Re: Swap Space

In my experience this should read: "... will likely crash the database.". The Linux kernel includes a process called "oom-killer", which in an Out-Of-Memory (OOM) situation will try to identify and forcefully kill idle memory hogs using some advanced algorithms.
Unfortunately with Oracle processes, the most important such as SMON in the database and OPMN in the oracle app server may seem perfect candidates for this as they tend to be idle most of the time and allocate a lot of memory. Killing these of course is not a good idea, but it does free up memory. It may also kill other vital but usually near-idle processes, causing the OS to become unresponsive.
The oom-killer logs its actions to syslogd, so you can always find a trail showing why the database crashed (assuming that /var is not full and you do log kernel messages).
I am not sure that kswapd pushes up the average load by itself, as this reflects the average number of runnable processes, but I am not a kernel expert by any measure so I may well be wrong here. Incidentally, a load of 40 doesn't strike me as awfully high on 8 (16?) cores. It works out to 2.5-5/core. I've seen dual core machines work quite happily, if not extremely slowly, with loads up to 10-15 per core. At any rate, the number of runnable processes is not a reliable measure for memory usage. 

Cheers,
Tony

On 26/03/10 6:26 AM, Martin Bach wrote:

Hi Bill,

just my 0.02 worth. The amount of swap the installer requires is a bit mad
with lots of memory like Howard already pointed out. The amount of swap
you need depends on your application, bear in mind that if the system runs
out of memory and swap it will crash. Before that it will become quite
unusable since the kswapd daemon(s) will agressively try to free memory.
This process can and will push the load average very high, and if no
substantial amount of memory can be freed causing kswapd to sleep again
your system is likely to become completely unresponsive to the point where
it will have to be rebooted. Just happened to me today-the last stats top
reported was a load average of 40 with a 8 dual core opteron box, but I
expect it went much higher than that.

So if your application is gentle on memory then you only need a little
amount of swap (just for that warm fuzzy feeling), but if you know that
the box is going to be hit hard you might want to add some extra swap
space for peace of mind. Once you did that, tune the application to use
less memory-there are plenty of articles out there helping you with that
task.

Hope that helps,

Martin


No the estimates by the installer get a bit mad with a large amount of
memory

On 25/03/2010, Bill Zakrzewski<bill@xxxxxxxxxxxx>  wrote:

I am installing Oracle 10.2.0.4.0 on a linux server running RH 5.x with

...

Thanks in advance,
Bill--
http://www.freelists.org/webpage/oracle-l


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Martin Bach
OCM 10g
http://martincarstenbach.wordpress.com

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