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

[Olivier Matz]

Hi,

On 11/21/2016 03:33 PM, Wiles, Keith wrote:
> 
>> On Nov 21, 2016, at 4:48 AM, Damjan Marion (damarion) <damarion at 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);


Regards,
Olivier