Re: Why separate pools? -- MIDRANGE-L


On 24/11/2004, at 11:17 AM, Sue Baker wrote:

Hey, both of you are right, but for different parts of the
equation.  Data can be accessed in any memory page regardless of
the memory pool it is in.  Program specific logic and buffers
cannot be accessed across memory pool boundaries, IIRC.

IE, if I've got 2 memory pools for my interactive work and user z
loads up pgma in pool 1 and then user y loads up pgma in pool 2,
there will be two copies of pgma in memory.  If user z reads
record 12345 and then user y also reads 12345, user y will read
from memory and not perform another fetch from disk.

I realise you have a us.ibm.com address but I'd bet money** this is not correct. Program code (logic) is no different from any other object--other than it is always demand paged. It is paged in to the pool containing the process that first requests the address containing part of the object. Once in main storage it is available to any other process that needs the same page-even if that process is in a different pool.

Program variables, open data paths, UFCBs, program buffers, etc. (all the stuff that used to be in the PAG) are local to the process and as a result remain in the pool to which the process is assigned but the code is shared.

If you have 2 memory pools for interactive work and user z runs pgma in pool 1 the the pages of the program required so far by user z will be paged into pool 1. If user y runs the same pgma then user y will share the code paged in by user z, even though user y is in pool 2, as long as the code is still in main storage (i.e., its pages have not been stolen by something else). If user y takes a different path through pgma from user z then new pages may be brought in for user y and will load into pool 2. If user z subsequently follows the alternate path through pgma then user z will share the pages in pool 2 as long as they are still resident. Both users will have completely separate program variables, ODPs, etc.

I suppose it is possible that the increase in main storage sizes has resulted in a change to the storage directories such that each pool has its own directory for program pages and thus could keep multiple copies of a program around but that seems contrary to the point of single level store.

**$100 AUD donated to midrange.com if I'm proved wrong. Bill Davidson has moved on but maybe Dan Hicks or Paul Remtema is still around. Bruce Vining probably knows too.

Regards,
Simon Coulter.
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