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    SubjectRe: [RFC PATCH 3/4] sched/topology: remove smt_gain
    * Vincent Guittot <vincent.guittot@linaro.org> [2018-09-05 11:11:35]:

    > On Wed, 5 Sep 2018 at 10:50, Srikar Dronamraju
    > <srikar@linux.vnet.ibm.com> wrote:
    > >
    > > * Vincent Guittot <vincent.guittot@linaro.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.

    --
    Thanks and Regards
    Srikar Dronamraju

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