On Fri, Jan 20, 2017 at 11:48:22AM +0000, Bruce Richardson wrote: > On Fri, Jan 20, 2017 at 11:24:40AM +0000, Ananyev, Konstantin wrote: > > > > > > From: Andrew Rybchenko [mailto:arybche...@solarflare.com] > > > Sent: Friday, January 20, 2017 10:26 AM > > > To: Yang, Zhiyong <zhiyong.y...@intel.com>; dev@dpdk.org > > > Cc: thomas.monja...@6wind.com; Richardson, Bruce > > > <bruce.richard...@intel.com>; Ananyev, Konstantin > > > <konstantin.anan...@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.
For what it's worth, I agree with Konstantin. Wrappers should be as thin as possible on top of PMD functions, they are not helpers. We could define a set of higher level functions for this purpose though. In the meantime, exposing and documenting PMD limitations seems safe enough. We could assert that RX/TX burst requests larger than the size of the target queue are unlikely to be fully met (i.e. PMDs usually do not check for completions in the middle of a TX burst). > > 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 > requests. > > No big deal to me either way though. > > /Bruce Right but there is a cost in doing so, as unlikely() as the additional code is. We should leave that choice to applications. -- Adrien Mazarguil 6WIND
