linux-kernel - Re: [RFC V2 03/12] mm: Change generic FALLBACK zonelist creation process

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Message-Id: <c174cdbe-fb7f-2c0f-7ac5-9f8719a06e0f@linux.vnet.ibm.com>
Date:   Wed, 1 Feb 2017 12:16:16 +0530
From:   Anshuman Khandual <khandual@...ux.vnet.ibm.com>
To:     John Hubbard <jhubbard@...dia.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Anshuman Khandual <khandual@...ux.vnet.ibm.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Cc:     mhocko@...e.com, vbabka@...e.cz, mgorman@...e.de,
        minchan@...nel.org, aneesh.kumar@...ux.vnet.ibm.com,
        bsingharora@...il.com, srikar@...ux.vnet.ibm.com,
        haren@...ux.vnet.ibm.com, jglisse@...hat.com,
        dan.j.williams@...el.com
Subject: Re: [RFC V2 03/12] mm: Change generic FALLBACK zonelist creation
 process

On 01/31/2017 12:55 PM, John Hubbard wrote:
> On 01/30/2017 05:57 PM, Dave Hansen wrote:
>> On 01/30/2017 05:36 PM, Anshuman Khandual wrote:
>>>> Let's say we had a CDM node with 100x more RAM than the rest of the
>>>> system and it was just as fast as the rest of the RAM.  Would we still
>>>> want it isolated like this?  Or would we want a different policy?
>>>
>>> But then the other argument being, dont we want to keep this 100X more
>>> memory isolated for some special purpose to be utilized by specific
>>> applications ?
>>
>> I was thinking that in this case, we wouldn't even want to bother with
>> having "system RAM" in the fallback lists.  A device who got its memory
>> usage off by 1% could start to starve the rest of the system.  A sane
>> policy in this case might be to isolate the "system RAM" from the
>> device's.
> 
> I also don't like having these policies hard-coded, and your 100x
> example above helps clarify what can go wrong about it. It would be
> nicer if, instead, we could better express the "distance" between nodes
> (bandwidth, latency, relative to sysmem, perhaps), and let the NUMA
> system figure out the Right Thing To Do.
> 
> I realize that this is not quite possible with NUMA just yet, but I
> wonder if that's a reasonable direction to go with this?

That is complete overhaul of the NUMA representation in the kernel. What
CDM attempts is to find a solution with existing NUMA framework and with
as little code change as possible.