Tuning zIIP Usage

This document contains suggestions to avoid unnecessary SRB/TCB switches and reduce CPU overhead for zIIP support and to improve the offload to the ZIIP.

You can use the Component Switch Statistics (see the Natural system command ZIIP) to view the list of components that cause TCB switches.

CPU Time Limit - FETCH Operations

The Natural profile/session parameter MT (maximum CPU time) has the effect that each time Natural starts a program at Level 1, a z/OS timer service request must be executed. This forces Natural to switch off from the zIIP. The default setting is MT=60 to prevent endless loops in Natural applications. Consequently, each time Natural falls back to a program level of 0, the timer is restarted. This also happens with the FETCH statement: each time a program is fetched, the timer must be reset.

The default setting is MT=60 to prevent endless loops in Natural applications. Software AG recommends to set MT=0 when running a session that executes many FETCH statements. This will prevent Natural from using timer macros and avoids unnecessary SRB/TCB switches.

3GL Program Calls

Natural needs to switch off zIIP usage each time a 3GL program is executed, because Natural does not know whether any z/OS service calls are issued in the external subprogram(s).

In addition, Natural is only allowed to offload Natural-written code to a zIIP. User-defined code written in any other language must not be offloaded.

Calling 3GL programs within a Natural loop, forces many SRB/TCB switches, and therefore much CPU overhead. A high share of 3GL code will reduce the offload capabilities of Natural sessions. Such sessions are not suitable for running zIIP-enabled.

Natural Multi-Fetch Record Retrieval

Executing an Adabas access statement in Natural, causes at least one WAIT SVC call and forces Natural to switch off from the zIIP. You can reduce the number of switches by exploiting the multi-fetch capabilities of Natural:

  Without Multi-Fetch With Multi-Fetch
Records read 100,000 100,000
Number of switches into SRB mode 100,000 12,500
Total enclave CPU time 5,389 ms 958 ms

For further information on the multi-fetch feature, see Multi-Fetch Clause, in Accessing Data in an Adabas Database in the Programming Guide.

Sort Processing - SORT Parameter

External sorts cannot run on the zIIP which forces Natural to switch off from the zIIP when a record is passed to or returned from the external sort.

You can avoid unnecessary switches by defining a sort work buffer as cache buffer using the WRKSIZE keyword subparameter of the Natural SORT profile parameter. Natural collects all records in the work buffer and passes them with a single SRB/TCB switch to the external sort during the input phase of the external sort. Natural retrieves the records from the external sort into the work buffer with a single SRB/TCB switch and passes them, record by record, to the Natural application during the output phase of the external sort.

Using the Natural SORT parameter reduces the number of SRB/TCB switches to zero.

In the example below, the size of the sort work buffer has been defined with 1000.

  Using External Sort Using Natural SORT
Records read 1,145 1,145
Number of switches into SRB mode 3 0
Total enclave CPU time 128 ms 51 ms

SORT Parameter

You can specify the Natural profile parameter SORT to bypass the external sort and use the internal Natural SORT, for example:


Print and Work File Caching

Any work file or printer usage causes I/O interrupts which force Natural to switch off from the zIIP. You can avoid unnecessary switches by defining cache buffers to be used for print and work file I/O processing. These cache buffers are used to keep the data in core as long as possible and read or write data in larger chunks.

The cache buffers are defined with the keyword subparameter PWCSIZE of the profile parameter ZIIP, for example:


The buffer sizes are interpreted in KB. They specify the print buffer, read buffer and write buffer. Buffer sizes of several 100 KB are sufficient.

Rerouting Primary I/O to a Cache Buffer

Primary I/O operations are not subject to caching. Instead, the size of the terminal I/O buffer is relevant. This buffer is flushed either when it is full or when an I/O is triggered by an INPUT statement.

You can use the Natural profile parameter MAINPR to separate program output from Natural system output and reroute primary output for CMPRINT to an additional printer that is processed with a cache buffer.

Example of Cache Buffer Usage

When your application creates a printout with a line size of 132 characters, a print cache of 132 KB will reduce the SRB/TCB switches for printing by a factor of 1000. This means, Natural will not switch for every line, but only once per 1000 lines or 20 pages.

  WRITE (1) Without Cache WRITE (1) With Cache
Lines written 10,000 10,000
Number of switches into SRB mode 10,003 10
Total WLM enclave CPU time 386 ms 68 ms

For the work file handling, you will get a similar result:

  READ WORK FILE (1) Without Cache READ WORK FILE (1) With Cache
Records read 10,000 10,000
Number of switches into SRB mode 10,478 5
Total WLM enclave CPU time 641 ms 305 ms

Monitoring the Cache Usage

You can monitor the usage of the cache buffers by using the Natural system command BUS.

Start of instruction setTo monitor buffer cache usage

  • Issue the following system command:


    A Buffer Usage Statistics report then outputs the sizes used by the cache buffers:

    12:50:28               ***** NATURAL BUS UTILITY *****              2012-04-03
    User SAG                 - Buffer Usage Statistics -            OpSYS z/OS
     No. Name    Type    Size      Used   Perc.  MaxUsed   Perc.  MaxSize   Perc.
     18  PCACHE    V   512000        32    0.0    511958  100.0
     21  WCACHEO   V   307200        32    0.0    306947   99.9  
     26  WCACHE01  V   512000        32    0.0    511992  100.0
    ThrdSize   Total  1382656    166593   12.0      2000   54.9    33480      1.6
      2000K  (in KB)   1351K      163K             742K             33K
    Nat9995 Natural session terminated normally.
    • PCACHE is the cache for all print output.

    • WCACHEO is the cache for the WRITE WORK FILE statement.

    • WCACHEnn is the cache for the READ WORK FILE statements.

For statements generating output (such as WRITE or WRITE WORK FILE), only one cache buffer is allocated, also for multiple files. For the READ WORK FILE statement, there is one buffer allocated per work file. The buffers are allocated only when used.

For detailed information on the Buffer Usage Statistics, refer to the relevant section in SYSTP Utility in the Utilities documentation.

Thread Size Specification - THSIZE Parameter

In batch mode and under TSO, Natural usually allocates an internal buffer with a GETMAIN or FREEMAIN requests to the operating system. Any storage request includes an SVC (supervisor call) and requires a switch back to TCB processing mode.

You can reduce the number of GETMAIN or FREEMAIN requests by specifying a thread size with the Natural profile parameter THSIZE. Natural will then allocate the specified amount of space with one GETMAIN and then serve all buffer requests from the allocated thread storage without calling the operating system again.

At the end of a Natural session, you can use the Buffer Usage Statistics report (see the example screen) to check the buffer and thread usage and determine whether the defined thread size (MaxUsed) is sufficient to allocate all buffers used by the session.