Re: [patch] Cache align futex hash buckets

[Ravikiran G Thirumalai]

On Tue, Feb 21, 2006 at 02:30:00PM +1100, Nick Piggin wrote:
> 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?

I don't know if having two (or more) spinlocks on the same cacheline is a good 
idea.  Right now, on a 128 B cacheline we have 10 spinlocks on the
same cacheline here!! Things get worse if two futexes from different nodes 
hash on to adjacent, or even nearly adjacent hash buckets.

> 
> 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.

Given a decent hash function.  I am not sure the hashing function is smart 
enough as of now.  Hashing is not a function of nodeid, and we have some 
instrumentation results which show hashing on NUMA is not good as yet, and
there are collisions from other nodes onto the same hashbucket; Nearby 
buckets have high hit rates too.

I think some sort of NUMA friendly hashing, where futexes from same nodes
hash onto a node local hash table, would be a decent solution here.
As I mentioned earlier, we are working on that, and we can probably allocate
the spinlock from nodelocal memory then and avoid this bloat.
We are hoping to have this as a stop gap fix until we get there.

Thanks,
Kiran

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