Saturday, January 31, 2015

DB2 for z/OS: Native SQL Procedures and STAY RESIDENT

A couple of days ago, I received from a DB2 for z/OS professional a question about stored procedures. This individual asked specifically about the STAY RESIDENT option of CREATE PROCEDURE (and ALTER PROCEDURE), and its effect on a native SQL procedure (i.e., a stored procedure that is written in SQL procedure language -- aka SQL PL -- and which runs in the DB2 database services address space). Less than an hour after the e-mail containing the query hit my in-box, another message from the same DB2 person arrived. The lead word in this second note was "Disregard," and that was followed by explanation of the discovery that STAY RESIDENT is not applicable to a native SQL procedure.

So, this question-asker had quickly found that STAY RESIDENT is only relevant to an external DB2 stored procedure (referring to a stored procedure that is written in a language such as COBOL and which runs in a DB2 stored procedure address space); still, I think that the question remains interesting because it leads to another: if STAY RESIDENT YES or NO cannot be specified on a CREATE or ALTER PROCEDURE statement tied to a native SQL procedure, what happens to the executable after the native SQL procedure has been called and then runs to completion?

In considering this question, consider the nature of the executable. For an external stored procedure, the associated executable is a load module. When the external stored procedure is first invoked, its load module will be copied from a load library on disk into a stored procedure address space in the z/OS LPAR in which the DB2 subsystem is running. After the stored procedure has run to completion, its load module will remain in memory in the stored procedure address space if the stored procedure was created with (or subsequently altered to have) the STAY RESIDENT YES specification; otherwise, the load module will be deleted from memory after the stored procedure has finished executing. For an external stored procedure, therefore, STAY RESIDENT YES is a performance tuning option that can be appropriately utilized for external stored procedure programs that are compiled and linked as reentrant and reusable (if a stored procedure program is not reentrant and reusable and is defined -- properly, in that case -- with STAY RESIDENT NO, module load time can be reduced by loading from the z/OS Virtual Lookaside Facility, aka VLF).

If we're talking about a native SQL procedure, that stored procedure's one and only executable is its DB2 package. To put it a slightly different way, for a native SQL procedure the package is the executable (as opposed to the external stored procedure situation, in which the package is only part of the executable story -- the other part being the stored procedure's load module). What can we say about the "stay resident" characteristic of a native SQL procedure's executable (its package)? We can say about that what we'd say for ANY DB2 package: it depends on the size of the DB2 skeleton pool, the nature of the thread through which the native SQL procedure is invoked, and the RELEASE specification of the SQL procedure's package. Hereinafter I'll expand on that statement.

When a native SQL procedure is first called (say, following the most recent START of a DB2 subsystem), its package will be loaded from the DB2 directory table space called SPT01 (often referred to as the skeleton package table) into a part of the DB2 EDM pool called the skeleton pool. The EDM pool is in the DB2 database services address space (aka DBM1). The size of the skeleton pool within the EDM pool is determined by the value of the  EDM_SKELETON_POOL parameter in the DB2 subsystem's ZPARM module.

After the package has been loaded into the skeleton pool, it is copied from there into another part of the DBM1 address space, called a local agent pool, that is associated with the thread through which the native SQL procedure's CALL got to DB2. That memory-to-memory copy operation is part of the process of allocating the package to the thread. 

When the native SQL procedure is done executing, the copy of the procedure's package in the thread's agent local pool in DBM1 will remain there, allocated to the thread, if the package was bound with RELEASE(DEALLOCATE) and if the thread persists through the commit that marks the end of a DB2 unit of work. Some thread types persist through commits, and some do not. Thread types that are persistent include CICS-DB2 protected entry thread, threads belonging to IMS pseudo-WFI regions, high-performance DBATs, and batch job threads (a DB2-accessing batch program could issue many commits, but its thread will persist until the job has finished executing).

If the native SQL procedure's package was bound (or rebound) with RELEASE(COMMIT), or if the thread through which the CALL came is non-persistent, the copy of the package in the agent local pool will be deleted. Does that mean that the package is no longer resident in memory? No. The "master" copy of the package remains in the skeleton pool section of the DB2 EDM pool, and it will again be copied from there to a thread's agent local pool as needed for execution via a thread.

So, in essence a native SQL procedure's executable will "stay resident" in memory by default. For optimal performance and CPU efficiency, the package can be bound with RELEASE(DEALLOCATE) and executed by way of a persistent thread -- that keeps the procedure's package not only in memory, but in the thread's own agent local pool.

How long will a native SQL procedure's package remain in the skeleton pool portion of the EDM pool, once it's been loaded there from the skeleton package table in the DB2 directory? A long time, if the skeleton pool is adequately sized. See, once a package is in the skeleton pool it will stay there (assuming that it's not invalidated through, for example, an ALTER of an object on which the package depends) until the DB2 subsystem is stopped (as it might be, on occasion, for application of software maintenance) or until DB2 steals the space in the skeleton pool occupied by the package in order to make room for another package that has to be brought in from SPT01. That space steal will only happen, of course, if the skeleton pool becomes full, and even then only space occupied by inactive packages is subject to stealing. Your DB2 monitor, via a statistics long report or an online display of EDM pool activity, can provide you with information that can help you to determine whether or not the skeleton pool is appropriately sized. In particular, check to make sure that you aren't getting any program failures caused by a full skeleton pool (such failures would indicate that the pool is undersized, and probably significantly so). Also check the number of free pages in the pool (I like to see a number of free page that is at least 10% of the total number of pages in the skeleton pool), and the skeleton pool hit ratio (the percentage of package allocation requests that were satisfied out of the skeleton pool, as opposed to requiring a package load from the skeleton package table in the DB2 directory -- I like to see a hit ratio of 99% or more for a production DB2 subsystem).

There you have it. As mentioned, what's true for a native SQL procedure's package, memory-residency-wise, is true for packages in general. What's different about a native SQL procedure is the fact that there is no executable beyond the package; so, if that package stays resident in memory, the native SQL procedure stays resident in memory.

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