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

[Mel Gorman <mgorman@suse.de>]

On Thu, Sep 04, 2014 at 11:29:29AM -0400, Johannes Weiner wrote:
> On Tue, Sep 02, 2014 at 04:21:43PM +0100, Mel Gorman wrote:
> > 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:
> > > > 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?
> 
> Is there a strong reason against defaulting both to node order?  Zone
> ordering, if anything, is a niche application.  We might even be able
> to remove it in the future.  We still have the backup of allowing the
> user to explicitely request zone ordering on the commandline, should
> someone depend on it unexpectedly.

Low memory depletion is the reason to default to zone order on 32-bit
NUMA. If processes on node 0 deplete the Normal zone from normal
activity then other nodes must keep reclaiming from Normal for all kernel
allocations. The problem is worse if CONFIG_HIGHPTE is not set.

A default of node-ordering on 32-bit NUMA increases low memory pressure
leading to increased reclaim and potentially easier to trigger OOM. I
expect this problem was worse in the past when the normal zone could be
filled with dirty pages under writeback.  However low memory pressure is
still enough of a concern that I'm wary of changing the default of 32-bit
NUMA without knowing who even cares about 32-bit NUMA.

-- 
Mel Gorman
SUSE Labs

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