How much of 4GB RAM I should be able to see?

wazow

I have compiled a kernel with himem support (4GB) and I believe I am running it on 4GB RAM machine (32 bit Xeon). Nevertheless top says:

col · Posted: Thu Nov 11, 2004 8:49 am Post subject:

is that what your bios says ?

wazow · Posted: Fri Nov 12, 2004 8:00 am Post subject:

piquadrat · Guru Joined: 18 Feb 2003 Posts: 301 Location: Switzerland

I'll try my best explaining this with my limited english:

a 32bit system can only address 4GByte of RAM (2^32 bytes). But since the PCI bus needs some address space to, it blends a window in the 4GByte address space (512 MBytes AFAIK) which essentially means that your BIOS can't address all of your RAM.

I'm not sure if there are workarounds for this problem for Xeons...

wazow · Posted: Fri Nov 12, 2004 12:52 pm Post subject:

col · Posted: Fri Nov 12, 2004 10:08 pm Post subject:

I plugged 4G of ram in and only saw 3.8 in the bios and in the kernel...the amounts were the same.

jebroman · Posted: Fri Nov 12, 2004 10:43 pm Post subject:

I've just installed a server with 4GB RAM. This is the output of meminfo:

system root # cat /proc/meminfo
MemTotal: 3960412 kB
MemFree: 1588560 kB
Buffers: 363996 kB
Cached: 1670564 kB
SwapCached: 0 kB
Active: 1170396 kB
Inactive: 995944 kB
HighTotal: 3080160 kB
HighFree: 1378304 kB
LowTotal: 880252 kB
LowFree: 210256 kB
SwapTotal: 3904552 kB
SwapFree: 3904552 kB
Dirty: 16 kB
Writeback: 0 kB
Mapped: 164808 kB
Slab: 190212 kB
Committed_AS: 246164 kB
PageTables: 1020 kB
VmallocTotal: 114680 kB
VmallocUsed: 14836 kB
VmallocChunk: 99788 kB

I'm using kernel 2.6.9 and as you can see I'm able to use all of the 4 GB of RAM. The Linux kernel can address 64GB RAM on a 32 bitIintel plattform using Intel PAE. As far as I know a single process can never be address more than 4GB on a 32-bit system. Because of the 3:1 memory split that the Linux kernel uses I think a single process can never be bigger than 3GB. (1GB revserved for kernel-stuff)

When I first installed my 4GB system I first actived the 64GB option in kernel. This caused some serious problemes though....I couldn't do anything without getting Segmentation Faults. Perhaps this occcured because of the Intel PAE (Physical Address Extension) which in essence allows the 32 Intel CPU to address more than 4GB of RAM.

I'm not sure why the seg faults occured, perhaps the hardware (HP DL 580) needs to activate Intel PAE?? Anyone seen anything similar?

Anyway..you should be able to use all of your 4GB of RAM.

/johan

wazow · Posted: Mon Nov 15, 2004 12:45 pm Post subject:

Are you guys giving any options to your kernels? What options?

(I now know that BIOS says 4GB)

jebroman · Posted: Mon Nov 15, 2004 5:56 pm Post subject:

Nope, no extra options to kernel....(during boot i persume?)

When compling the kernel i set:

CONFIG_HIGHMEM4G=y

That's it. No other magic to make it see the full 4GB of RAM

/j

wazow · Posted: Wed Nov 17, 2004 9:10 am Post subject:

jebroman, what kernel sources are you using? Can you also post the top of dmesg output corresponding to the one I posted above?

(yes, I meant boot options, too bad that you do not use them

I have to search for another fix then)

jebroman · Posted: Sun Nov 21, 2004 9:10 pm Post subject:

I use gentoo-dev sources (kernel 2.6.9-gentoo-r1). Dmesg doesn't give any information about memory.....sorry

wazow · Posted: Thu Nov 25, 2004 12:28 pm Post subject:

This puzzles me still a bit. I have recompiled the kernel (actually current hardened kernel -r10) with 64GB support and everything seems to be fine. The system sees the entire 4GB RAM. Anybody got a clue why this happens? According to kernel help (and common sense) it should be sufficient to turn on the 4GB option...

(I have also tested gentoo-dev-sources and it seems to present exactly the same puzzles).

semen · n00b Joined: 02 Jan 2005 Posts: 29

hi,

I had the same problem.
Only 1 GB memory was available instead of 2 GB.
After changing the value:

CONFIG_HIGHMEM4G=y

and recompiling the kernel 2.6.10-gentoo-r4
and reboot

everthing becomes fine.

dmesg | grep mem
Memory: 2074872k/2097024k available (2617k kernel code, 21344k reserved, 884k data, 200k init, 1179520k highmem)
highmem bounce pool size: 64 pages
Total HugeTLB memory allocated, 0
Freeing unused kernel memory: 200k freed

:wink:

zeek · Posted: Sat Jan 15, 2005 2:13 am Post subject:

drescherjm · Posted: Sat Jan 15, 2005 4:19 am Post subject:

I believe this is exactly what the 64GB option does (it enables PAE). You can not have a full 4GB of memory with 32 bit addressing because of the pci slots take over some of the upper addresses.
_________________
John

My gentoo overlay
Instructons for overlay

semen · n00b Joined: 02 Jan 2005 Posts: 29

The help of kernel make menuconfig says...

CONFIG_NOHIGHMEM:
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 more than 4 Gigabytes is used then answer "64GB" here. This │
selection turns Intel PAE (Physical Address Extension) mode on. │
PAE implements 3-level paging on IA32 processors. PAE is fully │
supported by Linux, PAE mode is implemented on all recent Intel │
processors (Pentium Pro and better). NOTE: If you say "64GB" here, │
then the kernel will not boot on CPUs that don't support PAE! │
The actual amount of total physical memory will either be │
auto detected or can be forced by using a kernel command line option such as "mem=256M". (Try "man bootparam" or see the documentation of your boot loader (lilo or loadlin) about how to pass options to the │ │ kernel at boot time.)

If unsure, say "off".

so long