DB2 UDB for z/OS Version 8 Performance Topics - IBM Redbooks

CPU reduction using in memory work file
CPU Time (s)
600
500
400
300
200
100
0
4232 4509 4416 4135 4457
4580 3016 4417 4485 4901
QUERY NO
Figure 3-22 In memory work file - CPU reduction
V8 NO IMWF
V8 64 MB
Performance improvements observed on queries from a workload with 64 MB of IMWF
compared with no IMWF:
► Up to 40% elapsed time reduction
► Up to 31% CPU time reduction. Top queries from the workload:
– Query 4580: CPU time NO IMWF = 560 sec. and with IMWF = 446 (reduction of 21%)
– Query 4485: CPU time NO IMWF = 559 sec. and with IMWF = 386 (reduction of 31%)
– Query 4901: CPU time NO IMWF = 553 sec. and with IMWF = 411 (reduction of 25%)
In practical environments 128 MB to 256 MB can be a good starting point, depending on the
workload using the snowflake data model.
Bigger improvements are anticipated in environments where work file data sets are shared
concurrently with other threads, such as for sorting.
Performance study on warehouse workload
Total elapsed time and total CPU time were measured for 79 star join queries in a highly
normalized data warehouse workload in V7 and V8.
Figure 3-23 shows the improvement in performance with sparse indexes, access path
changes and the use of in memory work file.
Chapter 3. SQL performance 63