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

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Date:   Wed, 5 Sep 2018 11:11:35 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     Srikar Dronamraju <srikar@...ux.vnet.ibm.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, 5 Sep 2018 at 10:50, Srikar Dronamraju
<srikar@...ux.vnet.ibm.com> wrote:
>
> * Vincent Guittot <vincent.guittot@...aro.org> [2018-09-05 09:36:42]:
>
> > >
> > > I dont know of any systems that have come with single threaded and
> > > multithreaded. However some user can still offline few threads in a core
> > > while leaving other cores untouched. I dont really know why somebody
> > > would want to do it.  For example, some customer was toying with SMT 3
> > > mode in a SMT 8 power8 box.
> >
> > In this case, it means that we have the same core capacity whatever
> > the number of CPUs
> > and a core with SMT 3 will be set with the same compute capacity as
> > the core with SMT 8.
> > Does it still make sense ?
> >
>
> To me it make sense atleast from  a power 8 perspective, because SMT 1 >
> SMT 2 > SMT 4 > SMT8. So if one core is configured for SMT 2 and other
> core is configured for SMT4; all threads being busy, the  individual
> threads running on SMT2 core will complete more work than SMT 4 core
> threads.

I agree for individual thread capacity but at core group level, the
core SMT 1 will have the same capacity as core group SMT 8 so load
balance will try to balance evenly the tasks between the 2 cores
whereas core SMT 8 > core SMT1 , isn't it ?

>
> --
> Thanks and Regards
> Srikar Dronamraju
>