Re: [dpdk-dev] mlx5 reta size is dynamic

["Nélio Laranjeiro" <nelio.laranjeiro@6wind.com>]

Hi Hanoch,

On Thu, Mar 22, 2018 at 09:02:19AM +0000, Hanoch Haim (hhaim) wrote:
> Hi Nelio, 
> I think you didn't understand me. I suggest to keep the RETA table
> size constant (maximum 512 in your case) and don't change its base on
> the number of configured Rx-queue.

It is even simpler, we can return the maximum size or a multiple of
RTE_RETA_GROUP_SIZE according to the number of Rx queues being used, in
the devop->dev_infos_get() as it is what the
rte_eth_dev_rss_reta_update() implementation will expect.
 
> This will make the DPDK API consistent. As a user I need to do tricks
> (allocate an odd/prime number of rx-queues) to get the RETA size
> constant at 512

I understand this issue, what I don't fully understand your needs.

> I'm not talking about changing the values in the RETA table which can
> be done while there is traffic. 

On MLX5 changing the entries of the RETA table don't affect the current
traffic, it needs a port restart to affect it and only for "default"
flows, any flow created through the public flow API are not impacted by
the RETA table.


>From my understanding, you wish to have a size returned by
devop->dev_infos_get() usable directly by rte_eth_dev_rss_reta_update().
This is why you are asking for a fix size?  So, if internally the PMD
starts with a smaller RETA table does not really matter, until the RETA
API works without any trick from the application side.  Is this correct?

Thanks,

> Thanks, 
> Hanoh
> 
> 
> -----Original Message-----
> From: Nélio Laranjeiro [mailto:nelio.laranjeiro@6wind.com] 
> Sent: Thursday, March 22, 2018 10:55 AM
> To: Hanoch Haim (hhaim)
> Cc: Yongseok Koh; dev@dpdk.org
> Subject: Re: [dpdk-dev] mlx5 reta size is dynamic
> 
> On Thu, Mar 22, 2018 at 06:52:53AM +0000, Hanoch Haim (hhaim) wrote:
> > Hi Yongseok,
> > 
> > 
> > RSS has a DPDK API,application can ask for the reta table size and 
> > configure it. In your case you are assuming specific use case and 
> > change the size dynamically which solve 90% of the use-cases but break 
> > the 10% use-case.
> > Instead, you could provide the application a consistent API and with 
> > that 100% of the applications can work with no issue. This is what 
> > happen with Intel (ixgbe/i40e) Another minor issue the rss_key_size 
> > return as zero but internally it is 40 bytes
> 
> Hi Hanoch,
> 
> Legacy DPDK API has always considered there is only a single indirection table aka. RETA whereas this is not true [1][2] on this device.
> 
> On MLX5 there is an indirection table per Hash Rx queue according to the list of queues making part of it.
> The Hash Rx queue is configured to make the hash with configured
> information:
>  - Algorithm,
>  - key
>  - hash field (Verbs hash field)
>  - Indirection table
> An Hash Rx queue cannot handle multiple RSS configuration, we have an Hash Rx queue per protocol and thus a full configuration per protocol.
> 
> In such situation, changing the RETA means stopping the traffic, destroying every single flow, hash Rx queue, indirection table to remake everything with the new configuration.
> Until then, we always recommended to any application to restart the port on this device after a RETA update to apply this new configuration.
> 
> Since the flow API is the new way to configure flows, application should move to this new one instead of using old API for such behavior.
> We should also remove such devop from the PMD to avoid any confusion.
> 
> Regards,
> 
> > Thanks,
> > Hanoh
> > 
> > -----Original Message-----
> > From: Yongseok Koh [mailto:yskoh@mellanox.com]
> > Sent: Wednesday, March 21, 2018 11:48 PM
> > To: Hanoch Haim (hhaim)
> > Cc: dev@dpdk.org
> > Subject: Re: [dpdk-dev] mlx5 reta size is dynamic
> > 
> > On Wed, Mar 21, 2018 at 06:56:33PM +0000, Hanoch Haim (hhaim) wrote:
> > > Hi mlx5 driver expert,
> > > 
> > > DPDK: 17.11
> > > Any reason mlx5 driver change the rate table size dynamically based 
> > > on the rx- queues# ?
> > 
> > The device only supports 2^n-sized indirection table. For example, if the number of Rx queues is 6, device can't have 1-1 mapping but the size of ind tbl could be 8, 16, 32 and so on. If we configure it as 8 for example, 2 out of 6 queues will have 1/4 of traffic while the rest 4 queues receives 1/8. We thought it was too much disparity and preferred setting the max size in order to mitigate the imbalance.
> > 
> > > There is a hidden assumption that the user wants to distribute the 
> > > packets evenly which is not always correct.
> > 
> > But it is mostly correct because RSS is used for uniform distribution. The decision wasn't made based on our speculation but by many request from multiple customers.
> > 
> > > /* If the requested number of RX queues is not a power of two, use the
> > >           * maximum indirection table size for better balancing.
> > >           * The result is always rounded to the next power of two. */
> > >           reta_idx_n = (1 << log2above((rxqs_n & (rxqs_n - 1)) ?
> > >                                            priv->ind_table_max_size :
> > >                                            rxqs_n));
> > 
> > Thanks,
> > Yongseok
> 
> [1] https://dpdk.org/ml/archives/dev/2015-October/024668.html
> [2] https://dpdk.org/ml/archives/dev/2015-October/024669.html
> 
> --
> Nélio Laranjeiro
> 6WIND

-- 
Nélio Laranjeiro
6WIND