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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ctrlaltdel

CTRLALTDEL(8)						       System Administration						     CTRLALTDEL(8)

NAME

       ctrlaltdel - set the function of the Ctrl-Alt-Del combination

SYNOPSIS

       ctrlaltdel hard|soft

DESCRIPTION

       Based  on examination of the linux/kernel/sys.c code, it is clear that there are two supported functions that the Ctrl-Alt-Del sequence can
       perform: a hard reset, which immediately reboots the computer without calling sync(2) and without any other preparation; and a soft  reset,
       which sends the SIGINT (interrupt) signal to the init process (this is always the process with PID 1).  If this option is used, the init(8)
       program must support this feature.  Since there are now several init(8) programs in the Linux community, please consult	the  documentation
       for the version that you are currently using.

       ctrlaltdel is usually used in the /etc/rc.local file.

FILES

       /etc/rc.local

SEE ALSO

       simpleinit(8), init(8)

AUTHOR

       Peter Orbaek (poe@daimi.aau.dk)

AVAILABILITY

       The ctrlaltdel command is part of the util-linux package and is available from ftp://ftp.kernel.org/pub/linux/utils/util-linux/.

util-linux							   October 1993 						     CTRLALTDEL(8)