Re: [dpdk-dev] ixgbe TX function selection

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

Hi Zoltan,

> -----Original Message-----
> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Zoltan Kiss
> Sent: Friday, March 18, 2016 1:33 PM
> To: Lu, Wenzhuo; Wu, Jingjing; dev@dpdk.org
> Subject: Re: [dpdk-dev] ixgbe TX function selection
> 
> 
> 
> On 18/03/16 00:45, Lu, Wenzhuo wrote:
> > Hi Zoltan,
> >
> >
> >> -----Original Message-----
> >> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Zoltan Kiss
> >> Sent: Friday, March 18, 2016 1:11 AM
> >> To: Wu, Jingjing; dev@dpdk.org
> >> Subject: Re: [dpdk-dev] ixgbe TX function selection
> >>
> >>
> >>
> >> On 10/03/16 07:51, Wu, Jingjing wrote:
> >>> Hi, Zoltan
> >>>
> >>>> -----Original Message-----
> >>>> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Zoltan Kiss
> >>>> Sent: Wednesday, March 2, 2016 3:19 AM
> >>>> To: dev@dpdk.org
> >>>> Subject: [dpdk-dev] ixgbe TX function selection
> >>>>
> >>>> Hi,
> >>>>
> >>>> I've noticed that ixgbe_set_tx_function() selects the non-SG function
> >>>> even if (dev->data->scattered_rx == 1). That seems a bit dangerous,
> >>>> as you can turn that on inadvertently when you don't set
> >>>> max_rx_pkt_len and buffer size in certain ways. I've learnt it in the
> >>>> hard way, as my segmented packets were leaking memory on the TX path,
> >>>> which doesn't cries if you send out segmented packets.
> >>>> How should this case be treated? Assert on the non-SG TX side for the
> >>>> 'next' pointer? Or turning on SG if RX has it? It doesn't seem to be
> >>>> a solid way as other interfaces still can have SG turned on.
> >>>>
> >>>
> >>> If you look into the ixgbe_set_tx_function, you will find tx function
> >>> selection is decided by the tx_flags on queue configure, which is
> >>> passed by rte_eth_txconf. So even you set dev->data->scattered_rx to
> >>> 1, if the tx_flags is ETH_TXQ_FLAGS_NOMULTSEGS, ixgbe_xmit_pkts_simple
> >>> is still selected as tx function. So, you'd better to set tx_flags=0, and have a try.
> >>
> >> You mean getting default_txconf from rte_eth_dev_info_get() and explicitly turn
> >> ETH_TXQ_FLAGS_NOMULTSEGS to 0? (filling tx_flags with zeros doesn't work
> >> very well) That's a way to solve it for me, but I'm rather talking about using
> >> defaults which doesn't cause memory leak quite easily.
> > Yes, ETH_TXQ_FLAGS_NOMULTSEGS only can be set to 1 when you know all your packets will not be segmented.
> > I think that means normally we should use full function path for TX, for we have no knowledge about if the packets will be
> segmented or not.
> > You don't need to set tx_flags to 0, only the ETH_TXQ_FLAGS_NOMULTSEGS bit should be 0, the other bits can be 1 if needed.
> 
> So can we agree that the default settings should set
> ETH_TXQ_FLAGS_NOMULTSEGS to 0?

I'd prefer to keep things as they are right now.
There always will be arguments and supporters for both alternatives: 
Should the fastest or the most comprehensive path be the default one.
Again default txq_flags can vary from one PMD to another. 
So, I think  the right behaviour for the app would be not to rely on default value
but set it up manually to the desired value. 
Konstantin

> 
> 
> >
> >>
> >>>
> >>>> Regards,
> >>>>
> >>>> Zoltan