Now that the new IBM® Power Systems™ S9XX and L922 servers have been released with support for Db2® in Power8 mode, let’s visit the benefits of SMT8 with the new larger cores available in these POWER9 systems. The POWER9 processor implements a more powerful mix of execution units and the vast resources in the POWER9 SMT8 core are best utilized with 8 threads per core.

To demonstrate the new benefits of SMT8 with Db2 products, we ran a series of analytical tests against the Db2 Warehouse product on a Red Hat Enterprise Linux (RHEL) 7.4 cluster, the Db2 BLU engine on RHEL 7.4, and the Db2 BLU engine on AIX® 7.1. All of the workloads were configured to maximize the throughput and CPU utilization and they all benefited from SMT8 versus SMT4 with results ranging from a 10% to 18% increase in performance, measured as queries per hour. The chart below shows the relative performance gains for the analytical workloads we executed. In these tests the throughput results (queries per hour) for RHEL and AIX using SMT8 were the same.

SMT8 vs SMT4 on POWER9

For transactional Db2 performance, we ran a series of OLTP tests on RHEL 7.4 and AIX 7.1 that showed a slight performance benefit of 2.5% when the system was under low utilization. However, as the workload density increased the benefits of SMT8 become clear as the difference in throughput performance increases to as high as 30%, as shown below.

Transactional Db2 performance OLTP test results

Linux on the POWER9 platform is configured to use SMT8 by default when you install the OS. You can check and change which mode you are using by issuing the following ppc64_cpu commands:

Checking SMT level
   ppc64_cpu –smt
    
Changing SMT level, where # is 1, 2, 4 or 8
   ppc64_cpu –smt=#
   

AIX on the POWER9 platform is configured to use SMT4 by default. In order to maximize the performance of the system, we need to change the SMT configuration by using the smtctl command as follows:

Checking SMT level
   smtctl
    
Changing SMT level now, where # is 1, 2, 4 or 8.  The system will run SMT4 after reboot
   smtctl -t # -w now
	
Changing SMT level and persist at reboot
   smtctl -t # -w boot
   bosboot -a
   

As a best practice, and especially for AIX, we recommend rebooting the system after changing SMT mode. Performance may suffer in some conditions if the SMT level is changed without a reboot.

More details about the ppc64_cpu command are available at: https://github.com/ibm-power-utilities/powerpc-utils and more details about smtctl can be found here: https://www.ibm.com/support/knowledgecenter/en/ssw_aix_71/com.ibm.aix.cmds5/smtctl.htm.

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