linux-kernel - Re: [PATCH v4 1/5] powerpc/smp: Enable Asym packing for cores on shared processor

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Message-ID: <20231115054249.GU2194132@linux.vnet.ibm.com>
Date:   Wed, 15 Nov 2023 11:12:49 +0530
From:   Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Mark Rutland <mark.rutland@....com>,
        Valentin Schneider <vschneid@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "ndesaulniers@...gle.com" <ndesaulniers@...gle.com>,
        linux-kernel@...r.kernel.org, Rohan McLure <rmclure@...ux.ibm.com>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Josh Poimboeuf <jpoimboe@...nel.org>
Subject: Re: [PATCH v4 1/5] powerpc/smp: Enable Asym packing for cores on
 shared processor

* Aneesh Kumar K.V <aneesh.kumar@...ux.ibm.com> [2023-11-15 10:57:08]:

> Srikar Dronamraju <srikar@...ux.vnet.ibm.com> writes:
> 
> > If there are shared processor LPARs, underlying Hypervisor can have more
> > virtual cores to handle than actual physical cores.
> >
> > Starting with Power 9, a big core (aka SMT8 core) has 2 nearly
> > independent thread groups. On a shared processors LPARs, it helps to
> > pack threads to lesser number of cores so that the overall system
> > performance and utilization improves. PowerVM schedules at a big core
> > level. Hence packing to fewer cores helps.
> >
> > For example: Lets says there are two 8-core Shared LPARs that are
> > actually sharing a 8 Core shared physical pool, each running 8 threads
> > each. Then Consolidating 8 threads to 4 cores on each LPAR would help
> > them to perform better. This is because each of the LPAR will get
> > 100% time to run applications and there will no switching required by
> > the Hypervisor.
> >
> 
> Will this patch consolidate things to first 8 threads or just the one
> Big core? /me continues to look at other patches and wonder whether 4/5
> should come before this? 

It will consolidate 1 thread per small core aka SMT domain or 2 threads per
Big core. If the load is such that there are more unbound threads than SMT
domains, asym packing will not kick-in.

4/5 would make sense only once we enable asym_packing above SMT domain.
> 
> 
> >
> > To achieve this, enable SD_ASYM_PACKING flag at CACHE, MC and DIE level
> > when the system is running in shared processor mode and has big cores.
> >
> > Signed-off-by: Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
> 
> 
> -aneesh

-- 
Thanks and Regards
Srikar Dronamraju