max swap is only 32 gig?

[abc_18]

According to HP-UX 11.x's /usr/share/doc/11.00RelNotes, table 2-1, "Capacities of Recent HP-UX Releases", the 64-bit flavor of HP-UX has a max of 4 TB of physical RAM.

However... according to SAM, the max swap is only 32 gig (swapchunks = 16384). Is this a bug? Or does it actually make sense to allow 4 TB of RAM, but limit swap to 32 gig?

Thanks in advance.

[Sridhar Bhaskarla]

Hi,

the limit to maxswapchunks is 16384. The default swchunk value is 2048k. So, the total swap if you do not alter swchunk value would be 32GB. But you can change swchunk value to more than 2048 if you need more swap.

-Sri

[Thayanidhi]

Even if it is possible you don't need swap of more than say 4G or 8G (what ever is your main memory).
You need to enable swap_mem in kernel, thats it.
I don't think HP needs crash dump of 32G.

TT

[Steven E. Protter]

There are some within the HP-UX community that say once you get to huge amounts of memory, the need for twice as much swap goes away.

Thats opinion.

The last OS install I did made clear that the minimum amount of swap required is half RAM.

So you seem to have pointed out a discrepency.

I think what people with big superdomes do is have smaller virutal machines, via vpars or some of those nice superdome tools. Then each virtual machine has so much memory, so much swap.

Just my penny and a half.

SEP

[Armin Kunaschik · Accepted solution]

HP-UX does not swap like Linux or AIX (which use late allocation policy = allocate swapspace only if necessary)
In HP-UX allocated RAM needs to be reserved (not used!) in swapspace in case it needs to be swapped out later.
Therefore you need swapspace even it is not used any time.

As far as I understood it's necessary to have
at least the amount of RAM as "reservable" to be able to use all RAM.
That means that the swap space + pseudo swap (swapmem_on=1) has to be equal or greater the amount of installed RAM.
So you don't need twice the RAM as swapspace but half is not enough... In my opinion RAM = swapspace (+ swapmem_on=1) should do the job.

There is a memory management whitepaper that describes this behavior in detail. I don't remember the location but it's somewhere in the knowledge base.

Hope this helps,
Armin

[Gary L. Paveza, Jr.]

As stated earlier, maxswapchunks is the number of chunks allowed - maximum is 16384. However, you can increase teh size of those chunks (we use 64k for our sizes). This allows for a greater size of your swap area. We have 128 GB of swap reserved for a 96 GB machine. I try to keep a 1-1 ratio when we get this large (I have a bit extract just for future memory additions). Good thing is that you can change the value of swchunk and not have to rebuild your swap since it's just used to map the space memory. Since swap isn't preserved between reboots there's no problem.