Re: [PATCH] futex: eliminate cache miss from futex_hash()

[Davidlohr Bueso]

On Wed, 09 Sep 2015, Rasmus Villemoes wrote:

> futex_hash() references two global variables: the base pointer
> futex_queues and the size of the array futex_hashsize. The latter is
> marked __read_mostly, while the former is not, so they are likely to
> end up very far from each other. This means that futex_hash() is
> likely to encounter two cache misses.
>
> We could mark futex_queues as __read_mostly as well, but that doesn't
> guarantee they'll end up next to each other (and even if they do, they
> may still end up in different cache lines). So put the two variables
> in a small singleton struct with sufficient alignment and mark that as
> __read_mostly.

This really doesn't have much practical effect -- not even on larger
boxes, where such things matter. For instance, I ran the patch on a
60-core IvyBridge with 'perf-bench futex', for which futex-hash
particularly benefits in good data layout (ie our current smp alignment).

http://linux-scalability.org/futex-__futex_data/

I think we should leave it as is.

Thanks,
Davidlohr
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