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It says in How many maximum CPUs does Ubuntu support by default? that there is a maximum number of 8 cores supported by default in Ubuntu 12.04 Desktop/Server 32-Bit. What is the way to increase the maximum number of cores from the default of 8 to a higher value, like 12?

The reason I ask is because I have just built a machine with 6 hyper threading cores. There were issues installing the 64-bit version of the software from a CD (it "panicked" and crashed several times). The 32-bit installed with no problem, but I only see 8 of the 12 cores. The BIOS is fine. If I turn hyper-threading off and reboot, I see 6 cores as expected.

I am nervous to try and compile a kernal, I am still new to all of this. Is there a config setting somewhere that I should change?

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It appears that in /usr/src/linux-source-3.2.0/debian.master/config/i386/config.common.i386, from the linux-source-3.2.0 package, the maximum number of CPUs on 32-bit is set to 8, set with CONFIG_NR_CPUS=8 in that file.

This is perhaps a bug, though you really should be using the 64-bit version, which does have the max number of CPUs configured to 256. The 64-bit installer should not be crashing. If it is, it could be a bad CD write, or possibly bad hardware, which the 32-bit installer doesn't hit due to small differences in how the hardware is used.

If you do wish to use 32-bit with more than 8 CPUs, it looks like you will need to recompile the kernel, after changing the config, or file a bug report and wait for a fix to get released as an update (assuming there isn't some very good reason for limiting it to 8, and it won't be changed; though it seems like the -pae kernel should have this higher than 8, even if the non-pae one doesn't).

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Dobey, thanks for the reply. I tried to install the 64-bit version again, but this time pressing shift when the splash screen came up, and then selecting "check disk for defects." There was, indeed, a defect on the disk! With a fresh download of the 64-bit ISO, and fresh disk, the install is working. I might add though that the 8 core limit is lame, especially considering that the 32-bit version is recommended on Ubuntu's page. –  Matthew Jul 19 '12 at 13:42
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@Matthew If you could please file a bug about the 8 core limit on 32-bit, that would be great. As for the recommended version, Server is 64-bit, and Desktop is 32-bit because many proprietary end-user features that people like to install, do not work on 64-bit, or do not work as well, such as Flash, Skype, and I believe some of the apps and games in the software center store. I agree it would be better if 64-bit were recommended in this day and age, but getting those proprietary apps, games, and plug-ins to 64-bit is not trivial. :) –  dobey Jul 19 '12 at 17:27
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