Re: Memory Performance - When is RAM over committed ?

Hi, Kenneth:

There are lots of things to consider, before you decide to "throw money at the problem" and just buy more real memory.

#1. how is the main storage you already have allocated? Do you have main storage "carved up" into a number of different storage pools, allocated to different subsystems, etc.? (Issue WRKSYSSTS and you can see how things are currently allocated.)
#2. do you have some memory permanently allocated to private "shared pools"? (Issue WRKSHRPOOL to see what the allocation looks like.)

#3. what is the value of QPFRADJ on your system? (Issue DSPSYSVAL QPFRADJ to see that.)

#4. what is the "degree of multiprogramming" (also called multiprogramming level) for each subsystem? That is, what is the maximum number of jobs allowed to be executing concurrently within each subsystem and memory pool? Allowing too many jobs to attempt to run simultaneously in a subsystem that is storage-constrained can cause "thrashing" where the system spends more and more time swapping pages in and out of real memory and less and less time actually accomplishing the real "work" of those jobs. You might try lowering the degree of multiprogramming by changing the "Maximum jobs in subsystem" parameter on the subsystem, or the "activity level" (ACTLVL parameter of the CHGSHRPOOL command.)

#5. You might also just need to move some real memory from one pool to another, where the target pool is currently "resource-starved" thus causing additional paging.

#6. Depending on what model of iSeries or System i you have, and how much main storage you currently have installed, you may or may not be able to add more real main storage to the configuration.

Only after doing the above kinds of "tuning" would I consider a recommendation to add more real main storage.
Hope that helps,

Mark S. Waterbury

> Graap, Kenneth wrote:

Which performance measurement value(s) do you think would provide the
best evidence that memory (RAM) is beginning to be over committed on the
iSeries?
Do you think an analysis of faulting as a percentage of total disk
operations would provide a good objective measure of this, or would some
other measurement combination be better?
I would like to be able to provide objective evidence to upper
management that it is time to purchase additional memory... Even though
I know from experience that adding memory to the iSeries is ALWAYS a
good idea if throughput and/or response times start to increase... Oh yes ... and please consider printing multiple copies of this e-mail
on green-bar paper.... <smile> P
Please consider the environment before printing this email
Kenneth
Kenneth E. Graap
Systems Administrator
NW Natural
keg@xxxxxxxxxxxxx
503-226-4211 x5537