zIIP is short for System z Integrated Information Processor. It's a so-called specialty engine for an IBM mainframe server. The zIIP raison d'etre is reduced cost of computing on the System z platform, accomplished in two ways: 1) zIIP engines cost less than general-purpose processors (often referred to as CPs -- short for central processors), and 2) zIIP engines do not factor into the cost of software that runs in z/OS LPARs. zIIP engines aren't new (they were introduced in 2006), but there are some relatively recent zIIP-related developments that are important from a DB2 for z/OS perspective. I hope that this blog entry will help mainframe DB2 people to keep their zIIP knowledge up to date.
The circle of zIIP-eligible work continues to expand
At many System z sites, the number one driver of zIIP utilization is execution of SQL statements that get to DB2 via the distributed data facility, or DDF (i.e., statements that come from DRDA requesters). Two factors emerged in recent years to boost zIIP offload for DDF workloads. First, there was an increase in zIIP eligibility for work executing under enclave SRBs in the DDF address space: a DB2 9 PTF took this zIIP offload percentage from about 55% to about 60%. Second, native SQL PL routines were introduced, also with DB2 9. SQL PL -- short for SQL procedure language -- is the language in which DB2 native SQL procedures are written. A native SQL procedure executes under the task of the application process that calls it. When the calling process is a DDF-connected application, the task is an enclave SRB in the DDF address space. In that case, a native SQL procedure is zIIP-offloadable to the tune of about 60%, as is true of SQL statements in general that are issued by DRDA requesters (in contrast, an external stored procedure always runs under its own TCB in a WLM-managed stored procedure address space, and so is not zIIP-eligible). A number of organizations have been engaged for some time in converting external DB2 stored procedures called by DRDA requesters to native SQL procedures, so as to boost redirection of work to zIIP engines.
This SQL PL-related zIIP offload booster was expanded in scope when, with DB2 10, user-defined functions as well as stored procedures could be written in SQL PL. UDFs coded in SQL PL, along with UDFs that have a RETURN statement that includes a scalar fullselect, are referred to as compiled SQL scalar UDFs, or, sometimes, as non-inline SQL scalar UDFs. Compiled SQL scalar UDFs, like inline SQL scalar UDFs and SQL table UDFs, run under the task of the UDF-invoking application process, just as do native SQL procedures. As noted previously, when the application process is a DRDA requester, the task in the z/OS system is a DDF enclave SRB, and that makes any invoked SQL UDF -- whether compiled scalar, inline scalar, or table -- about 60% zIIP-eligible.
Some IBM DB2 utility operations contribute to zIIP engine utilization. In a DB2 9 system, it's the index maintenance activity associated with LOAD, REORG, and REBUILD execution. DB2 10 added RUNSTATS processing to this mix. DB2 11 delivers still greater zIIP offload for RUNSTATS, plus zIIP offload for inline statistics generation performed as an option of LOAD, REBUILD INDEX, and REORG.
Other DB2-related zIIP utilization drivers include query parallelization (DB2 V8), prefetch read and database write operations (DB2 10), XML schema validation (DB2 10), log reads and writes (DB2 11), and clean-up of pseudo-deleted index entries (DB2 11).
And don't think that DB2 alone exercises zIIP engines. I'm seeing more and more Java programs running in z/OS systems -- certainly in WebSphere Application Server for z/OS, but also in the form of DB2 for z/OS Java stored procedures, and as batch processes and started tasks that utilize IBM's JZOS Toolkit. Java code executing in a z/OS system can be redirected to another type of specialty engine called a zAAP, but on a mainframe server with zIIPs and no zAAPs, Java programs will drive utilization of zIIPs by way of what's called zAAP-on-zIIP functionality.
The bottom-line message is this: DB2 itself, with each succeeding release, provides more and more opportunities for shifting work from general-purpose mainframe engines to zIIPs. Java programs, increasingly prevalent in z/OS systems, also boost zIIP utilization. That's good news from a System z cost-of-computing perspective, as it means that you can grow a mainframe workload without having to add general-purpose engines to a system; however, it also means that you need to keep an eye on zIIP engine utilization, because running those engines too hot could have a negative impact on application performance, as explained below.
Don't over-utilize zIIP engines
One thing that organizations have long liked about mainframe computers is the fact that you can run general-purpose engines at very high levels of utilization -- like 90% or more -- while still getting excellent application performance and throughput. The story is different for zIIP engines, and here's why: if a zIIP engine is not available when zIIP-eligible work is ready to be dispatched, that work can be directed instead to a general-purpose engine, but such redirection introduces a degree of delay. This delay can affect performance noticeably in a DB2 10 (or later) system, because (as previously noted) starting with DB2 10 prefetch processing became zIIP eligible. If prefetch reads are slowed because of overly high zIIP engine utilization, throughput for prefetch-intensive workloads (think batch, and decision support applications) can be negatively impacted. In a case of that nature, a slowdown in prefetch processing would show up in a DB2 monitor accounting long report (or an online display of thread detail data) as elevated "wait for other read" time (that is one of the so-called class 3 wait time categories).
Frankly, I'd start thinking about adding more zIIP capacity to a System z server if I saw zIIP utilization regularly reaching 60% or more.
July 2013: a zIIP capacity boost
With more and more zIIP-eligible work in z/OS systems, and given the importance of not over-utilizing zIIP engines, you'll want to make sure that zIIP capacity on your mainframe servers stays well ahead of demand for same. That task became easier with an important announcement that IBM made in July of 2013: on a zEC12 or zBC12 server, the number of zIIP engines can be up to two times the number of general-purpose engines (prior to that announcement, the number of zIIP engines on a server could not exceed the number of general-purpose engines). The more heavily weighted your DB2 workload is towards DDF-connected applications (and the more Java code you have running in z/OS LPARs), the more heavily weighted your processing capacity should be towards zIIP engines versus general-purpose engines.
zIIP engines are enabling organizations to grow z/OS system workloads in an ever more cost-effective manner. That said, you need to keep an eye on zIIP engine utilization. Yes, take actions to drive zIIP capacity utilization (so as to shift work off of general-purpose engines), but take actions as well to ensure that zIIP resources on your system are adequate to handle zIIP-eligible work in an optimal fashion.