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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
ish
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Raja G
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3 Answers3

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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%.

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

    enter image description here

  2. PAE

    enter image description here

Community
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ish
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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.

Colin Ian King
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I had serious troubles using 32-bit 12.04 with certain software (zfs filesystem), unveiling the vmalloc limitations of 32-bit systems ( I have tried both pae and non-pae and vmalloc and depmod params tuning, to death... but no success). That is probbably a special case, but could be given as an example. Limitation somewhere deep inside 32-bit kernels is 1GB, even if upto some 3.2 GB is usable.

Go for 64-bit if You can. I am pretty sure that Tou vcan... I guess that to date, it (=64bit cpu core) should be supported by almost any running system. Later "upgrade" is somehow uneasy (it means reinstall in fact, even if dpkg and copying /etc/ and /home/ may help bringing new installation running..) but still should be acceptable. I think that there are no problems to be afraid of with 64-bit, not even if You are forced to use old/closed source 32bit application or some libraries for some reason.

coro
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