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

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Date:	Fri, 8 Jan 2010 12:33:52 -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:

> I bet it won't be a problem. It's when things go cross-socket that they
> suck. So 16 cpu's across two sockets I wouldn't worry about.
>
> > > 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.
>
> Umm. That's going to be limited by the memset/memcpy, not the rwlock, I
> bet.

That may be true for a system with 2 threads. As the number of threads
increases so does the cacheline contention. In larger systems the
memset/memcpy is negligible.

> The benchmark in question literally did a single byte write to each page
> in order to show just the kernel component. That really isn't realistic
> for any real load.

Each anon fault also includes zeroing the page before its ready to be
written to. The cachelines will be hot after a fault and initialization of
any variables in the page will be fast due to that warming up effect.


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