Shared memory what are the limits

[Luke Bell]

Hi I am currently trying to merge two sap systems onto one server. I have manged to get to now start the restore one of the systems but have found i do not have enough shared memeory.

I am running on 10.20 and would like to know if theres any way of working out what the shared memory settings should be?

[Steven E. Protter]

You need to work out the minimums with the vendor based on how big the SAP installation is.

One good thing to do is run glance gpm and drill the menu until you see shared memory useage. This will give you figures on what you are actually using.

Do this on both servers.

Add the figures up from the kernels, shmmax and such. If you are close to peak utilization on either box, build in a fudge factor.

Note that shmmax can be set higher than 25% of system memory, which is defined as ram plus swap.

Any setting above 25% of system memory as defined above will be ignored.

SEP

[Fred Ruffet]

No limit... and that's the problem !

You may set your shmmax kernel parameter to the size of your RAM, as Oracle request. But be carefull not to size your SGA to big, and then have a part of it in swap...

Regards,

Fred

[Patrick Wallek]

You've got to keep in mind that HP-UX 10.20 is a 32-bit operating system and thus you essentially only have 4GB of total RAM available.

The shared memory pool is approximately 3.75GB and that is for ALL 32-bit applications.

Depending on the size of the SAP instances, 10.20 may not be a good choice for merging these. You are severely limited as to RAM usage.

[Luke Bell]

i wotn hit the limit of 32 bit memory addressing, the server only has 3.5 gb of memory althoguh thres more mery left on this box with one sap instance then there is in total on the other server.

I am thinking i just need to double the amount of shared memory as both servers have the same kernel config.

i belive you have to pay for glance so i wont be able to install that so unless theres other ways to find share dmemory usage????? i will have to suck it and see.

[Bill Hassell · Accepted solution]

Actually, you're hitting far smaller limits because of the 32bit shared memory map. For standard programs, it is just 1Gb for *ALL* programs that use shared memory. Compile the program for SHMEM_MAGIC and the limit for a single program can be extended to about 1.75Gb. To see the actual shared memory usage, use:

ipcs -bmop

However, you will very seldom get to that limit because of fragmentation. Shared memory in 10.20 is, well, shared by everything. And it cannot be shuffled around within the map. Once a segment is allocated, it is stuck in that address space until released.

To see the rather messy allocation of memory in the shared memory map, download a copy of shminfo from: ftp://contrib:9unsupp8@hprc.external.hp.com/sysadmin/programs/shminfo/
and you'll see all the small chunks that get allocated. So the size of RAM is unimportant because your programs are asking for a chunk that is too large for the current map. There may be several hundred megs free but the largest CONTIGUOUS piece might be just 100 megs and the SAP/Oracle tools want more.

So the 3.75Gb of RAM might have 1000megs free but it cannot be used in the shared memory map. The only way around this limitation is to reduce the SGA and other shared memory areas needed by the programs until it fits. This will be a severe performance hit for the programs but that is the nature of 10.20. Two instances of SAP on a 10.20 box is going to be really difficult to get working reliably. You'll have to be very careful with stopping and starting programs. Read the program management and memory management white papers in /usr/share/doc on your 10.20 system for the gory details.

Note that at 11.0 and higher, you can use memory windows for 32bit programs that can create separate shared memory maps for each SAP instance. There's no way to pay for Glance anymore because it 10.20 is way out of support.