From: David Gibson <david@gibson.dropbear.id.au>
To: Daniel Henrique Barboza <danielhb413@gmail.com>
Cc: qemu-ppc@nongnu.org, qemu-devel@nongnu.org, groug@kaod.org
Subject: Re: [PATCH v2 2/6] spapr_numa: forbid asymmetrical NUMA setups
Date: Mon, 28 Sep 2020 16:25:22 +1000 [thread overview]
Message-ID: <20200928062522.GB501872@yekko.fritz.box> (raw)
In-Reply-To: <5097b6b6-bd8a-fbac-af11-5e9c1e054559@gmail.com>
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On Sun, Sep 27, 2020 at 08:41:30AM -0300, Daniel Henrique Barboza wrote:
>
>
> On 9/26/20 4:49 AM, David Gibson wrote:
> > On Fri, Sep 25, 2020 at 09:41:02AM -0300, Daniel Henrique Barboza wrote:
> > >
> > >
> > > On 9/25/20 12:48 AM, David Gibson wrote:
> > > > On Thu, Sep 24, 2020 at 04:50:54PM -0300, Daniel Henrique Barboza wrote:
> > > > > The pSeries machine does not support asymmetrical NUMA
> > > > > configurations. This doesn't make much of a different
> > > > > since we're not using user input for pSeries NUMA setup,
> > > > > but this will change in the next patches.
> > > > >
> > > > > To avoid breaking existing setups, gate this change by
> > > > > checking for legacy NUMA support.
> > > > >
> > > > > Reviewed-by: Greg Kurz <groug@kaod.org>
> > > > > Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
> > > >
> > > > Having read the rest of the series, I realized there's another type of
> > > > configuration that PAPR can't represent, so possibly we should add
> > > > logic to catch that as well. That's what I'm going to call
> > > > "non-transitive" configurations, e.g.
> > > >
> > > > Node 0 1 2
> > > > 0 10 20 40
> > > > 1 20 10 20
> > > > 2 40 20 10
> > > >
> > > > Basically the closeness of 0 to 1 and 1 to 2 forces them all to be in
> > > > the same domain at every PAPR level, even though 0-2 is supposed to be
> > > > more expensive.
> > >
> > > Yes, this is correct. I'm not sure how to proceed in this case
> > > though. Should we error out?
> >
> > Given that we're already erroring on asymmetric configurations, I
> > think it makes sense to error for these as well.
>
> Thing is that asymmetrical configurations is an easy concept to enforce
> to the user - distance from A to B can't be different from B to A.
>
> In the example you gave above, with 3 NUMA nodes, is easy to spot where
> the non-transitivity rule would hit. I'm afraid that if we add 2-3 more
> NUMA nodes in the mix this will stop being straightforward, with more and
> more combinations hitting the 'non-transitivity' rule, and erroring out
> will end up being frustrating to the user.
>
> I'd say that we should report this in the documentation as one more
> limitation of the implementation (and PAPR). I wouldn't oppose with
> throwing a warning message either, letting the user know that the
> approximation will be less precise than it already would be in this
> case.
Hmm... yes, I see your point. Ok, let's go with your suggestion.
--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson
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next prev parent reply other threads:[~2020-09-28 6:34 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-09-24 19:50 [PATCH v2 0/6] pseries NUMA distance calculation Daniel Henrique Barboza
2020-09-24 19:50 ` [PATCH v2 1/6] spapr: add spapr_machine_using_legacy_numa() helper Daniel Henrique Barboza
2020-09-24 19:50 ` [PATCH v2 2/6] spapr_numa: forbid asymmetrical NUMA setups Daniel Henrique Barboza
2020-09-25 2:36 ` David Gibson
2020-09-25 3:48 ` David Gibson
2020-09-25 12:41 ` Daniel Henrique Barboza
2020-09-26 7:49 ` David Gibson
2020-09-27 11:41 ` Daniel Henrique Barboza
2020-09-28 6:25 ` David Gibson [this message]
2020-09-24 19:50 ` [PATCH v2 3/6] spapr_numa: translate regular NUMA distance to PAPR distance Daniel Henrique Barboza
2020-09-25 2:35 ` David Gibson
2020-09-25 12:44 ` Daniel Henrique Barboza
2020-09-24 19:50 ` [PATCH v2 4/6] spapr_numa: change reference-points and maxdomain settings Daniel Henrique Barboza
2020-09-25 2:38 ` David Gibson
2020-09-25 13:16 ` Greg Kurz
2020-09-24 19:50 ` [PATCH v2 5/6] spapr_numa: consider user input when defining associativity Daniel Henrique Barboza
2020-09-25 3:39 ` David Gibson
2020-09-25 14:42 ` Daniel Henrique Barboza
2020-09-24 19:50 ` [PATCH v2 6/6] specs/ppc-spapr-numa: update with new NUMA support Daniel Henrique Barboza
2020-09-25 3:43 ` David Gibson
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