netdev - Re: [PATCH rdma-next v2 2/3] RDMA/rw: Support threshold for registration vs scattering to local pages

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Message-ID: <20191008055317.GD5855@unreal>
Date:   Tue, 8 Oct 2019 08:53:17 +0300
From:   Leon Romanovsky <leon@...nel.org>
To:     Bart Van Assche <bvanassche@....org>
Cc:     Doug Ledford <dledford@...hat.com>,
        Jason Gunthorpe <jgg@...lanox.com>,
        Christoph Hellwig <hch@...radead.org>,
        RDMA mailing list <linux-rdma@...r.kernel.org>,
        Or Gerlitz <ogerlitz@...lanox.com>,
        Yamin Friedman <yaminf@...lanox.com>,
        Saeed Mahameed <saeedm@...lanox.com>,
        linux-netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH rdma-next v2 2/3] RDMA/rw: Support threshold for
 registration vs scattering to local pages

On Mon, Oct 07, 2019 at 03:22:30PM -0700, Bart Van Assche wrote:
> On 10/7/19 9:03 AM, Leon Romanovsky wrote:
> > On Mon, Oct 07, 2019 at 08:07:55AM -0700, Bart Van Assche wrote:
> > > On 10/7/19 6:59 AM, Leon Romanovsky wrote:
> > > >    /*
> > > > - * Check if the device might use memory registration.  This is currently only
> > > > - * true for iWarp devices. In the future we can hopefully fine tune this based
> > > > - * on HCA driver input.
> > > > + * Check if the device might use memory registration. This is currently
> > > > + * true for iWarp devices and devices that have optimized SGL registration
> > > > + * logic.
> > > >     */
> > >
> > > The following sentence in the above comment looks confusing to me: "Check if
> > > the device might use memory registration." That sentence suggests that the
> > > HCA decides whether or not to use memory registration. Isn't it the RDMA R/W
> > > code that decides whether or not to use memory registration?
> >
> > I'm open for any reasonable text, what do you expect to be written there?
>
> Hi Leon,
>
> How about the following (not sure whether this is correct)?
>
> /*
>  * Report whether memory registration should be used. Memory
>  * registration must be used for iWarp devices because of
>  * iWARP-specific limitations. Memory registration is also enabled if
>  * registering memory will yield better performance than using multiple
>  * SGE entries.
>  */

"Better performance" is relevant for mlx5 only, maybe others will use
this max_.. field to overcome their HW limitations.

Thanks

>
> Thanks,
>
> Bart.