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

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Date:	Fri, 8 Jan 2010 11:43:41 -0600 (CST)
From:	Christoph Lameter <cl@...ux-foundation.org>
To:	Linus Torvalds <torvalds@...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, Linus Torvalds wrote:

> That's a huge jump. It's clear that the spinlock-based rwsem's simply
> suck.  The speculation gets rid of some additional mmap_sem contention,
> but at least for two sockets it looks like the rwsem implementation was
> the biggest problem by far.

I'd say that the ticket lock sucks for short critical sections vs. a
simple spinlock since it forces the cacheline into shared mode.

> Of course, larger numbers of sockets will likely change the situation, but
> at the same time I do suspect that workloads designed for hundreds of
> cores will need to try to behave better than that benchmark anyway ;)

Can we at least consider a typical standard business server, dual quad
core hyperthreaded with 16 "cpus"? Cacheline contention will increase
significantly there.

> Because let's face it - if your workload does several million page faults
> per second, you're just doing something fundamentally _wrong_.

You may just want to get your app running and its trying to initialize
its memory in parallel on all threads. Nothing wrong with that.

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