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Subject[PATCH 3/3] FUTEX : NUMA friendly global hashtable
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[PATCH 3/3] FUTEX : NUMA friendly global hashtable

On NUMA machines, we should get better performance using a big futex
hashtable, allocated with vmalloc() so that it is spreaded on several nodes.

I chose a static size of four pages. (Very big NUMA machines have 64k page
size)

This patch should have a temporary effect, as most futexes are expected to be
stored in process private tables. We probably can drop it in five years :)


Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
---
kernel/futex.c | 26 +++++++++++++++++++++++---
1 file changed, 23 insertions(+), 3 deletions(-)--- linux-2.6.21-rc3/kernel/futex.c 2007-03-15 18:54:47.000000000 +0100
+++ linux-2.6.21-rc3-ed/kernel/futex.c 2007-03-15 18:54:47.000000000 +0100
@@ -152,8 +152,13 @@ struct futex_hash_bucket {
# define FUTEX_HASH_SLOTS 16
# define FUTEX_NOPRIVHASH /* no private hashtable, only one global */
#else
-# define FUTEX_HASH_SLOTS 256
-# define FUTEX_PRIVHASH_SLOTS 64
+# ifdef CONFIG_NUMA
+# define FUTEX_HASH_SLOTS ((4*PAGE_SIZE)/sizeof(struct futex_hash_bucket))
+# define FUTEX_PRIVHASH_SLOTS (4096 / sizeof(struct futex_hash_bucket))
+# else
+# define FUTEX_HASH_SLOTS 256
+# define FUTEX_PRIVHASH_SLOTS 64
+# endif
#endif

#define FUTEX_PRIVHASH_SIZE \
@@ -166,8 +171,14 @@ struct futex_hash_bucket {
* PTHREAD_PROCESS_PRIVATE futexes may be hashed into this table too if the
* owner process failed to allocate its private hashtable (or CONFIG_BASE_SMALL)
*
+ * On NUMA configs, table is allocated with vmalloc() to spread this hash table
+ * up to 4 nodes (we use 4 pages)
*/
+#ifdef CONFIG_NUMA
+static struct futex_hash_bucket *futex_queues __read_mostly;
+#else
static struct futex_hash_bucket futex_queues[FUTEX_HASH_SLOTS];
+#endif

/* Futex-fs vfsmount entry: */
static struct vfsmount *futex_mnt;
@@ -2051,7 +2062,16 @@ static int __init init(void)
return PTR_ERR(futex_mnt);
}

- for (i = 0; i < ARRAY_SIZE(futex_queues); i++) {
+#ifdef CONFIG_NUMA
+ /*
+ * vmalloc() is supposed to obey mempolicy and spread our 4 pages
+ * on several nodes
+ */
+ futex_queues = vmalloc(FUTEX_HASH_SLOTS * sizeof(*futex_queues));
+ if (!futex_queues)
+ panic("Failed to allocate futex hash table\n");
+#endif
+ for (i = 0; i < FUTEX_HASH_SLOTS; i++) {
INIT_LIST_HEAD(&futex_queues[i].chain);
spin_lock_init(&futex_queues[i].lock);
}
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