Using hugemem kernels on servers with 4GB memory or less

[Steven Mertens]

Hello,

I have a question regarding kernels compiled with different 'High Memory Support' settings.

At my workplace we use RHEL3 Update 6. Most of our systems have a maximum of 4GB physical memory installed.
Some new projects require that we have to upgrade our memory above 4GB.

The default smp kernel (2.4.21-37.ELsmp) doesn't support memory above 4GB. So we have to install the 'hugemem' kernel (2.4.21-37.ELhugemem) to make all the memory visible.

We want to use the same kernel on all our systems regardless of how much memory they have installed.

Is it a bad idea to use the 'hugemen' kernel on systems which contain 4GB memory or less ?
Are there some drawbacks if we do this .. performance issues,
others ... . All the cpu's we use support PAE so that's not an issue.

Thanks in advance.

Best Regards,

Steven Mertens

[Vipulinux]

Hi Steven

I have tried running SMP kernel on a single CPU machine without any issues.

Using Huge Mem kernel wont be an issue..the only thing is that the feature that hugemem kernel has wont be utilised.

Cheers

[Sung Oh]

I have some systems with less then 4GB and some systems with more then 16GB of memory and both cases, I have enabled "hugemem" from the same kernel and works fine. I did not see any performance issues or drawbacks.

Sung

[Vitaly Karasik_1]

As far as I see, there is no simple answer. At least different authors suggest different things:

1)http://www.puschitz.com/TuningLinuxForOracle.shtml "The hugemem kernel is not recommended on systems with less than 8GB of RAM due to some overhead issues in the kernel"

2) http://www.forum.psoft.net/archive/index.php/t-13787.html (well, it's about RHEL4) "For servers with RAM 1GB and more, we recommend to install the kernel-freevps-hugemem package"

3) http://www.redhat.com/docs/manuals/enterprise/RHEL-3-Manual/release-notes/es-x86/ RHEL3 release notes "The hugemem kernel is required in order to use all the memory in system configurations containing more than 16GB of memory. The hugemem kernel can also benefit configurations running with less memory (if running an application that could benefit from the larger per process user space, for example.)"

[Steven E. Protter]

Shalom Steven,

Nice name.

RH cluster suite required me to install the hugemen kernel in order to upgrade the rest of it when I went from 4.2 to 4.3

I simply had to have the thing installed, but could not see where it was being used and was CLEARLY not booting off of it.

As Vitaly says, I don't think the answer is simple. My gut tells me its not a good idea.

I ran several RH 7.x servers in the US without any problems doing just what you are doing.

The concept of having the same kernel on all machines is not really viable, long term. Here is an example why:

You have a 2.4 GHz Pentium server with one CPU, it will use the smp kernel but might have difficulties.

Your new 3.0 GHz server has HT Hyperthreading technology enabled and will get a 60% performance boost from using the smp kernel.

In my opinion, the right call in the last scenario is to use the EL kernel on teh older box and the smp kernel on the newer one.

Still, I undrstand your desire but politely recommend you re-think it.

SEP