I am running nmon64 on an AIX box and getting 99% of physical memory usage. When i brought it up to our 'Unix Admin' he said this was ok. We only really needed to worry if the machine began to swap. Is this true? I am an application developer and have always been taught the 99% memory utilization was a 'bad' thing. Does AIX give the memory back when my app needs it?

Also we have 4GB of memory on this machine and we were told that 1 GB off the top goes to the operating system. Does that sound right?

Actually, I don't know if it is bad if percent used of memory is 99%, but I want tell you about my system. I have AIX 4.3.3, and by using the nmon I note always the physical used memory is 99%. I thought, may be the reboot is the solution. When I reboot the system, the physical used memory go down to about 35%, but the this percentage grows until reach to 99% & 100% some times. Really, I want to know is this percentage normal or not?

Note: I have 1GB RAM.

apparently, there is an OS theory out there that if memory is not being used is is a bad thing. AIX is configured to consume the un-used memory for OS-level tasks (not sure what?) but 'earmarked' until an application needs it, and then it is released to the application if available. There are some schools of thought that this could actually improve performance. I am not sure how though, unless there is some initialization overhead that this model gets out of the way before your app needs the memory??? That is only a guess though, I would love for some OS experts to weigh in on this???

I also think the amount of memory that AIX is 'ear-marking' is configurable.

99-100% should be fine - see article