4GB memory issue in Centos 5

LinuxLover · 02-27-2009, 06:21 AM

I am using Centos 5.0 32bit system with simple kernel 2.6.18-53.el5 and have 4GB RAM in system.

I have some problems with system RAM detection, (I cant use kernel-PAE kernel)

This is system log

Code:

3200MB HIGHMEM available.
896MB LOWMEM available.
Memory for crash kernel (0x0 to 0x0) notwithin permissible range
  HighMem zone: 819200 pages, LIFO batch:31
Memory: 3103732k/4194304k available (2080k kernel code, 40104k reserved, 869k data, 220k init, 2227584k highmem)
Freeing initrd memory: 2345k freed

but free command shows only 3GB

Quote:

free -m
total used free shared buffers cached
Mem: 3034 2911 123 0 50 2566
-/+ buffers/cache: 293 2740

I have read that 32 bit linux support 4GB RAM then why free showing only 3GB.

Second what is

3200MB HIGHMEM available.
896MB LOWMEM available.

mean in above code.

I have also check

Quote:

cat config-2.6.18-53.el5 |grep -i MEM4
CONFIG_HIGHMEM4G=y

4GB support is enable in kernel.

theYinYeti · 02-27-2009, 07:50 AM

I don't know for Redhat, but with Mandriva the most compatible kernel (hence the default one) is named “kernel-desktop586” and can't handle over 880MB RAM (up to version 2008.1), or over 4GB (since version 2009.0). So people with more RAM need to install the non-586 kernel, namely “kernel-desktop”, instead. Maybe there's something similar with Redhat.

Apart from that, some of your RAM could be taken by the graphics chipset if it has no RAM of its own.

Yves.

johnsfine · 02-27-2009, 08:18 AM

Quote:

Originally Posted by LinuxLover

I have some problems with system RAM detection, (I cant use kernel-PAE kernel)

If you can't use PAE then you can't use the full 4GB.

Why can't you use kernel-PAE?

Quote:

3200MB HIGHMEM available.
896MB LOWMEM available.

I'm not 100% certain, but I think that 3200MB HIGHMEM is the total available to Linux without PAE and the 896 LOWMEM is included in the 3200.

Quote:

I have read that 32 bit linux support 4GB RAM then why free showing only 3GB.

Without PAE an OS can support 4GB of physical address space not 4GB of ram. The motherboard and BIOS use parts of the first 4GB of address space for things other than ram, so part of the first 4GB of ram must be outside the first 4GB of physical address space, and cannot be used without PAE.

Quote:

what is

3200MB HIGHMEM available.
896MB LOWMEM available.

mean in above code.

Part of ram (LOWMEM) is permanently mapped into the Kernel virtual address space. Certain direct uses of ram by the kernel are limited to that section of ram (I don't know the details of exactly what things can only be stored in LOWMEM).
There is no such limitation for ram used by processes.

Quote:

4GB support is enable in kernel.

That just means you have support for ram beyond the first 896MB. It does not mean you have support for a full 4GB of ram.