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Date:	Sun, 10 Dec 2006 10:34:16 -0500
From:	John Richard Moser <nigelenki@...cast.net>
To:	Arjan van de Ven <arjan@...radead.org>
CC:	linux-kernel@...r.kernel.org
Subject: Re: PAE/NX without performance drain?

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Arjan van de Ven wrote:
> On Sat, 2006-12-09 at 15:39 -0500, John Richard Moser wrote:
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>>
>> Apparently (as I've been told today) using a hardware NX bit in a 32-bit
>> x86 kernel requires PAE mode.  PAE mode is enabled with HIGHMEM64, which
>> is (apparently) extremely slow.
> 
> 
> it's not extremely slow. 
> 
> there is a minor performance delta, sure, but to be honest that's a
> benchmark thing more than a real life thing.
> 
> What did your measurements show that the slowdown was? And how did you
> measure this?

I didn't measure, I was told by various people on IRC.  Also Google has
some misleading facts:

 - Someone mentioned that HIGHMEM64G + 4G split costs 10%-30%.  I
   immediately took that as "4G split might be friggin' expensive."

 - In some discussions Ingo mentioned HIGHMEM64G re RT preemption being
   a huge performance delta; but I haven't found evidence that this
   isn't a bug in RT.

Too bad PAE can't be detected at boot time; someone else mentioned that
some recent Pentium M laptops (and anything older than PPro) don't boot
if PAE is on.  Making 2 copies of the functions would be a pain in the
ass; and using indirect addressing and function pointers would be...
slow.  In performance critical areas.

I want my hardware NX bit working in Ubuntu without having to recompile
my kernel dammit.  How do I swing this one past them?  The performance I
can argue; breaking 586 and some random laptops (I've been contacted by
someone who has several of these Pentium-M machines that die on PAE) I
can't do.

> 
> 
> 
> 

- --
    We will enslave their women, eat their children and rape their
    cattle!
                  -- Bosc, Evil alien overlord from the fifth dimension
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