feedback on PAE kernel

[skt_skt]

We had a stituation were the system was not detecting 20GB memory and later moved to 2.6.18-92.1.26.0.1.el5PAE to see the full memory (from 2.6.18-92.1.26.0.1.el5 kernel). I have given the old & new kernel info from grub.conf below.

from current uname -a

2.6.18-92.1.26.0.1.el5PAE #1 SMP Tue May 19 23:27:46 EDT 2009 i686 i686 i386 GNU/Linux

here is the /etc/grub part

title Enterprise Linux Enterprise Linux Server (2.6.18-92.1.26.0.1.el5PAE)
root (hd0,0)
kernel /vmlinuz-2.6.18-92.1.26.0.1.el5PAE ro root=/dev/vg00/lvol1 crashkernel=128M@16M
initrd /initrd-2.6.18-92.1.26.0.1.el5PAE.img
title Enterprise Linux Enterprise Linux Server (2.6.18-92.1.26.0.1.el5)
root (hd0,0)
kernel /vmlinuz-2.6.18-92.1.26.0.1.el5 ro root=/dev/vg00/lvol1 crashkernel=128M@16M
initrd /initrd-2.6.18-92.1.26.0.1.el5.img


is there a memory limit with el5 kernel? How is the feedback with PAE kernel?? We are using the PAE one for the first time in our environemnt and noticed a couple of system hangs and kernel panic in a few days.

I will be glad to see your responses

[Michal Kapalka (mikap)]

hi,

we are using PAE without any problem, of course the system have only 4 GB memory.

CentOS 5.2 for example :

[root@tux-nagios ~]# uname -a
Linux tux-nagios.xxx 2.6.18-92.1.18.el5PAE #1 SMP Wed Nov 12 10:02:30 EST 2008 i686 i686 i386 GNU/Linux
[root@tux-nagios ~]# strings /proc/meminfo |grep -i memto
MemTotal: 4147988 kB


in my opinion, if some server have more than 4 GB memmory, it will be better to use 64bit linux x86_64, of course it the application are supported on the OS.

mikap

[Matti_Kurkela]

We have been using PAE features with RHEL4 for a while now. (In RHEL 4 it was called the "hugemem" kernel.)

While PAE allows increasing the system memory up to 64 GB in a 32-bit system, each process is still restricted to an absolute maximum of 4 GB size. PAE also requires the OS to do quite a bit of tricky memory management with DMA transfers (=disk I/O, for example), so there may be an increased possibility of kernel bugs. However, we did not see any hang/panic problems with our PAE-enabled RHEL 4 systems.

A true 64-bit kernel would be able to handle larger-than-4 GB systems just as usual, with no tricks needed.

MK

[Ragu_3]

>> is there a memory limit with el5 kernel?

What is your CPU family/class? Here, Intel Xeons (multi-core + multi procs) machines are put in with the amd64/x86_64 port of the GNU/Linux system. They are performing better on RAM > 6GB, all OOM issues have disappeared after the OS port change. MySQL performs better on the HP HNs having > 16 CTs, Apache scales perfectly.

Even on Intel CPUs, particularly with the newer line of processors, installing the amd64/x86_64 port of the Linux kernel helps. PAE/normal kernels fail under certain conditions.

[skt_skt]

two years back we have been seeing a lot of kernel panicks with our RHEL AS 3 servers and most of the running with hugmem kernel. Later we shifted to SMP kernel. AFAIK, SMP one meant for multiprocessor systems and the the servers are stable till now.

Good to know PAE one is equivalent to hugemem.


Intel Xeon Quad core CPU i have. what OS port change are u mentioning here?

What is your CPU family/class? Here, Intel Xeons (multi-core + multi procs) machines are put in with the amd64/x86_64 port of the GNU/Linux system. They are performing better on RAM > 6GB, all OOM issues have disappeared after the OS port change. MySQL performs better on the HP HNs having > 16 CTs, Apache scales perfectly.

Even on Intel CPUs, particularly with the newer line of processors, installing the amd64/x86_64 port of the Linux kernel helps. PAE/normal kernels fail under certain conditions.

[Ragu_3]

>> Intel Xeon Quad core CPU. what OS port change are u mentioning?

Your Intel Xeon machine should have been installed with the x86_64 port of RHEL5, not the i686 port.

[skt_skt]

I accept 64bit is better.what is the reason or logic?

[loco_vikide]

we do run latest RHEL5.4 kernel-PAE without issues.

The PAE kernel should only be needed if you have more than 4GB of memory. If you only have 4GB of memory or less, it could affect your performance, as more low memory is allocated for kernel usage. It is optimized to run on large memory and is less stingy when managing some of the memory pools. You could always use the phoronix test suite and test a box with 2 with both kernels (regular vs PAE) and see the difference. It could be a 10-15% performance hit according to RHCE.

Cheers

[Steven E. Protter]

Shalom,

There are few reasons in these circumstances not to do the OS as x_64.

Application support to 32 bit applications is excellent, though those apps have memory limitations.

64 bit applications will be able to address more memory when needed with better performance.

Worth a look.

SEP