2016-03-09 16:17, Ananyev, Konstantin: > From: Thomas Monjalon [mailto:thomas.monjalon at 6wind.com] > > 2016-03-09 15:42, Ananyev, Konstantin: > > > From: Thomas Monjalon [mailto:thomas.monjalon at 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. > > One lcore can do TX for several queues/ports. > Let say we have N queues to handle, but right now traffic is going only > through > one of them. > That means we'll have to do N-1 useless calls and reduce number of cycles > available to send actual traffic.
OK, good justification, thanks. > > > 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, > > Exactly. > > >just a non sense. > > Look at what most of current DPDK examples do: they do check manually > does nb_pkts==0 or not, if not call tx_burst(). > For me it makes sense to move that check into the library function - > so each and every caller doesn't have to do it manually. > > > So I agree to keep it if we change the website to announce that DPDK > > accelerates the idle processing ;) > > That's fine by me, but at first I suppose you'll have to provide some data > showing that this approach slowdowns things, right? :) You got me
