Re: [dpdk-dev] Adding API to force freeing consumed buffers in TX ring

[Bruce Richardson <bruce.richardson@intel.com>]

On Mon, Nov 21, 2016 at 04:06:32PM +0100, Olivier Matz wrote:
> Hi,
> 
> On 11/21/2016 03:33 PM, Wiles, Keith wrote:
> > 
> >> On Nov 21, 2016, at 4:48 AM, Damjan Marion (damarion) <damarion@cisco.com> wrote:
> >>
> >>
> >> Hi,
> >>
> >> Currently in VPP we do memcpy of whole packet when we need to do 
> >> replication as we cannot know if specific buffer is transmitted
> >> from tx ring before we update it again (i.e. l2 header rewrite).
> >>
> >> Unless there is already a way to address this issue in DPDK which I’m not aware
> >> of my proposal is that we provide mechanism for polling TX ring 
> >> for consumed buffers. This can be either completely new API or 
> >> extension of rte_etx_tx_burst (i.e. special case when nb_pkts=0).
> >>
> >> This will allows us to start polling tx ring when we expect some 
> >> mbuf back, instead of waiting for next tx burst (which we don’t know
> >> when it will happen) and hoping that we will reach free_threshold soon.
> > 
> > +1
> > 
> > In Pktgen I have the problem of not being able to reclaim all of the TX mbufs to update them for the next set of packets to send. I know this is not a common case, but I do see the case where the application needs its mbufs freed off the TX ring. Currently you need to have at least a TX ring size of mbufs on hand to make sure you can send to a TX ring. If you allocate too few you run into a deadlock case as the number of mbufs  on a TX ring does not hit the flush mark. If you are sending to multiple TX rings on the same numa node from the a single TX pool you have to understand the total number of mbufs you need to have allocated to hit the TX flush on each ring. Not a clean way to handle the problems as you may have limited memory or require some logic to add more mbufs for dynamic ports.
> > 
> > Anyway it would be great to require a way to clean up the TX done ring, using nb_pkts == 0 is the simplest way, but a new API is fine too.
> >>
> >> Any thoughts?
> 
> Yes, it looks useful to have a such API.
> 
> I would prefer another function instead of diverting the meaning of
> nb_pkts. Maybe this?
> 
>   void rte_eth_tx_free_bufs(uint8_t port_id, uint16_t queue_id);
> 

Third parameter for a limit(hint) of the number of bufs to free? If the
TX ring is big, we might not want to stall other work for a long time
while we free a huge number of buffers.

	/Bruce