linux-kernel - Re: [RFC PATCH 0/5] futex: introduce an optimistic spinning futex

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Date:	Tue, 22 Jul 2014 10:39:17 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	Steven Rostedt <rostedt@...dmis.org>,
	Darren Hart <dvhart@...ux.intel.com>,
	Andy Lutomirski <luto@...capital.net>,
	Andi Kleen <andi@...stfloor.org>,
	Waiman Long <Waiman.Long@...com>,
	Ingo Molnar <mingo@...nel.org>,
	Davidlohr Bueso <davidlohr@...com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linux API <linux-api@...r.kernel.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	Jason Low <jason.low2@...com>,
	Scott J Norton <scott.norton@...com>,
	Robert Haas <robertmhaas@...il.com>
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?

Thanks,

	tglx

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