Re: [PATCH v2 1/2] ethdev: add buffered tx api

[Thomas Monjalon <thomas.monjalon@6wind.com>]

2016-03-09 15:42, Ananyev, Konstantin:
> From: Thomas Monjalon [mailto:thomas.monjalon@6wind.com]
> > 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.

If there is no packet, we have time to do a useless 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?

Because it may be slower when there are some packets
and will "accelerate" only the no-packet case.

We do not progress in this discussion. It is not a big deal, just a non sense.
So I agree to keep it if we change the website to announce that DPDK
accelerates the idle processing ;)