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

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Date:   Thu, 6 Sep 2018 11:21:29 +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 13:14, Srikar Dronamraju
<srikar@...ux.vnet.ibm.com> wrote:
>
> * Vincent Guittot <vincent.guittot@...aro.org> [2018-09-05 11:11:35]:
>
> > 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 ?
> >
>
> I believe that Core capacity irrespective of the number of threads
> should be similar. We wanted to give a small benefit if the core has
> multiple threads and that was smt_gain. Lets say we have 8 equal sw
> threads running on 2 cores; one being SMT 2 and other being SMT4.
> then 4 threads should be spread to each core. So that we would be fair
> to each of the 8 SW threads.

Do you mean that it would be the same with SMT 2 and SMT 8 ?
evenly spread the 8 SW threads between the 2 cores would be better
than 2 SW threads on core SMT 2 and 6 on core SMT8

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