netdev - Re: [for-next V2 10/10] RDMA/core: Provide RDMA DIM support for ULPs

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Date:   Thu, 4 Jul 2019 07:51:59 +0000
From:   Leon Romanovsky <leonro@...lanox.com>
To:     Sagi Grimberg <sagi@...mberg.me>
CC:     Idan Burstein <idanb@...lanox.com>,
        Saeed Mahameed <saeedm@...lanox.com>,
        "David S. Miller" <davem@...emloft.net>,
        Doug Ledford <dledford@...hat.com>,
        Jason Gunthorpe <jgg@...lanox.com>,
        Or Gerlitz <ogerlitz@...lanox.com>,
        Tal Gilboa <talgi@...lanox.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-rdma@...r.kernel.org" <linux-rdma@...r.kernel.org>,
        Yamin Friedman <yaminf@...lanox.com>,
        Max Gurtovoy <maxg@...lanox.com>
Subject: Re: [for-next V2 10/10] RDMA/core: Provide RDMA DIM support for ULPs

On Wed, Jul 03, 2019 at 11:56:04AM -0700, Sagi Grimberg wrote:
>
> > Hi Sagi,
> >
> > I'm not sharing your worries about bad out-of-the-box experience for a
> > number of reasons.
> >
> > First of all, this code is part of upstream kernel and will take time
> > till users actually start to use it as is and not as part of some distro
> > backports or MOFED packages.
>
> True, but I am still saying that this feature is damaging sync IO which
> represents the majority of the users. It might not be an extreme impact
> but it is still a degradation (from a very limited testing I did this
> morning I'm seeing a consistent 5%-10% latency increase for low QD
> workloads which is consistent with what Yamin reported AFAIR).
>
> But having said that, the call is for you guys to make as this is a
> Mellanox device. I absolutely think that this is useful (as I said
> before), I just don't think its necessarily a good idea to opt it by
> default given that only a limited set of users would take full advantage
> of it while the rest would see a negative impact (even if its 10%).
>
> I don't have  a hard objection here, just wanted to give you my
> opinion on this because mlx5 is an important driver for rdma
> users.

Your opinion is very valuable for us and we started internal thread to
challenge this "enable by default", it just takes time and I prefer to
enable this code to get test coverage as wide as possible.

>
> > Second, Yamin did extensive testing and worked very close with Or G.
> > and I have very high confident in the results of their team work.
>
> Has anyone tested other RDMA ulps? NFS/RDMA or SRP/iSER?
>
> Would be interesting to understand how other subsystems with different
> characteristics behave with this.

Me too, and I'll revert this default if needed.

Thanks