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eng_mohammedmostafa 04-26-2010 01:50 PM

How to know the type of kernel?

Please , my O.S is Linux Red Hat 32-bit.
I need to know the type of the kernel
( default or PAE or xen ) kernel.

How know this?

Thanks & regards,,

johnsfine 04-26-2010 02:30 PM

I think the kernel configuration will be in the text file
/boot/config-`uname -r`
Note those things that might look like single quotes are backticks, not quotes.

I think PAE support is controlled by CONFIG_HIGHMEM64G in that file, so you should be able to find out whether you have PAE with the command

grep 64G  /boot/config-`uname -r`
On my 32 bit Centos with PAE support, that gives the result

I don't know about xen.

eng_mohammedmostafa 04-26-2010 02:36 PM


I used this command grep 64G /boot/config-`uname -r`
It gave me CONFIG_HIGHMEM64G=y

It means that kernel is PAE in my O.S ?
Also , i need to know what about default kernel? How can i know it?

pixellany 04-26-2010 02:55 PM

uname -r applies to the currently running kernel. What do you mean by "default kernel"? (e.g. maybe you mean the GRUB default?)

johnsfine 04-26-2010 02:56 PM


Originally Posted by eng_mohammedmostafa (Post 3948431)
It gave me CONFIG_HIGHMEM64G=y

It means that kernel is PAE in my O.S ?



Also , i need to know what about default kernel? How can i know it?
I don't understand the question.

eng_mohammedmostafa 04-26-2010 02:59 PM


I searched in Web and i found that the kernel types are default or PAE or xen.
That's why i ask. May be i am wrong in my question ( Is there a kernel type called default )
If i am wrong , correct me

johnsfine 04-26-2010 03:09 PM


Originally Posted by eng_mohammedmostafa (Post 3948452)
I searched in Web and i found that the kernel types are default or PAE or xen.

You might be taking that claim out of context. I don't know whether that claim even applies to your version of Red Hat (I don't know what version of Red Hat you're using).

That claim implies that "default" kernels are not PAE. Your kernel is PAE, so it is not "default" (within the meaning of the info you found in your Web search).

If you want to ask follow up questions about this, please post the output of the command
uname -r

eng_mohammedmostafa 04-26-2010 05:26 PM

[root@rac1 ~]# uname -r

pixellany 04-26-2010 06:26 PM

Kernels have many attributes---I'm not sure that PAE or Xen would be considered "kernel types"---It think that it is more correct to say that kernel was "compiled to support <<something--eg PAE>>"

"default" is certainly not a type of kernel. "default" simply means the action that will be taken if you don't do anything. For example, if you have 2 kernels installed, you can set either one to be the default by editing the GRUB menu.lst file.

Shadow_7 04-26-2010 06:49 PM

uname -a

gives a bit more info about your currently running kernel.

/boot/config files might have more information related to whatever kernel you're running. A lot of which depends on how PAE or Xen differ (if they differ).

/boot/grub/menu.lst or /boot/grub/grub.cfg might be more informative relative to boot options. Or /etc/lilo.conf depending on how your system is setup.

jschiwal 04-26-2010 06:58 PM

If you look in your package manager, I am betting that you will see packages for the kernel supplied that are built with PAE or XEN support. They probably wouldn't be the one called default, and probably have PAE and XEN in their package names, which you can check for by looking in grub's menu.lst file and at the filenames in your /boot/ directory. Also examine the .config-* files in /boot/. This are the configuration files used to build the kernels. You can learn other things about how your kernel was built, such as whether particular devices are supported.

A kernel with default in it's name (the vmlinuz file) refers to the options that your distros regard as default. Default in grub's menu.list refers to which kernel is booted by default if you don't select another.

johnsfine 04-26-2010 07:26 PM


Originally Posted by eng_mohammedmostafa (Post 3948605)
[root@rac1 ~]# uname -r

I was expecting one of the more experienced Linux users here to comment on the age of 2.6.9.

I don't actually know where to look that up. But I expect it is rather old, because of the "hugemem".

If I understand correctly, hugemem is a kernel option in RHEL 4 that was dropped from RHEL 5, because using it is usually a bad idea. It is needed for 32 bit kernels for very large amounts of ram. I'm not sure exactly how much ram you can support with PAE without hugemem (somewhere around 12GB to 16GB). Hugemem also allows a slightly larger per process virtual address space than ordinary PAE.

Hugemem is a bad idea if you don't need its features (if you have 12GB or less and don't need the extra virtual space) because it has extra overhead and some compatibility issues.

Hugemem is usually a bad idea if you do need its features because a 64 bit kernel does a much better job than hugemem at supporting the kind of 32 bit applications that would need hugemem. If you are running such applications on a cpu that can't run a 64 bit kernel, you are probably doing major enough processing on obsolete enough hardware that buying a new computer would make more sense.

I don't know much about RHEL 4, including I don't know how you select PAE without hugemem. Especially I don't know why one would use RHEL 4. If you don't want to pay for RHEL 5, you can get Centos 5. I certainly would prefer Centos 5 to RHEL 4, even if the RHEL 4 still had prepaid support.

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