Messages in this thread | | | Date | Wed, 10 Sep 2003 12:07:44 +0100 | From | Jamie Lokier <> | Subject | Re: [PATCH] Split futex global spinlock futex_lock |
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Hu, Boris wrote: > Split futex global spinlock futex_lock into hash bucket spinlocks.
> +/* > + * Split the global futex_lock into every hash list lock. > + */ > +struct futex_hash_bucket { > + struct list_head chain; > + spinlock_t lock; > +};
Put "lock" first: it is always the first field accessed. That will save a few clock cycles on some systems.
I was going to suggest something about cache alignment, but of course that doesn't make sense. If the structure is made any larger, it might as well contain more hash buckets.
Thinking a little deeper, it occurs to me that for scalable SMP performance, you want:
(1 << FUTEX_HASHBITS) > some factor * SMP_CACHE_BYTES * NR_CPUS / sizeof (bucket)
To put it into perspective, consider a hypothetical 16-way P4. SMP_CACHE_BYTES is 128 on a P4. That's 24 cache lines in the whole hash table.
If there are only a few futexes in the table at any time, the dominant time for each operation is going to be the spinlock and associated cache line transfers, not traversing a bucket's list.
So that hypothetical box would have an effective hash table of only 24 buckets.
What I'm saying is:
If you're able to benchmark changes to the code on a big box, and you want to tune it's performance, try changing FUTEX_HASHBITS. Also try adding a dummy word into futex_hash_bucket, and use __cache_aligned_in_smp on the array so that no buckets straddle two cache lines.
Thought for the day... -- Jamie - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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