[CentOS] To PAE or not to PAE...

Thu Jul 22 22:13:24 UTC 2010
Kevin Krieser <k_krieser at sbcglobal.net>

On Jul 22, 2010, at 12:51 PM, Markus Falb wrote:

> On 22/07/2010 19:07, Warren Young wrote:
>> On 7/22/2010 3:25 AM, John Doe wrote:
>>> I have a 4GB pc and was wondering if it was worth going the PAE way to gain
>>> those exta 700MB...
>> Very few programs can use PAE to get at that extra RAM.  Can the
>> programs you run do this?
> What is a program supposed to do to get at "that extra RAM" then ?
> Just curious ;-)

If it is supported by your chipset, etc, then the extra RAM just goes into the pool of available RAM for system usage.  An extra 700MB of program space that doesn't have to be paged out, or discard clean pages, depending on overall memory pressure of the system.

But system calls are costlier, since the kernel has to do more work swapping into different address spaces.  For instance, on a 3GB RAM computer with a 32 bit OS, the OS can just access any physical RAM directly.  But since the OS is 32 bit too, it has more work to access "upper" RAM.