Re: [PATCH 1/3] tuntap: rx batching

[Jason Wang]

On 2016å11æ11æ 11:31, Michael S. Tsirkin wrote:
> On Fri, Nov 11, 2016 at 10:07:44AM +0800, Jason Wang wrote:
> > >
> > >
> > >On 2016å11æ10æ 00:38, Michael S. Tsirkin wrote:
> > > > >On Wed, Nov 09, 2016 at 03:38:31PM +0800, Jason Wang wrote:
> > > > > > >Backlog were used for tuntap rx, but it can only process 1 packet at
> > > > > > >one time since it was scheduled during sendmsg() synchronously in
> > > > > > >process context. This lead bad cache utilization so this patch tries
> > > > > > >to do some batching before call rx NAPI. This is done through:
> > > > > > >
> > > > > > >- accept MSG_MORE as a hint from sendmsg() caller, if it was set,
> > > > > > >    batch the packet temporarily in a linked list and submit them all
> > > > > > >    once MSG_MORE were cleared.
> > > > > > >- implement a tuntap specific NAPI handler for processing this kind of
> > > > > > >    possible batching. (This could be done by extending backlog to
> > > > > > >    support skb like, but using a tun specific one looks cleaner and
> > > > > > >    easier for future extension).
> > > > > > >
> > > > > > >Signed-off-by: Jason Wang<jasowang@xxxxxxxxxx>
> > > > >So why do we need an extra queue?
> > >
> > >The idea was borrowed from backlog to allow some kind of bulking and avoid
> > >spinlock on each dequeuing.
> > >
> > > > >   This is not what hardware devices do.
> > > > >How about adding the packet to queue unconditionally, deferring
> > > > >signalling until we get sendmsg without MSG_MORE?
> > >
> > >Then you need touch spinlock when dequeuing each packet.
> It runs on the same CPU, right? Otherwise we should use skb_array...

There could be multiple senders technically. Will try skb_array and see 
if there's any difference.

Thanks