Re: [patch] Cache align futex hash buckets

[Nick Piggin]

Christoph Lameter wrote:
> On Tue, 21 Feb 2006, 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?
>
>
> It does if you essentially have a 4k cacheline (because you are doing NUMA 
> in software with multiple PCs....) and transferring control of that 
> cacheline is comparatively expensive.

Instead of 1MB hash with 256 entries in it covering 256 cachelines, you
have a 1MB hash with 65536(ish) entries covering 256 cachelines.

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