kernel mem parameter

Question

As a last resort to my question, I am yet to try the mem parameter of kernel to force it to use the specified amount of RAM. Short Summary - I can only see 3.2GB RAM on a 64bit OS and am not sure ifs a hardware limitation, so wants to try as I found a post on Ubuntuforums.

My question is if its ok to play with my resident Ubuntu install or should I be using a live bootable usb? What values do I try (I have 6GB with only 3.2GB being usable) and how to keep it safe? I don't want to burn any of my hardware component at this point of time or make the system unbootable.

Running Ubuntu 11.10 with kernel 3.0.0-13-generic

Solution

Weird, that Ubuntuforums link wants a login but I can view random other threads without logging in.

Anyway I'm guessing you're referring to mem=xM in your bootloader. This is perfectly safe to try on your current install. For 6GB you want mem=6144M (6*1024).

Just try tacking it onto the end of the line with the other options, which will include such things as quiet splash ro root=UUID=$stuff.

For safety, I would not recommend you edit any config files to do this the first time. While booting, hold Shift to get into GRUB and then press e to edit the highlighted grub entry. You can move around with the cursor keys to find the line you want to edit; add mem=6144M onto the end and boot (see directions at the bottom of the screen).

If it works, add it to /etc/default/grub in the line GRUB_CMDLINE_LINUX_DEFAULT, and run sudo update-grub.

Comments

[Ashfame]

This only seems to be capping the RAM to use and not a way to force the kernel to use more RAM if it already isn't. I tried mem=3500M since I only have 4GB plugged in right now and it doesn't work. It do works if I cap it lower at 2GB mem=2048M. But your answer explains the question, so I will mark it as answer. Thanks!

[tanius]

Since you are probably on a x86 system (32 or 64 bit, does not matter), the mem parameter works slightly different from what @Caesium explained: "mem=nn[KMG] […] [X86] Work as limiting max address." (see). I had to use mem=6700M to lower free -m available memory from 5763 MiB to 5365 MiB. Found out by experimenting, since I did not know the size and location of reserved address space.