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

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Message-ID: <8d525d64-6da1-48c3-952d-8c6b0d541859@grimberg.me>
Date:   Wed, 3 Jul 2019 11:56:04 -0700
From:   Sagi Grimberg <sagi@...mberg.me>
To:     Leon Romanovsky <leonro@...lanox.com>
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

> 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.

> 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.