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

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Date:	Tue, 22 Jul 2014 04:31:14 +0200
From:	Mike Galbraith <umgwanakikbuti@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Darren Hart <dvhart@...ux.intel.com>,
	Andy Lutomirski <luto@...capital.net>,
	Peter Zijlstra <peterz@...radead.org>,
	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 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.

-Mike

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