linux-kernel - Re: [PATCH 2/2] x86,mm,64bit: Round up memory boundary for init_memory_mapping

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Message-ID: <20110227115350.GB16453@elte.hu>
Date:	Sun, 27 Feb 2011 12:53:50 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Tejun Heo <tj@...nel.org>
Cc:	Yinghai Lu <yinghai@...nel.org>, Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] x86,mm,64bit: Round up memory boundary for
 init_memory_mapping_high()


* Tejun Heo <tj@...nel.org> wrote:

> > will have problem with cross node conf. like 0-4g, 8-12g on node0, 4g-8g, 
> > 12g-16g on node1.
> 
> And how common are they?  This whole cruft is basically meaningless if 1GiB 
> mapping is supported, IOW, basically on all AMD 64s and all post-nehalem intels.  
> Why not just cite the limitation in the comment and stick to something simple?

Such complexity should be justified via very careful "perf stat --repeat" 
measurements.

I.e. showing 'before patch' and 'after patch' instruction, TLB miss and cycle 
counts, showing that a positive effect that goes beyond the noise of the measurement 
exists.

1GB mappings should be assumed as the common case - anything else probably does not 
matter from a future performance/scalability POV. For vmalloc() it might make sense 
- but even for those precise measurements should be done about the positive effect.

Thanks,

	Ingo
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