> Hi:
> I am running Redhat Linux Enterprise version 4 update 4 on a dual-core
> 4G memory machine. There are many references on the web talking about
> increasing default user address space to 3.5 G however lacking specific
> instructions. My questions:
>
> 1. What is the specific steps to be done for the kernel to support 3.5 G
> address space?
> 2. Do I need to re-compile kernel to make this happen? If so, any
> specific instruction?
2. Yes, you need to re-compile the kernel. Instructions all over the web.
Basically, "cd /usr/src/linux", make sure a reasonable default .config
file is installed (the distribution should supply one for its default
kernel), "make menuconfig" or "make xconfig", change the options you
want changed, then "make" and "make install". The latter *usually*
works; the usual worst case is that it installs it somewhere other than
your boot loader is looking and rebooting will find the old kernel.
The fun comes when you left an option out of your new kernel that you
need to boot - like the hard drive controller! Then you need to go
back to an old, known working kernel.
It's not at all difficult, but you do need to be careful; a mistake
can be awkward to recover from if you don't plan ahead.
1. First of all, that's not necessarily a good idea. Doing that would limit
you to 384 MB of kernel memory, after the usual 128 MB deduction for
PCI devices. That has to fit the kernel binary, all page tables, inode
cache, network buffers, and so on. For some workloads, that can be
a bottleneck.
If your application is heavily biased toward file data that the kernel
doesn't have to look at, such as databases, it might be okay.
A much better thing would be to take advantage of the fact that
every multi-core processor I've heard of (IBM's POWER4, Sun Niagara,
and a few by some companies you may not have heard of like Intel and
AMD) is a 64-bit processor. So you can run a 64bit kernel and get
terabytes of user address space. Even 32-bit applications get a full
4G of address space, as the 64-bit kernel doesn't need to share.
That would make your user application happier *and* the kernel happier.
It would increase kernel data structure size, but it's still usually a
net win.
1b. If you really want to do it, it's not a normally selectable option,
but you can add it to arch/i386/Kconfig by following the pattern of
the others. You need CONFIG_PAGE_OFFSET=0xE0000000, and you need to
be sure that CONFIG_HIGHMEM64G is turned OFF. (If it's on, you're
using PAE, and its 3-level page table structure requires using a
1G boundary.)
Then configure the kernel, select CONFIG_EMBEDDED under "General setup",
then your memory split under "Processor type and features".
Compile, install the new kernel, and reboot.
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