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thrashing; ratio real/virtual memory

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mdet

Technical User
Jun 11, 2001
53
DE
Hi,
I'm looking for information regarding thrashing.
I know what thrashing is and I know how to find out whether a system is trashing or not. What I need is some statement or procedure to calculate thrashing in advance.
In other words:
Provided the mere number of running processes isn't a problem (i.e. is not high) then how much memory might these processes consume until the system starts to thrash.
I guess there only will be a rough "rule of thumb" for that but maybe there are admins who experienced thrashing and do remember the circumstances.
We're running an application server whose resident memory part is constantly growing and I'm asked to give a tip how long the system will be still working.
Don't ask me whether this is correct IT-Management.
We're running Solaris 8

Any idea is well appreciated.
 
I not sure there is a rule of thumb. You really need to know your system.

When I am evaluating my memory utilization I look at: the number of aplication users and how much the average application session takes. I then figure my average middleware utilization (i.e. Oracle SGA, MQ, bea, etc.) And finally the base operating system. Generally your base OS and middleware will remain fairly constant (unless an overzealos DBA tripples the SGA). The key is getting a good idea of how the application consumes memory. If the average app session keeps growing then you need to figure what is causing the growth and do a basic analysis to figure average rate of growth. In the end you can figure it like this:

real mem - (os util + middleware) = avail for app

Divide what's left for app sessions by the average number of sessions. Compute the current session utilization and average growth and you can get a rough idea of when you will start cratering memory. Don't forget to account for peak utilization overhead and for growth of the number of sessions (or other app variants as may apply)

Like I said after 12 years I still don't have anything better. I hope that atleast gives you a place to start.
 
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