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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LEARN ABOUT DEBIAN

bootcdflopcp

BOOTCD(1)							   bootcd utils 							 BOOTCD(1)

NAME

       bootcdflopcp - copy changes made after booting from bootcd to floppy

SYNOPSIS

       bootcdflopcp [-v] [-d <device>]

DESCRIPTION

       bootcdflopcp  will  copy changes made in ram to the floppy disk.  bootcdflopcp will be available as soon as your system is running from cd.
       The floppy has to have a filesystem already. (See mke2fs or mformat).  If you have to boot from floppy, because your cd-drive or bios  does
       not  support  to  boot from cd a msdos filesystem is used to run syslinux.  When bootcdflopcp is called it searches for differences between
       RAM and CD.  For each different file, it checks if it is listed in the files ignore, remove or change on floppy. If it is listed in  change
       it  will  be  saved to change.tgz on floppy. If it is listed in remove the file will be removed from ram next boot time. If it is listed in
       ignore it will be ignored. If it is not listed at all you will be interactively asked what to do.

OPTIONS

       -v  The option "-v" (verbose) adds messages on running.

       -d <device>
	   Use another device instead of "/dev/fd0" to save changes.

FILES

       FLOPPY:/remove
	      If a file is listed here the file will be deleted from ram next boot time.

       FLOPPY:/change
	      If a file is listed here bootcdflopcp will save it in change.tgz.

       FLOPPY:/ignore
	      If a file is listed here bootcdflopcp will ignore changes to this file.

       FLOPPY:/change.tgz
	      Here all changed files are stored in gzipped tar format.

SEE ALSO

       bootcd(1), bootcd2disk(1), bootcdwrite(1)

AUTHOR

       This manual page was written by Bernd Schumacher <bernd.schumacher@hp.com>, for the Debian GNU/Linux system (but may be used by others).

							   Wed Feb 23 00:00:00 EET 2000 						 BOOTCD(1)