From the kernel build system help:
Linux can use up to 64 Gigabytes of physical memory on x86 systems. However, the address space of 32-bit x86 processors is only 4 Gigabytes large. That means that, if you have a large amount of physical memory, not all of it can be "permanently mapped" by the kernel. The physical memory that's not permanently mapped is called "high memory".
If you are compiling a kernel which will never run on a machine with more than 1 Gigabyte total physical RAM, answer "off" here (default choice and suitable for most users). This will result in a "3GB/1GB" split: 3GB are mapped so that each process sees a 3GB virtual memory space and the remaining part of the 4GB virtual memory space is used by the kernel to permanently map as much physical memory as possible.
If the machine has between 1 and 4 Gigabytes physical RAM, then answer "4GB" here.
If choosing "off" gives each process a 3GB usable address space, why should I want to turn it on if my machine only has 2GB?
If I understand correctly, running in 4GB mode means that I need to remap the kernel space every time I enter kernel context, which imposes some performance penalty. One should think I would like to avoid it if it's not necessary.This means that only going above 3GB would cause me to want to turn this on, and even then, only if I have a single app that needs more than 3GB of memory.
Am I wrong here? Is this a bug in the documentation?
Shachar
-- Shachar Shemesh Lingnu Open Source Consulting ltd. http://www.lingnu.com/
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