Re: [patch] Cache align futex hash buckets

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Ravikiran G Thirumalai wrote:
Following change places each element of the futex_queues hashtable on a different cacheline. Spinlocks of adjacent hash buckets lie on the same cacheline otherwise.


It does not make sense to add swaths of unused memory into a hashtable for
this purpose, does it?

For a minimal, naive solution you just increase the size of the hash table.
This will (given a decent hash function) provide the same reduction in
cacheline contention, while also reducing collisions.

A smarter solution might have a single lock per cacheline, and multiple hash
slots. This could make the indexing code a bit more awkward though.

Signed-off-by: Ravikiran Thirumalai <[email protected]>
Signed-off-by: Shai Fultheim <[email protected]>

Index: linux-2.6.16-rc2/kernel/futex.c
===================================================================
--- linux-2.6.16-rc2.orig/kernel/futex.c	2006-02-07 23:14:04.000000000 -0800
+++ linux-2.6.16-rc2/kernel/futex.c	2006-02-09 14:04:22.000000000 -0800
@@ -100,9 +100,10 @@ struct futex_q {
 struct futex_hash_bucket {
        spinlock_t              lock;
        struct list_head       chain;
-};
+} ____cacheline_internodealigned_in_smp;
-static struct futex_hash_bucket futex_queues[1<<FUTEX_HASHBITS]; +static struct futex_hash_bucket futex_queues[1<<FUTEX_HASHBITS] + __cacheline_aligned_in_smp; /* Futex-fs vfsmount entry: */
 static struct vfsmount *futex_mnt;
-
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