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Message-ID: <20130930104408.GW3081@twins.programming.kicks-ass.net>
Date: Mon, 30 Sep 2013 12:44:08 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Ingo Molnar <mingo@...nel.org>, Waiman Long <Waiman.Long@...com>,
Ingo Molnar <mingo@...e.hu>,
Andrew Morton <akpm@...ux-foundation.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
Rik van Riel <riel@...hat.com>,
Peter Hurley <peter@...leysoftware.com>,
Davidlohr Bueso <davidlohr.bueso@...com>,
Alex Shi <alex.shi@...el.com>,
Tim Chen <tim.c.chen@...ux.intel.com>,
Andrea Arcangeli <aarcange@...hat.com>,
Matthew R Wilcox <matthew.r.wilcox@...el.com>,
Dave Hansen <dave.hansen@...el.com>,
Michel Lespinasse <walken@...gle.com>,
Andi Kleen <andi@...stfloor.org>,
"Chandramouleeswaran, Aswin" <aswin@...com>,
"Norton, Scott J" <scott.norton@...com>
Subject: Re: [PATCH] rwsem: reduce spinlock contention in wakeup code path
On Sat, Sep 28, 2013 at 12:33:36PM -0700, Linus Torvalds wrote:
> The old rwlock's really have been a disappointment - they are slower
> than spinlocks, and seldom/never end up scaling any better. Their
> main advantage was literally the irq behavior - allowing readers to
> happen without the expense of worrying about irq's.
So in part that is fundamental to the whole rw-spinlock concept.
Typically lock hold times should be short for spinlock type locks and if
your hold times are short, the lock acquisition times are significant.
And a read acquisition is still a RMW operation on the lock, thus read
locks are still entirely bound by the cacheline transfer of the lock
itself.
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