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Date:	Tue, 22 Jul 2014 04:31:14 +0200
From:	Mike Galbraith <>
To:	Steven Rostedt <>
Cc:	Thomas Gleixner <>,
	Darren Hart <>,
	Andy Lutomirski <>,
	Peter Zijlstra <>,
	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 Mon, 2014-07-21 at 21:34 -0400, Steven Rostedt wrote:

> I was telling Robert that if futexes get optimistic spinning, he should
> reconsider their use of userspace spinlocks in favor of this, because
> I'm pretty sure that they will see a great improvement.

My (dated) experience with pgsql says you're likely right.

Once upon a time, preempting a userspace spinlock owner caused pgsql to
collapse into a quivering heap.  The scheduler trying to hand the CPU
back to a preempted task instead of selecting what was strictly speaking
the most deserving task afterward (LAST_BUDDY) let pgsql+oltp scale nice
and flat instead of self destructing, but the root cause of collapse was
spinners lacking knowledge.   With spin done in kernel, you can know
when spinning is a waste of perfectly good cycles.


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