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