A machine equipped with 4 Gig RAM is so lagging running Linux PAE kernel and Windows 7 x64

I have a machine equipped with Intel motherboard DG965RY that has 4G RAM.  The machines work as usual running Windows Vista for years.

I installed Windows 7 x64 on the machine recently and the OS works extremely slow.  The CPU is Intel(R) Core(TM)2 CPU 6600 @ 2.40Ghz.  It should support 64 bits instruction sets as seens in BIOS page showing EM64T.  I first thought it could be CPU or motherboard that doesn’t support x64 OS well.  I then re-install the machine with Windows 7 x86 and it works like a charm.

2 Days later, I try to install AsteriskNOW backed by CentOS 5.5 (kernel 2.6.18-194.3.1.e15PAE) i386 on the same machine.  Again, the machine running the kernel 2.6.18-194.3.1.el5PAE is so lagging.  However, it works as usual if running kernel 2.6.18-194.3.1.el5.

I google for the different between PAE kernel and non PAE kernel.  PAE stand for Physical Address Extension.  A clue sparks on my mind suddenly after knowing from the PAE term that it might be something to do with the memory.  I remember that 32 bits machine have limitation on memory address in Windows Vista x86.  It may only detect 3.5G from 4G RAM installed.

I unplug a 2G RAM from motherboard slot and left 2G RAM to the machine and attempt to run PAE kernel.  It works like a charm.  A good news follow is Windows 7 x64 works extremely smooth with this 3 years old machine too.

I quickly browse the Intel BIOS update page and found there is update regarding addressing issues of 4G RAM.  I download and update the BIOS to the latest version, plug the 2G RAM back to motherboard and boot the machine with 4G RAM.  Both Linux PAE kernel and Windows 7 x64 works smoothly as expected.