LinuxQuestions.org
Download your favorite Linux distribution at LQ ISO.
Go Back   LinuxQuestions.org > Forums > Linux Forums > Linux - Software > Linux - Kernel
User Name
Password
Linux - Kernel This forum is for all discussion relating to the Linux kernel.

Notices


Reply
  Search this Thread
Old 10-23-2006, 02:42 AM   #1
apnakon
LQ Newbie
 
Registered: Oct 2006
Posts: 1

Rep: Reputation: 0
Issues with VM in RedHat Enterprise 2.4.21-37.0.1.ELhugemem


HI All,

I have an interesting problem with the VM management routines in Red Hat Enterprise 2.4.21-37.0.1.ELhugemem and would greatly appreciate any insights.

The systems we are running here are on the above O/S, supporting Oracle 9.2.0.7 databases. We are using the Oracle Clustered File System (OCFS) version 1.14.

The problem is an interesting one. It stems from the behaviour of the Linix VM in that as much free RAM as possible is consumed by the pagecache. This results in the free heap becoming very small in relation to the available RAM. I understand that this is the normal behaviour of the Linux VM sub-system, but it has some serious negative side effects for the configuration we are running.

It would appear that OCFS (Oracle Clustered File System) does some internal checks during its operation based on the amount of Free Memory in the system. It also appears (from watching it in operation) that it is using the amount of Free Memory to determine how much RAM is actually available in the system.

Because the Free Heap (as is shown in either vmstat or /proc/meminfo) is so low, OCFS goes into a CPU spin before it can get the resource it requires.
This can have two very serious negative consequences :

1. The whole RAC cluster will slow up while the affected node is in a CPU spin or
2. The cluster manager will fail entirely and the RAC will go down.

The solution, as I see it, is to force the Red Hat Linux 3 VM to release more memory sooner into the free heap. Unfortunately, I can not find a way to do this. I have tried different setting in the /proc/sys/vm files (bdflush, pagecache, kswapd, etc.) but with no success. It appears to me that the Red Hat Linux 3 VM completely ignores any of these parameters, which is very disappointing.

If all of this seems a bit far fetched, I had exactly the same issue on our previous system which used Red Hat Linux 2.1. In that situation, I was able to write a program that automatically sensed the low RAM situation and would force the VM to return large amounts of RAM to the free heap by setting the pagecache parameters to : 10 20 30 for a short while (a few minutes) before resetting the pagecache to the default 1 50 90.

The new Red Hat V3 Linux pagecache defaults to : 1 15 30, and the parameters are used in a completely different way, so that avenue (as far as I can tell) is closed to me. Further, this version of the kernel completely ignors any different values set in 'pagecache'.

I would appreciate any help/guidance with this issue. There must be a solution, and I am more then willing to learn about it from someone with a greater knowledge of the Linux VM than myself.

As an aside, I have tuned VM's for 'big iron' Unix systems, and have never seen this behaviour - so am quite surprised by it.

Many Thanks in Advance,

Adrian.
 
  


Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is Off
HTML code is Off



Similar Threads
Thread Thread Starter Forum Replies Last Post
Disabling HIGHMEM in kernel 2.6.9-34.0.1.ELhugemem rolando1981 Linux - Enterprise 1 09-26-2006 12:29 PM
Red Hat + Elhugemem jwholey Linux - Enterprise 1 06-17-2006 07:49 AM
display horizantly shifted for redhat 9.0, redhat enterprise and fedora core dudely Linux - Enterprise 3 08-13-2005 12:28 PM
display horizantly shifted for redhat 9.0, redhat enterprise and fedora core dudely Linux - Software 3 08-12-2005 10:18 AM
display horizantly shifted for redhat 9.0, redhat enterprise and fedora core dudely Linux - Newbie 1 08-11-2005 04:11 PM

LinuxQuestions.org > Forums > Linux Forums > Linux - Software > Linux - Kernel

All times are GMT -5. The time now is 12:09 AM.

Main Menu
Advertisement
My LQ
Write for LQ
LinuxQuestions.org is looking for people interested in writing Editorials, Articles, Reviews, and more. If you'd like to contribute content, let us know.
Main Menu
Syndicate
RSS1  Latest Threads
RSS1  LQ News
Twitter: @linuxquestions
Open Source Consulting | Domain Registration