Swap Poll

[Paul F. Carlson]

Hello, I am taking a poll:

Two systems, an rp7410 and an L2000, each currently with 12 gigs of memory. The 7410 will get an additional 8 gigs of memory, the L2000 will get an additional 4 gigs of memory. What would you do with swap:

a) Add 4 gigs of secondary swap to the L2000 and 8 gigs of secondary swap to the rp7410.

b) Turn on swapmem

Note: Each system currently has a 1:1 swap to memory ratio.

[Pete Randall · Accepted solution]

Are you using swap at all now? If not, adding more RAM will make it even less likely. I would leave swap alone.


Pete

[A. Clay Stephenson]

There are no enough data. Do you anticipate any sigficant paging out after the memory upgrade? If not, then I would enable pseudoswap and allocate no more device swap.

In fact, on the majority of my with large amounts of memory, I allocate 1GB or so of primary swap and then add enough addional swap to come up to 25% of memory and enable pseudoswap. I then monitor the systems. As long as you have available unallocated PE's in a VG and have already adjusted maxswapchunks to your largest anticipated need then it's so easy to add additional swap that it's not worth worrying about.

[James R. Ferguson]

Hi Paul:

Answer = "B". I'd enable pseudoswap (swapmem_on=1) and monitor the environment before configuring more device swap space.

Regards!

...JRF...

[A. Clay Stephenson]

Oh, and unless you increase swap space OR enable pseudoswap, your systems will not utilize the addional memory (at least as process space) because unless pseudoswap is enabled, virtual address space is limited to total device and filesystem swapspace. That's actually what pseudoswap does; it simply allows the OS to count 75% of memory as though it were swap space -- although it is never actually used as swap.

[Paul F. Carlson]

Thanks for your replies.

Pete and Clay: Neither system is currently paging out, and I wouldn't anticipate this changing after the memory upgrade.

Bunnies for all!

[Geoff Wild]

Depends - what are the systems running? SAP?

I would have 1 GB of swap per 1 GB of ram....

Rgds...Geoff

[Paul F. Carlson]

Hi Geoff,

No SAP. One system is running PeopleSquish (PeopleSoft) and Oracle, the other is running Oracle and another 3rd party app.

[Marcel Boogert_1]

Hi,

I once learned you have to answer these 4 questions:

1. How much main mem do you have? -> 20GB and 16GB

2. How much data will your application(s) want in mem at any one time? -> You should answer...

3. How much application data will be locked in mem? -> You should answer...

4. How does the virtual memory system decide when it´s time to start paging-out?

If you use pseudo-swap the minimum amount of memory would be 1/8 or real memory.

Regards, MB.

[DCE]

At a minimum - B.

If you have less physical swap than memory Without psuedoswap enabled), you can run insto situations where you will be unable to use all of the memory, because of the way the at HP reserves swap.

If you are not currently hitting swap, but just reserving it, then B is the obvious answer.

Also, if you start actually swapping out 4GB or more, your system performance is going to degrade very fast. Unless you have an app (such as SAP) that requires large amount of physical swap, psuedoswap is the best way to increase the swap in your curent configuration.

[Steven E. Protter]

a) Add 4 gigs of secondary swap to the L2000 and 8 gigs of secondary swap to the rp7410.

unless you are paging, you don't need to do this.

b) Turn on swapmem

The lowest swap to memory ratio allowed in HP-UX is half of ram. If your system is not paging, going to 1 to 1 may simply be a waist of disk space.

You need data on system use.

http://www.hpux.ws/system.perf.sh

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