Built in ram limit for 64 bit Post: 302118101

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Operating Systems Linux Built in ram limit for 64 bit Post 302118101 by Perderabo on Friday 18th of May 2007 11:25:33 AM

05-18-2007

Administrator Emeritus

I don't have any experience with x86_64 cpu's and I have never used more than 2 GB Ram on any pc. But after reviewing the memory management sections of my copy "Understanding the Linux Kernel", I may have a handle on this. The Linux kernel needs to use a CPU feature called PAE to access more than 4GB of memory with a 32 bit kernel. This pushes the paging system into a 3 level model (instead of a 2 level model) resulting in the performance hit you mentioned. A 64 bit kernel needs a 3 level model up to kernel version 2.6.10. But this imposed constraints on how memory could be allocated. Starting with version 2.6.11, a four level paging model is used with x86_64 cpu's. It is not clear from the book if it would be possible to not use PAE on a x86_64 with 4 GB or less of memory. But reading between the lines a bit, I gather that the answer is "no". Support for non-PAE kernels seems to be intended for very old cpu's and they won't do 64 bit. Besides large RAM configurations, PAE is required to use the new NX flag which marks some memory regions as non-executable. This closes a security problem. Linux 2.6.11 supports the NX feature.

So a 64-bit kernel may have less decisions for you to make thus "taking care of things".

Perderabo

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linux-version

LINUX-VERSION(1)					      General Commands Manual						  LINUX-VERSION(1)

NAME

       linux-version - operate on Linux kernel version strings

SYNOPSIS

       linux-version compare VERSION1 OP VERSION2

       linux-version sort [--reverse] [VERSION1 VERSION2 ...]

       linux-version list [--paths]

DESCRIPTION

       linux-version operates on Linux kernel version strings as reported by uname -r and used in file and directory names.  These version strings
       do not follow the same rules as Debian package version strings and should not be compared as such or as arbitrary strings.

       compare VERSION1 OP VERSION2
	      Compare version strings, where OP is a binary operator. linux-version returns success (zero result) if the  specified  condition	is
	      satisfied, and failure (nonzero result) otherwise.  The valid operators are: lt le eq ne ge gt

       sort [--reverse] [VERSION1 VERSION2 ...]
	      Sort the given version strings and print them in order from lowest to highest.  If the --reverse option is used, print them in order
	      from highest to lowest.

	      If no version strings are given as arguments, the version strings will instead be read from standard input, one per line.  They  may
	      be suffixed by arbitrary text after a space, which will be included in the output.  This means that, for example:

	      linux-version list --paths | linux-version sort --reverse

	      will list the installed versions and corresponding paths in order from highest to lowest version.

       list [--paths]
	      List kernel versions installed in the customary location.  If the --paths option, show the corresponding path for each version.

AUTHOR

       linux-version and this manual page were written by Ben Hutchings as part of the Debian linux-base package.

								   30 March 2011						  LINUX-VERSION(1)