linux-kernel - Re: DISCONTIGMEM is deprecated

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Message-ID: <20190421211604.GN18914@techsingularity.net>
Date:   Sun, 21 Apr 2019 22:16:04 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Mike Rapoport <rppt@...ux.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mikulas Patocka <mpatocka@...hat.com>,
        James Bottomley <James.Bottomley@...senpartnership.com>,
        linux-parisc@...r.kernel.org, linux-mm@...ck.org,
        Vlastimil Babka <vbabka@...e.cz>,
        LKML <linux-kernel@...r.kernel.org>, linux-arch@...r.kernel.org
Subject: Re: DISCONTIGMEM is deprecated

On Sun, Apr 21, 2019 at 06:26:07AM -0700, Matthew Wilcox wrote:
> On Sun, Apr 21, 2019 at 09:38:59AM +0300, Mike Rapoport wrote:
> > On Fri, Apr 19, 2019 at 07:05:21AM -0700, Matthew Wilcox wrote:
> > > On Fri, Apr 19, 2019 at 10:43:35AM +0100, Mel Gorman wrote:
> > > > DISCONTIG is essentially deprecated and even parisc plans to move to
> > > > SPARSEMEM so there is no need to be fancy, this patch simply disables
> > > > watermark boosting by default on DISCONTIGMEM.
> > > 
> > > I don't think parisc is the only arch which uses DISCONTIGMEM for !NUMA
> > > scenarios.  Grepping the arch/ directories shows:
> > > 
> > > alpha (does support NUMA, but also non-NUMA DISCONTIGMEM)
> > > arc (for supporting more than 1GB of memory)
> > > ia64 (looks complicated ...)
> > > m68k (for multiple chunks of memory)
> > > mips (does support NUMA but also non-NUMA)
> > > parisc (both NUMA and non-NUMA)
> > 
> > i386 NUMA as well
> 
> I clearly over-trimmed.  The original assumption that Mel had was that
> DISCONTIGMEM => NUMA, and that's not true on the above six architectures.
> It is true on i386 ;-)

32-bit NUMA systems should be non-existent in practice. The last NUMA
system I'm aware of that was both NUMA and 32-bit only died somewhere
between 2004 and 2007. If someone is running a 64-bit capable system in
32-bit mode with NUMA, they really are just punishing themselves for fun.

-- 
Mel Gorman
SUSE Labs