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Date:   Wed, 1 Nov 2023 14:56:13 +0100
From:   Michal Hocko <mhocko@suse.com>
To:     Johannes Weiner <hannes@cmpxchg.org>
Cc:     Gregory Price <gourry.memverge@gmail.com>,
        linux-kernel@vger.kernel.org, linux-cxl@vger.kernel.org,
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        weixugc@google.com, apopple@nvidia.com, tim.c.chen@intel.com,
        dave.hansen@intel.com, shy828301@gmail.com,
        gregkh@linuxfoundation.org, rafael@kernel.org,
        Gregory Price <gregory.price@memverge.com>
Subject: Re: [RFC PATCH v3 0/4] Node Weights and Weighted Interleave
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On Tue 31-10-23 12:22:16, Johannes Weiner wrote:
> On Tue, Oct 31, 2023 at 04:56:27PM +0100, Michal Hocko wrote:
[...]
> > Is there any specific reason for not having a new interleave interface
> > which defines weights for the nodemask? Is this because the policy
> > itself is very dynamic or is this more driven by simplicity of use?
> 
> A downside of *requiring* weights to be paired with the mempolicy is
> that it's then the application that would have to figure out the
> weights dynamically, instead of having a static host configuration. A
> policy of "I want to be spread for optimal bus bandwidth" translates
> between different hardware configurations, but optimal weights will
> vary depending on the type of machine a job runs on.

I can imagine this could be achieved by numactl(8) so that the process
management tool could set this up for the process on the start up. Sure
it wouldn't be very dynamic after then and that is why I was asking
about how dynamic the situation might be in practice.

> That doesn't mean there couldn't be usecases for having weights as
> policy as well in other scenarios, like you allude to above. It's just
> so far such usecases haven't really materialized or spelled out
> concretely. Maybe we just want both - a global default, and the
> ability to override it locally. Could you elaborate on the 'get what
> you pay for' usecase you mentioned?

This is more or less just an idea that came first to my mind when
hearing about bus bandwidth optimizations. I suspect that sooner or
later we just learn about usecases where the optimization function
maximizes not only bandwidth but also cost for that bandwidth. Consider
a hosting system serving different workloads each paying different QoS.
Do I know about anybody requiring that now? No! But we should really
test the proposed interface for potential future extensions. If such an
extension is not reasonable and/or we can achieve that by different
means then great.
-- 
Michal Hocko
SUSE Labs