linux-kernel - Re: [PATCH 00/10] x86: Reduce memory and intra-node effects with large count NR

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Message-Id: <200801161834.39746.nickpiggin@yahoo.com.au>
Date:	Wed, 16 Jan 2008 18:34:39 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Andi Kleen <ak@...e.de>
Cc:	Ingo Molnar <mingo@...e.hu>, travis@....com,
	Andrew Morton <akpm@...ux-foundation.org>,
	Christoph Lameter <clameter@....com>,
	Jack Steiner <steiner@....com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 00/10] x86: Reduce memory and intra-node effects with large count NR_CPUs

On Monday 14 January 2008 22:30, Andi Kleen wrote:

> In general there are more scaling problems like this (e.g. it also doesn't
> make sense to scale kernel threads for each CPU thread for example).

I think in a lot of ways, per-CPU kernel threads scale OK. At least
they should mostly be dynamic, so they don't require overhead on
smaller systems. On larger systems, I don't know if there are too
many kernel problems with all those threads (except for userspace
tools sometimes don't report well).

And I think making them per-CPU can be much easier than tuning some
arbitrary algorithm to get a mix between parallelism and footprint.

For example, I'm finding that it might actually be worthwhile to move
some per-node and dynamically-controlled thread creation over to the
basic per-CPU scheme because of differences in topologies...

Anyway, that's just an aside.

Oh, just while I remember it also, something funny is that MAX_NUMNODES
can be bigger than NR_CPUS on x86. I guess one can have CPUless nodes,
but wouldn't it make sense to have an upper bound of NR_CPUS by default?
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