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

[Kulasek, TomaszX]

> -----Original Message-----
> From: Ananyev, Konstantin
> Sent: Tuesday, February 2, 2016 14:50
> To: Kulasek, TomaszX <tomaszx.kulasek at intel.com>; dev at dpdk.org
> Subject: RE: [dpdk-dev] [PATCH 1/2] ethdev: add buffered tx api
> 
> Hi Tomasz,
> 
> > -----Original Message-----
> > From: Kulasek, TomaszX
> > Sent: Tuesday, February 02, 2016 10:01 AM
> > To: Ananyev, Konstantin; dev at dpdk.org
> > Subject: RE: [dpdk-dev] [PATCH 1/2] ethdev: add buffered tx api
> >
> > Hi Konstantin,
> >
> > > -----Original Message-----
> > > From: Ananyev, Konstantin
> > > Sent: Friday, January 15, 2016 19:45
> > > To: Kulasek, TomaszX; dev at dpdk.org
> > > Subject: RE: [dpdk-dev] [PATCH 1/2] ethdev: add buffered tx api
> > >
> > > Hi Tomasz,
> > >
> > > >
> > > > +		/* get new buffer space first, but keep old space around
> */
> > > > +		new_bufs = rte_zmalloc("ethdev->txq_bufs",
> > > > +				sizeof(*dev->data->txq_bufs) * nb_queues, 0);
> > > > +		if (new_bufs == NULL)
> > > > +			return -(ENOMEM);
> > > > +
> > >
> > > Why not to allocate space for txq_bufs together with tx_queues (as
> > > one chunk for both)?
> > > As I understand there is always one to one mapping between them
> anyway.
> > > Would simplify things a bit.
> > > Or even introduce a new struct to group with all related tx queue
> > > info togetehr struct rte_eth_txq_data {
> > > 	void *queue; /*actual pmd  queue*/
> > > 	struct rte_eth_dev_tx_buffer buf;
> > > 	uint8_t state;
> > > }
> > > And use it inside struct rte_eth_dev_data?
> > > Would probably give a better data locality.
> > >
> >
> > Introducing such a struct will require a huge rework of pmd drivers. I
> don't think it's worth only for this one feature.
> 
> Why not?
> Things are getting more and more messy here: now we have a separate array
> of pointer to queues, Separate array of queue states, you are going to add
> separate array of tx buffers.
> For me it seems logical to unite all these 3 fields into one sub-struct.
> 

I agree with you, and probably such a work will be nice also for rx queues, but these two changes impacts on another part of dpdk. While buffered tx API is more client application helper.

For me these two thinks are different features and should be made separately because:
1) They are independent and can be done separately,
2) They can (and should) be reviewed, tested and approved separately,
3) They are addressed to another type of people (tx buffering to application developers, rte_eth_dev_data to pmd developers), so another people can be interested in having (or not) one or second feature

Even for bug tracking it will be cleaner to separate these two things. And yes, it is logical to unite it, maybe also for rx queues, but should be discussed separately.

I've made a prototype with this rework, and the impact on the code not related to this particular feature is too wide and strong to join them. I would rather to provide it as independent patch for further discussion only on it, if needed.

> >
> >
> > > > +/**
> > > > + * @internal
> > > > + * Structure used to buffer packets for future TX
> > > > + * Used by APIs rte_eth_tx_buffer and rte_eth_tx_buffer_flush  */
> > > > +struct rte_eth_dev_tx_buffer {
> > > > +	struct rte_mbuf *pkts[RTE_ETHDEV_TX_BUFSIZE];
> > >
> > > I think it is better to make size of pkts[] configurable at runtime.
> > > There are a lot of different usage scenarios - hard to predict what
> > > would be an optimal buffer size for all cases.
> > >
> >
> > This buffer is allocated in eth_dev shared memory, so there are two
> scenarios:
> > 1) We have prealocated buffer with maximal size, and then we can set
> > threshold level without restarting device, or
> > 2) We need to set its size before starting device.
> 
> >
> > Second one is better, I think.
> 
> Yep, I was thinking about 2) too.
> Might be an extra parameter in struct rte_eth_txconf.
> 

Struct rte_eth_txconf is passed to ethdev after rte_eth_dev_tx_queue_config, so we don't know its value when buffers are allocated.
I'm looking for another solution.

> >
> > Tomasz