linux-kernel - Re: [RFC][PATCH 6/8] mm: handle_speculative

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Date:	Fri, 8 Jan 2010 13:36:47 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Christoph Lameter <cl@...ux-foundation.org>
cc:	Peter Zijlstra <peterz@...radead.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Minchan Kim <minchan.kim@...il.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"hugh.dickins" <hugh.dickins@...cali.co.uk>,
	Nick Piggin <nickpiggin@...oo.com.au>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [RFC][PATCH 6/8] mm: handle_speculative_fault()

On Fri, 8 Jan 2010, Christoph Lameter wrote:
> 
> I'd say that the ticket lock sucks for short critical sections vs. a
> simple spinlock since it forces the cacheline into shared mode.

Btw, I do agree that it's likely made worse by the fairness of the ticket 
locks and the resulting extra traffic of people waiting for their turn. 
Often for absolutely no good reason, since in this case the rwlock itself 
will then be granted for reading in most cases - and there are no 
ordering issues on readers.

We worried about the effects of fair spinlocks when introducing the ticket 
locks, but nobody ever actually had a load that seemed to indicate it made 
much of a difference, and we did have a few cases where starvation was a 
very noticeable problem.

				Linus
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