Re: RHEL 4 improvement in use of swap? (compared to RHEL 3)

[Brian Long <brilong cisco com>]

On Fri, 2005-10-28 at 12:17 +0200, Leos Bitto wrote:

> About a year ago we found an issue with RHEL 3 AS, which we considered 
> serious - however, when I called Red Hat support, I was assuerd that the 
> behaviour was absolutely normal. What happened was a server with 4 CPUs, 
> 2 GB RAM and 2 GB swap, which started swapping terribly after some disk 
> activity appeared, and kept swapping ever since then, until we rebooted it.
> 
> We have an in-house developed Java application, which is running using 
> Sun's JVM 1.4. This application receives authentication requests from 
> some black-box device and has to respond quickly, otherwise that 
> black-box device considers the authentication requests failed.
> 
> We were able to trigger the issue simply by analyzing some large logs 
> using simple grep commands. What happened was that the server cached the 
> logs in RAM and forced the application to swap and thus slow down 
> unacceptably. What bothered us was that the application never returned 
> to the original speed, even after we stopped accessing the logs. Even 
> after several hours it did not recover, so we had to reboot the server.
> 
> We solved it by turning off the swap - fortunatelly we had enough RAM. 
> This way the RAM does not get filled with disk cache and the application 
> is not forced to slow down due to swapping and everything works fine. 
> When we tried this application on RHEL 4, it worked much better - no 
> swapping, even with heavy disk activity.

Leos,

This was a problem in the original RHEL 3 settings
for /proc/sys/vm/pagecache.  In one of the updates, Red Hat changed the
defaults to "1 15 100" (I forget the original defaults).  Have you tried
re-enabling swap with U6?  Because of this VM setting, the newer kernels
should not exhibit the behavior you describe.

/Brian/

-- 
       Brian Long                      |         |           |
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