Muli Ben-Yehuda wrote:
On Tue, Jun 15, 2004 at 11:19:42PM +0300, Shachar Shemesh wrote:
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?
Because Linux only direclty maps the first 896mb of physical
memory. If your machine has 2GB of phys. memory, and you choose
"off", your kernel will only recognize the first 896mb of memory. The
documentatino text leaves something to be desired...
I'll say. I still didn't understand it, though.
Is it one of the options in himem support? I have there "off", "4GB", and "64GB".
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.
That's 4:4, which is something different. It is used when you need 4gb
of address space for either the kernel or userspace (or both).
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?
You're mixing virtual memory (the available address space) and Linux's
physical memory handling. To recap, only the first 896mb is directly
mapped by the kernel, and for the rest (anything up to 4GB of physical
memory) you need "highmem support". Then the kernel will use the
physical memory between 896mb-4gb by mapping it "on the fly" whenever
it needs to be used. This does incur some run time overhead, but only
for the memory between 896mb-4gb (ZONE_HIGHMEM), which will not be
used at all otherwise...
You lost me totally.
Let's see what I understand. I get 3GB of virtual memory, no matter what I compile in. If I want 4, I need a "4:4" patch, which is presumably not there by default.
If I compiled himem support to "off", I get just 1GB (well, less) of physical memory easily accessed. What's the difference? If I have a machine with 4GB of physical memory, and himem support compiled "off", and I ask for 2.5GB of heap memory, what is going to happen? Do I get failure? Do I swap? Is the memory going to be used, but with some performance penalty per context-switch?
Cheers, Muli
Even more confused than before,
Shachar
-- Shachar Shemesh Lingnu Open Source Consulting ltd. http://www.lingnu.com/
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