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 HPUX

dma32_pool_size

dma32_pool_size(5)						File Formats Manual						dma32_pool_size(5)

NAME

       dma32_pool_size - the amount of memory to reserve for the 32-bit DMA pool

VALUES

   Minimum
   Maximum
   Default
       Specify an integer value.

DESCRIPTION

       This  tunable  specifies  the amount of memory in the first 4GB of the physical address space to be reserved for DMA for 32-bit cards.  The
       memory is set aside at boot time and cannot be used for other purposes.	This is required only on some  Itanium	platforms.   On  platforms
       where the 32-bit pool is not needed, this value is ignored.

       The value of this tunable should be set based on the number of 32-bit cards in the system and the type of cards.

   Related Parameters
       None

WARNINGS

       All  HP-UX kernel tunable parameters are release specific.  This parameter may be removed or have its meaning changed in future releases of
       HP-UX.

       Installation of optional kernel software, from HP or other vendors, may cause changes to tunable  parameter  values.   After  installation,
       some  tunable  parameters may no longer be at the default or recommended values.  For information about the effects of installation on tun-
       able values, consult the documentation for the kernel software being installed.	For information about optional kernel  software  that  was
       factory installed on your system, see at

AUTHOR

       was developed by HP.

SEE ALSO

       None

Itanium(R)-based System Only				     Tunable Kernel Parameters						dma32_pool_size(5)