> > > > This patch tries to implement XDP for tun. The implementation was
> > > > split into two parts:
> > > >
> > > > - fast path: small and no gso packet. We try to do XDP at page level
> > > >     before build_skb(). For XDP_TX, since creating/destroying queues
> > > >     were completely under control of userspace, it was implemented
> > > >     through generic XDP helper after skb has been built. This could be
> > > >     optimized in the future.
> > > > - slow path: big or gso packet. We try to do it after skb was created
> > > >     through generic XDP helpers.
> > > >
> > > > Test were done through pktgen with small packets.
> > > >
> > > > xdp1 test shows ~41.1% improvement:
> > > >
> > > > Before: ~1.7Mpps
> > > > After:  ~2.3Mpps
> > > >
> > > > xdp_redirect to ixgbe shows ~60% improvement:
> > > >
> > > > Before: ~0.8Mpps
> > > > After:  ~1.38Mpps
> > > >
> > > > Suggested-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
> > > > Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
> > > Looks OK to me now :)
> > >
> > > Out of curiosity, you say the build_skb() is for "small packets", and it
> > > seems you are always reserving the 256B regardless of XDP being
> > > installed.  Does this have no performance impact on non-XDP case?
> > Have a test, only less than 1% were noticed which I think could be ignored.
> >
> > Thanks
> What did you test btw?

Pktgen

>   The biggest issue would be with something like
> UDP with short packets.

Note that we do this only when sndbuf is INT_MAX. So this is probably 
not an issue. The only thing matter is more stress to page allocator, 
but according to the result of pktgen it was very small that could be 
ignored.

Thanks