[dpdk-dev] [PATCH v2 1/2] ethdev: add buffered tx api

[Ananyev, Konstantin]

> -----Original Message-----
> From: Thomas Monjalon [mailto:thomas.monjalon at 6wind.com]
> Sent: Wednesday, March 09, 2016 3:27 PM
> To: Ananyev, Konstantin
> Cc: Kulasek, TomaszX; dev at dpdk.org
> Subject: Re: [dpdk-dev] [PATCH v2 1/2] ethdev: add buffered tx api
> 
> 2016-03-09 15:23, Ananyev, Konstantin:
> > >
> > > 2016-03-09 13:36, Ananyev, Konstantin:
> > > > > > +   if (to_send == 0)
> > > > > > +           return 0;
> > > > >
> > > > > Why this check is done in the lib?
> > > > > What is the performance gain if we are idle?
> > > > > It can be done outside if needed.
> > > >
> > > > Yes, that could be done outside, but if user has to do it anyway,
> > > > why not to put it inside?
> > > > I don't expect any performance gain/loss because of that -
> > > > just seems a bit more convenient to the user.
> > >
> > > It is handling an idle case so there is no gain obviously.
> > > But the condition branching is surely a loss.
> >
> > I suppose that condition should always be checked:
> > either in user code prior to function call or inside the
> > function call itself.
> > So don't expect any difference in performance here...
> > Do you have any particular example when you think it would?
> > Or are you talking about rte_eth_tx_buffer() calling
> > rte_eth_tx_buffer_flush() internally?
> > For that one - both are flush is 'static inline' , so I expect
> > compiler be smart enough to remove this redundant check.
> >
> > > So why the user would you like to do this check?
> > Just for user convenience - to save him doing that manually.
> 
> Probably I've missed something. If we remove this check, the function
> will do nothing, right? How is it changing the behaviour?

If we'll remove that check, then 
rte_eth_tx_burst(...,nb_pkts=0)->(*dev->tx_pkt_burst)(...,nb_pkts=0)
will be called.
So in that case it might be even slower, as we'll have to do a proper call.
Of course user can avoid it by:

If(tx_buffer->nb_pkts != 0)
	rte_eth_tx_buffer_flush(port, queue, tx_buffer);

But as I said what for to force user to do that?
Why not to  make this check inside the function? 
Konstantin