mcclnx mcc wrote:
we plan to setup our ORACLE database server (32 bits DB) and use dell r900 server. This server can put up to 128GB RAM. We are thinking use 32 bits CENTOS 4.7 or 5.2. My concern about CENTOS 5.2 is it only support up to 16 GB RAM on 32 bits O.S. Any suggestion?
To get beyond 4 GB of RAM with a 32-bit Intel CPU, you have to turn on PAE mode, which is a very ugly hack, invented several years ago now. It dates back to the old Pentium Pro! It was created before inexpensive 64-bit CPUs were available. Systems being built today should not use PAE mode, IMHO. If you need more than 4 GB of RAM today, use a 64-bit OS and applications. That will let you address the full memory capability of that server. In PAE mode, the system runs a little slower due to the extra overhead of the more complicated memory management scheme, and it can't really address that 16 GB all at once. It's kind of a like the old DOS days when we had 32-bit CPUs but could only address 64 KB of memory at a time, and could only access 640 KB without resorting to PAE-like hacks called EMS and XMS. These workarounds are best consigned to the dustbin of history.
Not true. The comparison of PAE to EMS/XMS is completely bogus, the technologies aren't alike at all. PAE does *NOT* involve any bank switching; a system using PAE can "address that 16 GB all at once". Comparing PAE to EMS/XMS has the same level of validity as comparing a .NET or Java virtual machine to a shell script interpreter.
http://en.wikipedia.org/wiki/Physical_Address_Extension#Page_table_structures
There is a, usually very small, performance penalty to PAE due to the using a three level verses a two level page directory.