[dpdk-dev] [RFC] lib/librte_ether: consistent PMD batching behavior

[Yang, Zhiyong]

From: Andrew Rybchenko [mailto:arybchenko at solarflare.com]
Sent: Friday, January 20, 2017 6:26 PM
To: Yang, Zhiyong <zhiyong.yang at intel.com>; dev at dpdk.org
Cc: thomas.monjalon at 6wind.com; Richardson, Bruce <bruce.richardson at intel.com>; Ananyev, Konstantin <konstantin.ananyev at intel.com>
Subject: Re: [dpdk-dev] [RFC] lib/librte_ether: consistent PMD batching behavior

On 01/20/2017 12:51 PM, Zhiyong Yang wrote:

The rte_eth_tx_burst() function in the file Rte_ethdev.h is invoked to

transmit output packets on the output queue for DPDK applications as

follows.



static inline uint16_t

rte_eth_tx_burst(uint8_t port_id, uint16_t queue_id,

                 struct rte_mbuf **tx_pkts, uint16_t nb_pkts);



Note: The fourth parameter nb_pkts: The number of packets to transmit.

The rte_eth_tx_burst() function returns the number of packets it actually

sent. The return value equal to *nb_pkts* means that all packets have been

sent, and this is likely to signify that other output packets could be

immediately transmitted again. Applications that implement a "send as many

packets to transmit as possible" policy can check this specific case and

keep invoking the rte_eth_tx_burst() function until a value less than

*nb_pkts* is returned.



When you call TX only once in rte_eth_tx_burst, you may get different

behaviors from different PMDs. One problem that every DPDK user has to

face is that they need to take the policy into consideration at the app-

lication level when using any specific PMD to send the packets whether or

not it is necessary, which brings usage complexities and makes DPDK users

easily confused since they have to learn the details on TX function limit

of specific PMDs and have to handle the different return value: the number

of packets transmitted successfully for various PMDs. Some PMDs Tx func-

tions have a limit of sending at most 32 packets for every invoking, some

PMDs have another limit of at most 64 packets once, another ones have imp-

lemented to send as many packets to transmit as possible, etc. This will

easily cause wrong usage for DPDK users.



This patch proposes to implement the above policy in DPDK lib in order to

simplify the application implementation and avoid the incorrect invoking

as well. So, DPDK Users don't need to consider the implementation policy

and to write duplicated code at the application level again when sending

packets. In addition to it, the users don't need to know the difference of

specific PMD TX and can transmit the arbitrary number of packets as they

expect when invoking TX API rte_eth_tx_burst, then check the return value

to get the number of packets actually sent.



How to implement the policy in DPDK lib? Two solutions are proposed below.



Solution 1:

Implement the wrapper functions to remove some limits for each specific

PMDs as i40e_xmit_pkts_simple and ixgbe_xmit_pkts_simple do like that.

IMHO, the solution is a bit better since it:
 1. Does not affect other PMDs at all
 2. Could be a bit faster for the PMDs which require it since has no indirect
     function call on each iteration
 3. No ABI change



Solution 2:

Implement the policy in the function rte_eth_tx_burst() at the ethdev lay-

er in a more consistent batching way. Make best effort to send *nb_pkts*

packets with bursts of no more than 32 by default since many DPDK TX PMDs

are using this max TX burst size(32). In addition, one data member which

defines the max TX burst size such as "uint16_t max_tx_burst_pkts;"will be

added to rte_eth_dev_data, which drivers can override if they work with

bursts of 64 or other NB(thanks for Bruce <bruce.richardson at intel.com><mailto:bruce.richardson at intel.com>'s

suggestion). This can reduce the performance impacting to the lowest limit.

I see no noticeable difference in performance, so don't mind if this is finally choosen.
Just be sure that you update all PMDs to set reasonable default values, or may be
even better, set UINT16_MAX in generic place - 0 is a bad default here.
(Lost few seconds wondering why nothing is sent and cannot stop)

Agree with you, 0 is not a good default value.   I recommend 32 by default here, of course, The driver writers
Can configure it as they expect before starting to sending packets.

Zhiyong