[dpdk-dev] [PATCH 1/3] ethdev: add max burst size to device info

[Ananyev, Konstantin]

> -----Original Message-----
> From: Shreyansh Jain [mailto:shreyansh.jain at nxp.com]
> Sent: Tuesday, December 12, 2017 1:44 PM
> To: Ananyev, Konstantin <konstantin.ananyev at intel.com>; Matan Azrad <matan at mellanox.com>; Nikhil Agarwal
> <nikhil.agarwal at linaro.org>
> Cc: dev at dpdk.org; Hunt, David <david.hunt at intel.com>; nikhil.agarwal at nxp.com; hemant.agrawal at nxp.com; Yigit, Ferruh
> <ferruh.yigit at intel.com>
> Subject: Re: [PATCH 1/3] ethdev: add max burst size to device info
> 
> On Tuesday 12 December 2017 04:33 PM, Ananyev, Konstantin wrote:
> >
> >
> >> -----Original Message-----
> >> From: dev [mailto:dev-bounces at dpdk.org] On Behalf Of Matan Azrad
> >> Sent: Tuesday, December 12, 2017 10:46 AM
> >> To: Nikhil Agarwal <nikhil.agarwal at linaro.org>; dev at dpdk.org
> >> Cc: Hunt, David <david.hunt at intel.com>; nikhil.agarwal at nxp.com; hemant.agrawal at nxp.com; Yigit, Ferruh <ferruh.yigit at intel.com>
> >> Subject: Re: [dpdk-dev] [PATCH 1/3] ethdev: add max burst size to device info
> >>
> >> Hi Nikhil
> >>
> >>> -----Original Message-----
> >>> From: dev [mailto:dev-bounces at dpdk.org] On Behalf Of Nikhil Agarwal
> >>> Sent: Tuesday, December 12, 2017 12:05 PM
> >>> To: dev at dpdk.org
> >>> Cc: david.hunt at intel.com; nikhil.agarwal at nxp.com;
> >>> hemant.agrawal at nxp.com; ferruh.yigit at intel.com
> >>> Subject: [dpdk-dev] [PATCH 1/3] ethdev: add max burst size to device info
> >>>
> >>> Currently, if the  rte_eth_rx_burst() function returns a value less than
> >>> *nb_pkts*, the application will assume that no more packets are present..
> >>>
> >>> Some of the hw queue based hardware can only support smaller burst for RX
> >>> and TX and thus break the expectation of the rx_burst API.
> >>>
> >>
> >> Doesn't such like devices PMDs should try to retrieve multiple HW burst to adjust the asked received  packet number?
> >
> > Same thought here...
> > Can't that limitation be hidden inside PMD by calling HW burst multiple times?
> 
> This might be required in some cases for performance.
> It is possible that for each request containing N buffers, if the PMD
> fetches all N (more than its preferred burst_size), cache misses reduce
> the performance - especially for SoC with limited cache size.
> 
> Also, a complete cycle of
> application->driver->hardware->driver->application can help driver
> prefetch buffers - which, in case of hw burst looping, might be too
> little to complete the prefetch cycle.
> 
> To summarize, indeed this is for performance specific cases and the idea
> that @Matan gave for renaming 'perf_buf_size' to highlight this, sounds
> logical.
> 
> > Also if I am not mistaken - it would increase size of struct rte_eth_dev_info, right?
> > If so, then it means ABI breakage.
> 
> Yes, deprecation notice should have been sent - if we continue with the
> dev_info change. To me that looks as one of the option. Maybe, someone
> on the list has a better idea.

In theory there is at least 2 free bytes at the end of struct rte_eth_desc_lim.
Might be it would help a bit.
Konstantin

> 
> > Konstantin
> >
> >>
> >>> This patch adds support to provide the maximum burst size that can be
> >>> supported by a given PMD. The dev_info is being memset to '0' in
> >>> rte_ethdev library. The value of '0' indicates that any value for burst size can
> >>> be supported i.e. no change for existing PMDs.
> >>>
> >>> The application can now use the lowest available max_burst_size value for
> >>> rte_eth_rx_burst.
> >>>
> >>
> >> If you are talking about performance, maybe the right field to expose is something like "perf_burst_size" or "preferred_burst_size".
> >> I also suggest to expose different fields for RX and for TX.
> >> Maybe the rte_eth_rx\tx_burst() descriptions should be updated.
> >>
> >> Thanks
> >> Matan.
> >>
> >>> Signed-off-by: Nikhil Agarwal <nikhil.agarwal at linaro.org>
> >>> ---
> >>>   lib/librte_ether/rte_ethdev.h | 1 +
> >>>   1 file changed, 1 insertion(+)
> >>>
> >>> diff --git a/lib/librte_ether/rte_ethdev.h b/lib/librte_ether/rte_ethdev.h
> >>> index 341c2d6..3ab6f02 100644
> >>> --- a/lib/librte_ether/rte_ethdev.h
> >>> +++ b/lib/librte_ether/rte_ethdev.h
> >>> @@ -1047,6 +1047,7 @@ struct rte_eth_dev_info {
> >>>   	/** Configured number of rx/tx queues */
> >>>   	uint16_t nb_rx_queues; /**< Number of RX queues. */
> >>>   	uint16_t nb_tx_queues; /**< Number of TX queues. */
> >>> +	uint16_t max_burst_size; /**< MAX burst size, 0 for no limit. */
> >>>   };
> >>>
> >>>   /**
> >>> --
> >>> 2.7.4
> >
> >