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Date:	Tue, 22 Jul 2014 10:39:17 +0200 (CEST)
From:	Thomas Gleixner <>
To:	Peter Zijlstra <>
cc:	Steven Rostedt <>,
	Darren Hart <>,
	Andy Lutomirski <>,
	Andi Kleen <>,
	Waiman Long <>,
	Ingo Molnar <>,
	Davidlohr Bueso <>,
	Heiko Carstens <>,
	"" <>,
	Linux API <>,
	"" <>,
	Jason Low <>,
	Scott J Norton <>,
	Robert Haas <>
Subject: Re: [RFC PATCH 0/5] futex: introduce an optimistic spinning futex

On Tue, 22 Jul 2014, Peter Zijlstra wrote:
> Anyway, there is one big fail in the entire futex stack that we 'need'
> to sort some day and that is NUMA. Some people (again database people)
> explicitly do not use futexes and instead use sysvsem because of this.
> The problem with numa futexes is that because they're vaddr based there
> is no (persistent) node information. You always end up having to fall
> back to looking in all nodes before you can guarantee there is no
> matching futex.
> One way to achieve it is by extending the futex value to include a node
> number, but that's obviously a complete ABI break. Then again, it should
> be pretty straight fwd, since the node number doesn't need to be part of
> the actual atomic update part, just part of the userspace storage.

So you want per node hash buckets, right? Fair enough, but how do you
make sure, that no thread/process on a different node is fiddling with
that "node bound" futex as well?



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