For a 32-bit kernel, what are the pros/cons of PAE vs. non-PAE?

I am using Ubuntu 12.04 32-bit. My kernels are PAE kernels. I know these are the kernels we can use for 32-bit OS which have 4 GB RAM, to utilize all 4GB of RAM.

My system is a 32-bit system. So I thought I don't need them. What are the advantages and disadvantages I am going have if I keep running PAE kernels?

raja@badfox:~$ uname -a
Linux badfox 3.2.0-24-generic-pae #39-Ubuntu SMP Mon May 21 18:54:21 UTC 2012 i686 i686 i386 GNU/Linux

Short answer: Go 64-bit if you can, which is faster than both PAE/non-PAE 32-bit; if you cannot, non-PAE may be 1-2% faster than PAE.

In theory PAE has a slight overhead over non-PAE...

• The big advantage of PAE is allowing a 32-bit processor/system to use more than 4GB of RAM
• But this requires slightly more overhead over non-PAE, which can lead to slightly decreased performance.
  • Here's a very simple explanation: in non-PAE mode, a 32-bit CPU must lookup (access) two tables to access a physical memory address; in PAE-mode, it must lookup three tables to do so. The one additional lookup requires some (very small) extra time, thus imposing additional overhead.
  • At the end of this answer are two images from the Wikipedia PAE article, illustrating the above point.
  • NX/XD bit: The PAE kernel also supports the No-eXecute/eXecute-Disable bit on 64-bit processors; this can help prevent some kinds of virus/malicious attacks (buffer overflows), but IMO this doesn't matter much when choosing 32-bit kernels for Ubuntu.

...but in practice this overhead is negligible (almost nothing)...

• Phoronix has done a number of tests over the years which show that on systems with 4GB or less, the PAE kernel may be at most approximately 5% slower than the non-PAE kernel. This is only for a specific test application; the usual difference is less than 1%.
  • Tests for 12.04 LTS - 8 GB system!
  • Tests for 11.04 - 4 and 8GB systems

and 64-bit almost always beats BOTH 32-bit kernels - go for it!

• The one thing those Phoronix benchmarks above tell you is that 64-bit is king -- even if you have less than 4GB of RAM (although I'd recommend 1GB as the minimum)
• If you have a 64-bit processor -- generally anything after 2006 except Intel Atoms -- you are probably losing performance by using a 32-bit kernel!

Comparing non-PAE vs. PAE page table accesses:

1. Non-PAE

   

2. PAE

   

For some real-world data, here are some benchmarks I gathered for i386, i386-pae and amd64 installs on an Atom based netbook and Sandybridge based laptop:

http://kernel.ubuntu.com/~cking/power-benchmarking/blueprint-foundations-p-64bit-by-default/hpmini-and-x220-tests/results-3/results.txt

..may give some insight into pros and cons.