[CentOS] PAE or use 64-bit?

From: centos-bounces at centos.org on behalf of John Doe
>From: Filipe Brandenburger <filbranden at gmail.com>
>> > > Which would you recommend for a 4GB+ machine?
>> > > A 32-bit install with PAE-enabled kernel or just use 64-bit?
>> > If you are not doing anything that needs a 64-bit address space, then
>> > 32-bit w/PAE will work just fine.
>> Yes, but I would recommend you to go with 64-bit OS anyways.
>> Even though PAE might work reasonably well if you have 4GB or 6GB or
>> 8GB and with your current applications, 64-bit will keep working well
>> when you need to upgrade your machine to 16GB or 32GB and run
>> additional applications or newer versions that need to address more
>> than 4GB per process.
>
>I also read somewhere that enabling PAE would take some of the memory away from some components (PCI? Bus?)...
>If you have 4GB, without PAE, you get 3.3GB.  With PAE, you get 4GB.
>And it seemed to mean decreased performances.
>Anyone can confirm this?
>
>JD
>

This is true, in a 4GB setup and PAE, you will see a 3/1 split where three GB are accessible and one GB is used by the kernel to do the mapping. So, if you have 6GB you really have 5, since PAE needs the first GB for addressing to the higher registers. To accomplish this, you will notice a slight performance loss (around 2 to 5 percent overall) to handle it, since 64bit addressing requires a few extra operations in a 32bit CPU versus a native 64bit chip. This is why if you can, just go with a 64bit install if you have larger RAM configurations, it performs better all 'round.

      

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