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

["Tom G. Christensen" <tgc statsbiblioteket dk>]

Brian Long wrote:
> 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). 
>
Atleast on my workstation those are not the default settings:
[tgc throll tgc]$ cat /proc/sys/vm/pagecache
1       15      30
[tgc throll tgc]$ uname -a
Linux throll 2.4.21-37.ELsmp #1 SMP Wed Sep 7 13:28:55 EDT 2005 i686 
i686 i386 GNU/Linux
[tgc throll tgc]$ cat /etc/redhat-release
Red Hat Enterprise Linux WS release 3 (Taroon Update 6)
[tgc throll tgc]$

> Have you tried
> re-enabling swap with U6?  Because of this VM setting, the newer kernels
> should not exhibit the behavior you describe.
For me U6 has made no difference.

-tgc