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

Bruce Richardson bruce.richardson at intel.com
Fri Jan 20 12:48:22 CET 2017

On Fri, Jan 20, 2017 at 11:24:40AM +0000, Ananyev, Konstantin wrote:
> > 
> > From: Andrew Rybchenko [mailto:arybchenko at solarflare.com]
> > Sent: Friday, January 20, 2017 10:26 AM
> > 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
> I also would prefer solution number 1 for the reasons outlined by Andrew above.
> Also, IMO current limitation for number of packets to TX in some Intel PMD TX routines
> are sort of artificial:
> - they are not caused by any real HW limitations
> - avoiding them at PMD level shouldn't cause any performance or functional degradation.
> So I don't see any good reason why instead of fixing these limitations in
> our own PMDs we are trying to push them to the upper (rte_ethdev) layer.
> Konstantin
The main advantage I see is that it should make it a bit easier for
driver writers, since they have a tighter set of constraints to work
with for packet RX and Tx. The routines only have to handle requests for
packets in the range 0-N, rather than not having an upper bound on the
request. It also then saves code duplicating with having multiple
drivers having the same outer-loop code for handling arbitrarily large

No big deal to me either way though.


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