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Joined: Feb 18, 2009
Posts: 3
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Posted Apr 18, 2009 at 8:06:53 PM
Subject: Limiting the RAM Size
Hi Guys, I am fairly new to linux. My system has 8 Gb RAM. When I run the 'top' command it shows RAM usage as 3GB, on startup is shows it as 8 Gb. I don't know why is it showing as only 3Gb total by top command? What I am trying to do is, I have to run some benchmark test and for that I have to limit the RAM size. Someone told me that we can change the RAM size from startup. When I press F2 key to enter into the BIOS setup, it shows the RAM size over there but it does not allow me to modify it. I am not sure if this is the right way to modify the RAM size. I don't know if it matters but I am using Ubuntu 8.10 32 bit Can someone please guide me on this? Thanks a lot in advance.
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Reed
Joined Feb 07, 2008
Posts: 757

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Posted: Apr 18, 2009 10:58:47 PM
Subject: Limiting the RAM Size
It's because you're running 32 bit. You need to run a 64 bit system to access more RAM.
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blinky
Joined Jan 08, 2009
Posts: 51

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Posted: Apr 18, 2009 11:00:36 PM
Subject: Limiting the RAM Size
On 32 bit systems you need to run a PAE enabled kernel or run a 64 bit distribution.

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kevinchkin
Joined Feb 18, 2009
Posts: 3

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Posted: Apr 19, 2009 12:22:58 AM
Subject: Limiting the RAM Size
Thanks for the reply guys. Yes I did installed PAE enabled kernel and not it is using full RAM. That solves one of the two problems. :) But other one is still unsolved :( as I still can't figure out how to manually specify the RAM size to be used. I added ramdisk_size=1000000000 in /boot/grub/menu.lst (in ubuntu generic 11). I did this because I want to limit the RAM size to 1 Gb. But when I do this and reboot the computer and then when I run "top" command it says total memory as 3Gb ? When I remove this ramdisk_size=1000000000 from /boot/grub/menu.lst, total RAM is once again 8Gb. I don't understand what's going on. I thought after specifying the size it will show total ram size as 1Gb. Please guide me through guys. Thanks a lot in advance
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Rubberman
Joined Jul 30, 2007
Posts: 943
Location:40 miles west of Chicago

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Posted: Apr 20, 2009 4:54:52 AM
Subject: Limiting the RAM Size
I'm not sure, but it sounds like the ramdisk driver is interfering with the PAE extensions. It may be possible to tell it to locate the ramdisk at an absolute memory location so as to avoid this problem, but I'm not sure. I just find it's easier to run in full 64-bit mode, and only run 32bit applications when appropriate (such as Firefox).

Sometimes real fast is almost as good as real time. Remember, Google is your friend!

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kevinchkin
Joined Feb 18, 2009
Posts: 3

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Posted: Apr 20, 2009 9:21:11 PM
Subject: Limiting the RAM Size
Thanks a lot for helping out you guys. I finally found the way to limit it. Just in case if someone wants, here is how you can limit the RAM size on ubuntu: go to /boot/grub/menu.lst and there you'll see something like title Ubuntu 8.10, **** uuid ------ kernel /boot/vmlinuz-2.6.** ro quiet splash just after splash put mem=1G (for 1 GB RAM) So th final line would look like kernel /boot/vmlinuz-2.6.** ro quiet splash mem=1G where mem=1G says that limit RAM to 1GB :)
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Rubberman
Joined Jul 30, 2007
Posts: 943
Location:40 miles west of Chicago

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Posted: Apr 22, 2009 3:53:34 AM
Subject: Limiting the RAM Size
Since you can put 8GB in your system, I assume it has a 64-bit capable CPU. I would suggest that you install the x86_64 version of your OS instead of the 32-bit one you have now. Even with PAE, you don't get to take full advantage of the capabilities of your system.

Sometimes real fast is almost as good as real time. Remember, Google is your friend!

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