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Date: Fri, 8 Jan 2010 12:33:52 -0600 (CST)
From: Christoph Lameter <cl@linux-foundation.org>
Subject: Re: [RFC][PATCH 6/8] mm: handle_speculative_fault()
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To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>, KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>, Minchan Kim <minchan.kim@gmail.com>, "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>, "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>, "linux-mm@kvack.org" <linux-mm@kvack.org>, "hugh.dickins" <hugh.dickins@tiscali.co.uk>, Nick Piggin <nickpiggin@yahoo.com.au>, Ingo Molnar <mingo@elte.hu>
List-ID: <linux-mm.kvack.org>

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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