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Wednesday, February 13, 2008

Shall We use /PAE and /3GB switches together views

Physical Address Extension (PAE) refers to a feature of x86 and x86-64 processors that allows the OS to see more than regular 4 gigs of physical RAM on a 32-bit machine.To do that we need to specify /PAE switch in the boot.ini file and reboot the machine so that this setting takes effect.

So when you enable PAE ,the physical address size is increased from 32 bits to 36 bits which is equal to 64 GB. Note that the 32-bit size of virtual address is never changed, and it remains 4 GB.The OS uses PTE ( Page Table Entries) to map this 4 GB address space onto the 64 GB of total memory.Now any APP which needs access to more than 4 GiB of memory,has to have some special mechanism and on MS Windows this mechanism is called Address Windowing Extensions (AWE), while on Unix-like systems a variety of tricks are used, such as using mmap() to map regions of a file into and out of the address space as neededbut not yet adopted as a standard.

AWE is an MS API that allows the apps to access more physical memory than available in VAS ..The process through which it maps the VAS to physical memory is called as Windowing ...Not all the applications are AWE enabled ..but SQL Server is ..However , for doing that we need to have the locked pages in memory privilege enabled .One more benefit of using AWE in SQL Server is that the memory being allocated using AWE will not be written in Page files ..So,AWE can prevent Windows from swapping sensitive data from RAM to pagefile.

Now , I am pretty sure that we all know /3GB setting but just wanted to convey that with this setting the physical memory limit on Windows Server 2003 is 16 GB only ..So even if you have more than 16GB RAM you cannot see/use it :D...thats because it trims the PTE table rows ..

Now comes the real question ....Should we use /PAE and /3GB switch together .....well one negative factor i have already mentioned above. Secondly just for 1 GB I dont feel it would be worth .....Its set on my machine and i have no issues :)..

However , I have seen a few cases where these 2 combinations creates situations like Blue screen or black screen on machine upon boot up .....that is something because the OS goes for a toss while going through ZEROING activity ..But i think this stuff still needs some good research ...So, I do not say dont use it , but surely test it first .

So , summary is that I am not totally against it but i wont prefer it ....
Your choice :D .......

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