Re: [PATCH] mm: page_alloc: Default to node-ordering on 64-bit NUMA machines

[Mel Gorman]

On Tue, Sep 02, 2014 at 09:51:20AM -0400, Johannes Weiner wrote:
> On Mon, Sep 01, 2014 at 01:55:51PM +0100, Mel Gorman wrote:
> > Zones are allocated by the page allocator in either node or zone order.
> > Node ordering is preferred in terms of locality and is applied automatically
> > in one of three cases.
> > 
> >   1. If a node has only low memory
> > 
> >   2. If DMA/DMA32 is a high percentage of memory
> > 
> >   3. If low memory on a single node is greater than 70% of the node size
> > 
> > Otherwise zone ordering is used to preserve low memory. Unfortunately
> > a consequence of this is that a machine with balanced NUMA nodes will
> > experience different performance characteristics depending on which node
> > they happen to start from.
> > 
> > The point of zone ordering is to protect lower nodes for devices that require
> > DMA/DMA32 memory. When NUMA was first introduced, this was critical as 32-bit
> > NUMA machines commonly suffered from low memory exhaustion problems. On
> > 64-bit machines the primary concern is devices that are 32-bit only which
> > is less severe than the low memory exhaustion problem on 32-bit NUMA. It
> > seems there are really few devices that depends on it.
> > 
> > AGP -- I assume this is getting more rare but even then I think the allocations
> > 	happen early in boot time where lowmem pressure is less of a problem
> > 
> > DRM -- If the device is 32-bit only then there may be low pressure. I didn't
> > 	evaluate these in detail but it looks like some of these are mobile
> > 	graphics card. Not many NUMA laptops out there. DRM folk should know
> > 	better though.
> > 
> > Some TV cards -- Much demand for 32-bit capable TV cards on NUMA machines?
> > 
> > B43 wireless card -- again not really a NUMA thing.
> > 
> > I cannot find a good reason to incur a performance penalty on all 64-bit NUMA
> > machines in case someone throws a brain damanged TV or graphics card in there.
> > This patch defaults to node-ordering on 64-bit NUMA machines. I was tempted
> > to make it default everywhere but I understand that some embedded arches may
> > be using 32-bit NUMA where I cannot predict the consequences.
> 
> This patch is a step in the right direction, but I'm not too fond of
> further fragmenting this code and where it applies, while leaving all
> the complexity from the heuristics and the zonelist building in, just
> on spec.  Could we at least remove the heuristics too?  If anybody is
> affected by this, they can always override the default on the cmdline.

I see no problem with deleting the heuristics. Default node for 64-bit
and default zone for 32-bit sound ok to you?

-- 
Mel Gorman
SUSE Labs
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