Re: [PATCH v4] zone_reclaim is always 0 by default

[Robin Holt]

On Tue, Jun 09, 2009 at 11:37:55AM +0100, Mel Gorman wrote:
> On Tue, Jun 09, 2009 at 04:55:07AM -0500, Robin Holt wrote:
> > On Mon, Jun 08, 2009 at 12:50:48PM +0100, Mel Gorman wrote:
> > 
> > Let me start by saying I agree completely with everything you wrote and
> > still disagree with this patch, but was willing to compromise and work
> > around this for our upcoming x86_64 machine by putting a "value add"
> > into our packaging of adding a sysctl that turns reclaim back on.
> > 
> 
> To be honest, I'm more leaning towards a NACK than an ACK on this one. I
> don't support enough NUMA machines to feel strongly enough about it but
> unconditionally setting zone_reclaim_mode to 0 on x86-64 just because i7's
> might be there seems ill-advised to me and will have other consequences for
> existing more traditional x86-64 NUMA machines.

I was sort-of planning on coming up with an x86_64 arch specific function
for setting zone_reclaim_mode, but didn't like the direction things
were going.

Something to the effect of...
--- 20090609.orig/mm/page_alloc.c       2009-06-09 06:51:34.000000000 -0500
+++ 20090609/mm/page_alloc.c    2009-06-09 06:55:00.160762069 -0500
@@ -2326,12 +2326,7 @@ static void build_zonelists(pg_data_t *p
        while ((node = find_next_best_node(local_node, &used_mask)) >= 0) {
                int distance = node_distance(local_node, node);
 
-               /*
-                * If another node is sufficiently far away then it is better
-                * to reclaim pages in a zone before going off node.
-                */
-               if (distance > RECLAIM_DISTANCE)
-                       zone_reclaim_mode = 1;
+               zone_reclaim_mode = arch_zone_reclaim_mode(distance);
 
                /*
                 * We don't want to pressure a particular node.

And then letting each arch define an arch_zone_reclaim_mode().  If other
values are needed in the determination, we would add parameters to
reflect this.

For ia64, add

static inline ia64_zone_reclaim_mode(int distance)
{
	if (distance > 15)
		return 1;
}

#define	arch_zone_reclaim_mode(_d)	ia64_zone_reclaim_mode(_d)


Then, inside x86_64_zone_reclaim_mode(), I could make it something like
	if (distance > 40 || is_uv_system())
		return 1;

In the end, I didn't think this fight was worth fighting given how ugly
this felt.  Upon second thought, I am beginning to think it is not that
bad, but I also don't think it is that good either.

Thanks,
Robin
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