When it comes to analyzing the performance of a Db2 for z/OS system and associated applications, I think of a set of concentric circles. The outermost circle represents the operational environment in which Db2 is processing work - that would be the z/OS LPAR (logical partition) in which the Db2 subsystem runs. The next circle within that outer one represents the Db2 subsystem itself - its buffer pool configuration, EDM pool, RID pool, lock manager, recovery log, etc. The innermost of these concentric circles relates to the applications that access Db2-managed data. With the three circles in mind, I take an "outside-in" approach to Db2 system and application tuning. In other words, I begin with a look at the z/OS system within which Db2 is running, then I turn to the Db2 subsystem itself and lastly I analyze application-centric information. The reason for this approach? If the z/OS system in which Db2 is running is constrained in some way, there's a good chance that Db2 subsystem and application tuning actions will yield little positive impact. Similarly, if the Db2 subsystem is operating in a constrained fashion then application tuning actions may not do much good.
So, if assessing the operation of a z/OS system is important prior to turning to Db2 subsystem and/or application performance analysis, how do you determine whether the z/OS LPAR in question is running in a constrained or an unconstrained way? I do this based on examination of information in two RMF reports: the Summary report and the CPU Activity report. If you support a Db2 for z/OS system, you should be familiar with the content of these reports - in particular, some key metrics provided by the reports. In this blog entry I'll point out those key metrics and explain how I use them.
[Note: I am referring to reports generated by IBM's RMF z/OS monitor because RMF is the z/OS monitor with which I am most familiar. If your organization uses another vendor's z/OS monitor, that monitor might be able to generate reports similar to the RMF reports that are the subject of this blog entry - if need be, check with the vendor on that.]
The RMF CPU Activity report
RMF, by default (you can change this), carves the time period covered by a CPU Activity report into 15-minute intervals (so, if you requested a report for a one-hour time period you will see within that report four sub-reports, each providing information for a 15-minute part of the overall one-hour time period). Within a given 15-minute interval you will see, for the z/OS LPAR for which the report was requested, information that looks something like this (I highlighted two important values in red):
---CPU--- ---------------- TIME % ----------------
NUM TYPE ONLINE LPAR BUSY MVS BUSY PARKED
0 CP 100.00 87.03 86.85 0.00
1 CP 100.00 77.76 77.68 0.00
2 CP 100.00 83.88 83.78 0.00
3 CP 100.00 87.07 86.91 0.00
4 CP 100.00 76.23 76.14 0.00
5 CP 100.00 76.79 76.71 0.00
6 CP 100.00 80.45 80.35 0.00
7 CP 100.00 73.29 73.24 0.00
8 CP 100.00 63.83 69.22 0.00
9 CP 100.00 57.78 62.95 0.00
A CP 100.00 35.28 48.33 17.01
TOTAL/AVERAGE 72.67 75.16
12 IIP 100.00 66.63 58.68 0.00
13 IIP 100.00 26.70 23.42 0.00
14 IIP 100.00 9.21 8.07 0.00
3E IIP 100.00 0.00 ----- 100.00
TOTAL/AVERAGE 25.64 26.86
Here is an explanation of what you see in the report snippet above:
Note that for this LPAR, there are two MVS BUSY values for each zIIP engine. Why is that? Well, it indicates that the zIIP engines are running in SMT2 mode. SMT2 is short for simultaneous multi-threading 2, with the "2" meaning that z/OS can dispatch two pieces of work simultaneously to the one zIIP core. Running a zIIP engine in SMT2 mode does not double the engine's capacity (each of the two pieces of work dispatched to the one zIIP core will not run as fast as would be the case if the zIIP engine were running in "uni-thread" mode), but for a transactional workload SMT2 can enable a zIIP engine to deliver around 25-40% more throughput versus uni-thread mode (think of a one-way, single-lane road with a speed limit of 60 miles per hour versus a one-way, 2-lane road with a speed limit of 45 miles per hour - the latter will get more cars from A to B in a given time period if there's enough traffic to take advantage of the two lanes). For more information on SMT2 mode for zIIPs, see the entry on that topic that I posted to this blog a few years ago.
- NUM - This is the ID of a given "engine" (processor core) configured for the LPAR.
- TYPE - CP is short for central processor (typically referred to as a "general-purpose engine"); IIP is short for integrated information processor (typically referred to as a "zIIP engine").
- LPAR BUSY - Engine utilization from the LPAR perspective.
- MVS BUSY - I think of this as the busy-ness of the physical engine - if the engine is used exclusively (or almost exclusively) by the LPAR in question, the LPAR busy and MVS busy numbers should be very similar.
- PARKED - This is the extent to which an engine's capacity is NOT available to the LPAR during the 15-minute interval (so, if the engine is seen to be 75% parked then the LPAR has access to 25% of that engine's processing capacity). When engines in a mainframe "box" (sometimes called a CEC - short for central electronic complex) are shared between several LPAR's it's not unusual to see a non-zero parked value for at least some of an LPAR's engines).
OK, to the values highlighted in red in the report snippet:
Below the information shown in the report snippet above, you'll see a sideways bar chart that looks something like this (again, I've highlighted some key information in red):
- TOTAL/AVERAGE MVS BUSY for the general-purpose engines (75.16 in the report snippet) - As a general rule, application performance - especially for transactional applications (e.g., CICS-Db2, IMS TM-Db2, Db2 DDF) - will be optimal if average MVS busy for an LPAR's general-purpose engines does not exceed 80%. When average MVS busy for the general-purpose engines exceeds 90%, you can see a negative impact on the performance of Db2-accessing applications in the form of what is labeled "not accounted for" time in a Db2 monitor-generated accounting long report or an online monitor display of Db2 thread detail information. Not-accounted-for time is in-Db2 elapsed time that is not CPU time and not one of the "known" Db2 wait times (those are so-called class 3 wait times, such as wait for database synchronous read, wait for lock/latch, wait for update/commit processing, etc.). It's literally elapsed time, related to SQL statement execution, for which Db2 cannot account. In my experience, in-Db2 not-accounted-for time is most often a reflection of wait-for-dispatch time, which itself is indicative of CPU contention. I'm generally not too concerned about not-accounted-for time as long as it's less than 10% of in-Db2 elapsed time for an application workload - particularly when it's a higher-priority transactional workload (you might tolerate a higher percentage of not-accounted-for time for a lower-priority batch workload). If not-accounted-for time exceeds 10% of in-Db2 elapsed time (again, especially for a higher-priority transactional workload), that would be a matter of concern for me, indicating that CPU contention is negatively impacting application throughput.
- TOTAL/AVERAGE MVS BUSY for the zIIP engines (26.86 in the report snippet) - How "hot" can you run zIIP engines before zIIP engine contention becomes a concern? That depends on how many zIIP engines the LPAR has (and, to a lesser extent, whether or not the zIIPs are running in SMT2 mode). The more zIIP engines an LPAR has, the higher the average MVS busy figure for those engines can go before zIIP contention becomes an issue (in the example shown above, the LPAR has three zIIP engines that are running in SMT2 mode - in such a situation average MVS busy for the zIIP engines could probably go to 40-50% without zIIP contention becoming an issue). And when does zIIP contention become an issue? When the zIIP spill-over percentage gets too high, as explained in an entry I posted a few years ago to this blog. [Note: the report snippet shows four zIIP engines, but the fourth - the one identified as processor number 3E - is 100% parked from the LPAR's perspective. That means the LPAR had no access to zIIP processor 3E's capacity, so in effect the LPAR had three zIIP engines during the time interval.]
-----------------------DISTRIBUTION OF IN-READY WORK UNIT QUEUE-
NUMBER OF 0 10 20 30 40 50 60 70
WORK UNITS (%) |....|....|....|....|....|....|....|....
<= N 55.9 >>>>>>>>>>>>>>>>>>>>>>>>>>>>
= N + 1 3.5 >>
= N + 2 3.1 >>
= N + 3 3.5 >>
<= N + 5 5.5 >>>
<= N + 10 10.9 >>>>>>
<= N + 15 5.7 >>>
<= N + 20 4.2 >>>
<= N + 30 3.1 >>
<= N + 40 1.5 >
<= N + 60 1.3 >
<= N + 80 0.4 >
<= N + 100 0.2 >
<= N + 120 0.1 >
<= N + 150 0.2 >
> N + 150 0.2 >
N = NUMBER OF PROCESSORS ONLINE UNPARKED (16.8 ON AVG)
With regard to the report snippet above, the first thing to which I want to draw your attention is the bottom-line information about "N". We see that, for this LPAR during this 15-minute interval, N = 16.8. What does that mean? It means that the LPAR had "16.8 processor targets to which pieces of work could be dispatched." Why do I use the phrase "processor targets" instead of "processors?" It's because we tend to think of "mainframe processors" as meaning "mainframe engines," and that's not quite the case here. This report snippet goes with the first one we looked at (the second snippet appears right after the first one in the source RMF CPU Activity report), and you might recall that the first snippet showed that the LPAR's three zIIP engines are running in SMT2 mode. For that reason, those three zIIP engines are counted as six processor targets to which pieces of work can be dispatched. Thus, when the report shows that N = 16.8, we can say that 6 of the 16.8 relate to the LPAR's zIIP engines. That leaves 10.8 (16.8 - 6 = 10.8). We've accounted for the zIIP engines, so the 10.8 number relates to general-purpose engines. Where does that 10.8 come from? Refer again to the first report snippet. You'll see that the LPAR had 10 general-purpose processors that were not at all parked (i.e. that were 0% parked from the LPAR's perspective). An 11th general-purpose engine, identified as processor number A, was 17.01% parked during the interval. That means that 83% of the capacity of general-purpose processor number A (that's a hexadecimal A) was available to the LPAR during the time interval. That 83% is equivalent to 0.83, and RMF rounds 0.83 down to 0.8, and that's where the ".8" of N = 16.8 comes from. So, then, the LPAR had 6 zIIP "targets" to which work could be dispatched (3 engines, each running in SMT2 mode), and 10.8 general-purpose targets to which work could be dispatched, and that's why we have N = 16.8.
With N now understood, we can turn our attention to the other bit of information I highlighted in red: <= N 55.9. What does that mean? It means that for 55.9% of the time in the 15-minute report interval, the number of "in and ready" tasks (i.e., the number of tasks ready for dispatch) was less than or equal to the number of processor targets to which pieces of work in the LPAR could be dispatched. When that is true - when the number of in-and-ready tasks is <= N - there is essentially nothing in the way of CPU constraint, because an in-and-read task won't have to wait in line to get dispatched to a processor. In my experience, when the <= N figure is above 80%, the LPAR is very unconstrained in terms of processing capacity. A figure between 50% and 80% is indicative of what I'd call moderate CPU constraint, and performance (particularly in terms of throughput) is likely not impacted much by a lack of processing capacity. When the figure is below 50%, I'd say that CPU constraint could be impacting throughput in a noticeable way, and if it's below 10% the performance impact of CPU constraint for the LPAR could be severe. As previously mentioned, the Db2 performance impact of a CPU-constrained system is typically apparent in elevated levels of in-Db2 not-accounted-for time, as seen in a Db2 monitor-generated accounting long report or an online monitor display of Db2 thread detail information.
One more thing about an RMF CPU Activity report: the number of engines configured for an LPAR - something that is shown in the report - should be balanced by an adequate amount of memory (aka real storage) so that the LPAR's processing power can be fully exploited to maximize application performance. For a z/OS LPAR in which a production Db2 workload runs, my rule of thumb, based on years of analyzing system and application performance data, is this: the LPAR should have at least 20 GB of memory per engine with which it is configured. The first report snippet included above shows that the LPAR has 13.8 engines: 10.8 general-purpose engines (as previously mentioned, the ".8" relates to an engine that is about 20% parked from the LPAR's perspective) and 3 zIIP engines (and for balanced-configuration purposes, I count physical zIIP cores - I don't double-count a zIIP engine because it is running in SMT2 mode). I'd round the 13.8 to 14 (the nearest integer) and say that on that basis the LPAR should have at least 14 X 20 GB = 280 GB of memory. If that seems like a lot to you, it shouldn't - mainframe memory sizes are getting bigger all the time, and real storage resources in the hundreds of GB are no longer unusual for production z/OS LPARs, especially those in which Db2 workloads run (the biggest real storage size I've personally seen for a z/OS LPAR is about 1100 GB).
The RMF Summary report
An RMF Summary report is smaller in size than a CPU Activity report - typically, one line of information for each 15 minute interval within the report time period. An RMF Summary report for a one-hour period could look something like what you see below (I removed some columns so that I could use a big-enough-to-read font size - the really important column is the one on the far right, with the numbers highlighted in green):
NUMBER OF INTERVALS 4 TOTAL LENGTH OF INTERVALS 00.59.58
-DATE TIME INT ... JOB JOB TSO TSO STC ... SWAP DEMAND
MM/DD HH.MM.SS MM.SS ... MAX AVE MAX AVE MAX ... RATE PAGING
11/03 09.15.00 15.00 ... 83 72 96 92 371 ... 0.00 0.00
11/03 09.30.00 14.59 ... 85 68 98 95 369 ... 0.00 0.00
11/03 09.45.00 15.00 ... 75 68 95 92 363 ... 0.00 0.00
11/03 10.00.00 14.59 ... 82 70 94 91 365 ... 0.00 0.00
-TOTAL/AVERAGE ... 85 69 98 93 371 ... 0.00 0.00
So, what's the LPAR's demand paging rate? It's the rate, per second, at which pages that had been moved by z/OS from real to auxiliary storage (to make room for other pages that needed to be brought into real storage) were brought back into real storage on-demand (i.e., because some process needs to access the page). Why is the demand paging rate important? Here's why: it is, in my opinion, the best indicator of whether or not memory usage can be expanded without putting too much pressure on the LPAR's real storage resource. Here's what I mean by that: suppose you have a Db2 buffer pool that has a total read I/O rate (synchronous reads + sequential prefetch reads + list prefetch reads + dynamic prefetch reads, per second) that's higher than you like - maybe the total read I/O rate for the pool is north of 1000 per second, and you want to bring that down substantially to boost application performance and CPU efficiency (every read I/O eliminated saves CPU and elapsed time). The best way to lower a buffer pool's total read I/O rate is to make the pool larger. Can you do that without putting too much pressure on the LPAR's real storage resource? Here's what I'd say: If the LPAR's demand paging rate is consistently less than 1 per second, you have a green light for using more memory to get a performance boost. If the LPAR's demand paging rate is consistently greater than 1 per second, I'd hold off on using more memory until the LPAR is configured with additional real storage. This goes for any action that would increase memory usage by DB2 - besides enlarging a buffer pool, that could be a RID pool or a sort pool or an EDM pool size increase, or increasing the use of RELEASE(DEALLOCATE) packages with threads that persist through commits, or whatever. Before doing something that will increase memory usage, check the LPAR's demand paging rate.
That's it for now. If you haven't had a look at these two RMF reports before, get them for an LPAR of interest to you and give them a look-see - a z/OS systems programmer at your site would probably be happy to generate the reports for you. Knowing the key utilization and configuration characteristics of the z/OS LPAR in which a Db2 subsystem runs is an important part of effective performance management of the Db2 environment.