linux-kernel - Re: [RFC][PATCH 00/13] percpu rwsem -v2

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Date:	Thu, 2 Jul 2015 11:41:55 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Daniel Wagner <wagi@...om.org>, Ingo Molnar <mingo@...nel.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Tejun Heo <tj@...nel.org>, Ingo Molnar <mingo@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	der.herr@...r.at, Davidlohr Bueso <dave@...olabs.net>,
	Rik van Riel <riel@...hat.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Jeff Layton <jlayton@...chiereds.net>
Subject: Re: [RFC][PATCH 00/13] percpu rwsem -v2

On Wed, Jul 01, 2015 at 02:54:59PM -0700, Linus Torvalds wrote:
> On Tue, Jun 30, 2015 at 10:57 PM, Daniel Wagner <wagi@...om.org> wrote:
> >
> > And an attempt at visualization:
> >
> > http://monom.org/posix01/sweep-4.1.0-02756-ge3d06bd.png
> > http://monom.org/posix01/sweep-4.1.0-02769-g6ce2591.png
> 
> Ugh. The old numbers look (mostly) fairly tight, and then the new ones
> are all over the map, and usually much worse.
> 
> We've seen this behavior before when switching from a non-sleeping
> lock to a sleeping one. The sleeping locks have absolutely horrible
> behavior when they get contended, and spend tons of CPU time on the
> sleep/wakeup management, 

Right, I'm just not seeing how any of that would happen here :/ The read
side would only ever block on reading /proc/$something and I'm fairly
sure that benchmark doesn't actually touch that file.

In any case, I will look into this, I've just not had time yet..
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