Re: [patch] Cache align futex hash buckets

[Nick Piggin]

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 <kiran@xxxxxxxxxxxx>
> Signed-off-by: Shai Fultheim <shai@xxxxxxxxxxxx>
>
> 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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