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

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Date:   Tue, 31 Jan 2017 10:04:11 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     John Hubbard <jhubbard@...dia.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 01/30/2017 11:25 PM, John Hubbard wrote:
> 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?

In the end, I don't think the kernel can make the "right" decision very
widely here.

Intel's Xeon Phis have some high-bandwidth memory (MCDRAM) that
evidently has a higher latency than DRAM.  Given a plain malloc(), how
is the kernel to know that the memory will be used for AVX-512
instructions that need lots of bandwidth vs. some random data structure
that's latency-sensitive?

In the end, I think all we can do is keep the kernel's existing default
of "low latency to the CPU that allocated it", and let apps override
when that policy doesn't fit them.