Hello Nick and all,
I remember this was discussed some months ago, but it still seems that 
on 2.6.10, SD_NODES_PER_DOMAIN is statically defined to value 6.
This is not what is expected on Bull ia64 platforms, based on modules of 
4 bricks of 4 cpus each.
Using the cpu hot-plug mechanism to re-define dynamically the 
sched-domains looks heavy (please correct me if I am wrong).
Hereafter a trivial patch that allows to setup SD_NODES_PER_DOMAIN at 
configuration time or at boot time.
Boot time parameter should be helpfull, as on 32*ways machine based on 2 
modules of 4 bricks of 4 cpus each, it allows to build either 1 
(SD_NODES_PER_DOMAIN=8) or 2 (SD_NODES_PER_DOMAIN=4) NUMA sched-domain 
levels.
Thanks in advance for your comments.

diff --exclude-from /home15/xb/proc/patch.exclude -Nurp 
/tmp/linux-2.6.10/arch/ia64/Kconfig linux-2.6.10/arch/ia64/Kconfig
--- /tmp/linux-2.6.10/arch/ia64/Kconfig    2004-12-24 22:35:29.000000000 
+0100
+++ linux-2.6.10/arch/ia64/Kconfig    2005-02-15 17:07:46.741070673 +0100
@@ -168,6 +168,17 @@ config NUMA
       Access).  This option is for configuring high-end multiprocessor
       server systems.  If in doubt, say N.
 
+config SD_NODES_PER_DOMAIN
+    int "Number of nodes per base sched_domains"
+    default "4" if IA64_DIG
+    default "6"
+    help
+      Number of nodes per base sched_domains.
+      Should be 6 for SGI platforms.
+      Should be 4 for DIG platforms.
+      This value can be provided at boot time using the sd_nodes_per_domain
+      boot parameter.
+       
 config VIRTUAL_MEM_MAP
     bool "Virtual mem map"
     default y if !IA64_HP_SIM
diff --exclude-from /home15/xb/proc/patch.exclude -Nurp 
/tmp/linux-2.6.10/arch/ia64/kernel/domain.c 
linux-2.6.10/arch/ia64/kernel/domain.c
--- /tmp/linux-2.6.10/arch/ia64/kernel/domain.c    2004-12-24 
22:35:40.000000000 +0100
+++ linux-2.6.10/arch/ia64/kernel/domain.c    2005-02-15 
15:04:08.964794354 +0100
@@ -13,7 +13,14 @@
 #include <linux/init.h>
 #include <linux/topology.h>
 
-#define SD_NODES_PER_DOMAIN 6
+int sd_nodes_per_domain = CONFIG_SD_NODES_PER_DOMAIN;
+
+static int __init set_sd_nodes_per_domain(char *str)
+{
+    get_option(&str, &sd_nodes_per_domain);
+    return 1;
+}
+__setup("sd_nodes_per_domain=", set_sd_nodes_per_domain);
 
 #ifdef CONFIG_NUMA
 /**
@@ -78,7 +85,7 @@ static cpumask_t __devinit sched_domain_
     cpus_or(span, span, nodemask);
     set_bit(node, used_nodes);
 
-    for (i = 1; i < SD_NODES_PER_DOMAIN; i++) {
+    for (i = 1; i < sd_nodes_per_domain; i++) {
         int next_node = find_next_best_node(node, used_nodes);
         nodemask = node_to_cpumask(next_node);
         cpus_or(span, span, nodemask);
@@ -159,7 +166,7 @@ void __devinit arch_init_sched_domains(v
 
 #ifdef CONFIG_NUMA
         if (num_online_cpus()
-                > SD_NODES_PER_DOMAIN*cpus_weight(nodemask)) {
+                > sd_nodes_per_domain*cpus_weight(nodemask)) {
             sd = &per_cpu(allnodes_domains, i);
             *sd = SD_ALLNODES_INIT;
             sd->span = cpu_default_map;

-- 

	Sincères salutations.