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Subject: Re: [RFC][PATCH 0/8] Migrate Pages in lieu of discard
To:     Shakeel Butt <shakeelb@google.com>
Cc:     Dave Hansen <dave.hansen@linux.intel.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux MM <linux-mm@kvack.org>,
        Yang Shi <yang.shi@linux.alibaba.com>,
        David Rientjes <rientjes@google.com>,
        Huang Ying <ying.huang@intel.com>,
        Dan Williams <dan.j.williams@intel.com>
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Message-ID: <fdfeb902-e363-f8b5-4c63-df224b9817fa@intel.com>
Date:   Tue, 30 Jun 2020 12:31:05 -0700
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On 6/30/20 12:25 PM, Shakeel Butt wrote:
>> Since there is never a demotion path out of the set of
>> all nodes, the common case would be that there is no demotion path out
>> of a reclaim node set.
>>
>> If that's true, I'd say that the kernel still needs to consider
>> migrations even within the set.
> In my opinion it should be a user defined policy but I think that
> discussion is orthogonal to this patch series. As I understand, this
> patch series aims to add the migration-within-reclaim infrastructure,
> IMO the policies, optimizations, heuristics can come later.

Yes, this should be considered to add the infrastructure and one
_simple_ policy implementation which sets up migration away from nodes
with CPUs to more distant nodes without CPUs.

This simple policy will be useful for (but not limited to) volatile-use
persistent memory like Intel's Optane DIMMS.

> BTW is this proposal only for systems having multi-tiers of memory?
> Can a multi-node DRAM-only system take advantage of this proposal? For
> example I have a system with two DRAM nodes running two jobs
> hardwalled to each node. For each job the other node is kind of
> low-tier memory. If I can describe the per-job demotion paths then
> these jobs can take advantage of this proposal during occasional
> peaks.

I don't see any reason it could not work there.  There would just need
to be a way to set up a different demotion path policy that what was
done here.