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HP-UX and SWAP space


Title: HP-UX and SWAP space
Document: 922782577
Keywords: HPUX,swap

Q: On HP-UX do you need as much swap space as you have RAM? I was once told that if you only have 1/2 as much swap as RAM then you would only be using half your RAM. By swap, i am not including psuedo swap.

A: The correct answer is that you need as much swap as all the processes that run at the same time. How do you predict that? Generally it isn't practical to do this as it is difficult to predict the worst case scenario.

Please note that HP-UX is a virtual memory system which means that swap (plus pseudo swap if enabled) constitutes the available memory for processes. RAM is a special case of swap where programs can actually run.

Without pseudo swap, then your statement is accurate. 500 megs of RAM with only 100 megs of swap means you can only utilize 100 megs of your RAM. Since it is not possible to swap everything out at the same time, the concept of pseudo swap (an overallocation policy) allows 3/4 of RAM in megs to be added to disk swap for mapping purposes. In the above example, 100 megs fo disk swap plus 375 megs of RAM (total=475) is your virtual memory addressing space. The 3/4 RAM area is never used for swapping, it is an arbitrary number that represents a typical busy system with RAM occupied with processes.

Always enable swapmem_on. Add swap as you need it or for one-time jumps, borrow some space from a filesystem with swapon.

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