linux-kernel - Re: [RFC PATCH 3/4] sched/topology: remove smt

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Date:   Wed, 29 Aug 2018 16:42:01 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     qais.yousef@....com
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 3/4] sched/topology: remove smt_gain

On Wed, 29 Aug 2018 at 16:08, Qais Yousef <qais.yousef@....com> wrote:
>

> You beat me to it, I got confused by smt_gain recently when I stumbled
> on it as I found out on ARM it's not used and had to spend sometime to
> convince myself it's not really necessary to use it.
>
> It was hard to track the history of this and *why* it's needed.
>
> The only 'theoretical' case I found smt_gain can be useful is when you
> have asymmetric system, for example:
>
> Node_A: 1 Core 2 Threads
> Node_B: 1 Core 4 Threads
>
> Then with smt_gain the group_capacity at the core level will be limited
> to smt_gain. But without smt_gain Node_B will look twice as powerful as
> Node_A - which will affect balancing AFAICT causing Node_B's single core
> to be oversubscribed as the 4 threads will still have to share the same
> underlying hardware resources. I don't think in practice such systems
> exists, or even make sense, though.

Yes I came to the same conclusion

>
> So +1 from my side for the removal.

Thanks

>
>
> --
> Qais Yousef
>