[Novalug] PAE extensions, any negative aspects?

[Peter Larsen]

On Thu, 2008-11-13 at 11:15 -0500, Jon LaBadie wrote:
> Are there any downsides to using the running
> the PAE enabled kernel version?
> 
> I presume memory access has slightly more overhead.
> A search on the internet turned up pages that were
> mostly 3-6 years old but did not indicate any
> significant problems.  Those that did say perform-
> ance was affected said "1-10%" but cited no data.

I use PAE and don't see an issue. Here's the details about it:
http://en.wikipedia.org/wiki/Physical_address_extension. This is a CPU
feature.

Of course there's "overhead" when the computer creates virtual liniary
address spaces. But all OS's have done that ever since the invention of
Virtual memory. PAE allows 36 instead 32 bit address space and increases
the size of the memory mapping. But it would be foolish to talk about a
performance hit? Unless of course you feel that anything mapping
physical memory is a performance issue, in which fall no OS will pass
muster.

If you're running memory heavy systems like video editing or huge
databases there may in some circumstances be an overhead because of the
additional lookup. But you have to consider, that page tables aren't
"relooked" every time there's an address. If you go from 0x00AA to
0x00AB the same tables remain in effect and it's a simple increase. Does
the OS need more space to do this, certainly. But you get at least 1GB
extra memory space for a single program.

It's worth remembering that this solves the problem of the largest
SINGLE allocated program. It's been possible for a long time to use more
than 4GB on 32bit machines. Just not for the same program.

-- 
Peter Larsen <plarsen at famlarsen.homelinux.com>
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