SMT settings

Simultaneous multithreading (SMT) allows separate instruction streams, or threads, to run concurrently on the same physical processor, or core.

The IBM® POWER7® processor under SUSE Linux Enterprise Server 11 SP1 supports the following SMT modes:

ST: In this mode, 1 thread runs on the core
SMT2: In this mode, 2 threads run on the core concurrently
SMT4: In this mode, 4 threads run on the core concurrently

In ST mode, the thread associated with the core is known as the primary thread. In SMT2 mode, the additional thread is known as a secondary thread. In SMT4 mode, the next two additional threads are known as tertiary threads.

In this test, 23 of the spinning hard disks attached to System B are replaced by 52 73 GB SAS SSD devices for main database space.

Determining what SMT mode to run in is dependent on the threading capability of the workload and the overall utilization of the machine. SMT threads are not equivalent in their processing capabilities. The following tables of SMT mode test results show how SMT can be effective as the system becomes highly utilized.

SMT test results

The Table 1shows that enabling SMT2 helps the workload since the added processing capacity of the secondary threads provides the workload extra processing threads of which the workload takes advantage. Enabling SMT4 mode, however, causes some of the execution threads to run on the tertiary threads which are not as powerful, having the effect of reducing the performance of the workload.

Table 1. SMT comparison under low utilization (Workload 4)
SMT mode	Performance metric	Processor utilization
ST	100	68
SMT2	106	44
SMT4	94	23

The Table 2 shows that as the workload uses more of the machine capacity, SMT4 mode has a positive impact on the performance of the workload. In this situation, the workload has enough threads of execution and work to perform that the tertiary threads are utilized to provide extra processing capacity. As more work is added, SMT4 can provide even further performance improvements.

Table 2. SMT comparison under high utilization (Workload 4)
SMT mode	Performance metric	Processor utilization
ST	100	98
SMT2	116	71
SMT4	118	53

SMT conclusions

Selecting an appropriate SMT mode can provide benefits to many types of workloads. You must understand your workload when evaluating SMT. Less utilized systems or workloads that are not highly multi-threaded may not benefit from enabling SMT.

Highly utilized systems or workloads that are highly multi-threaded, as the Sybase IQ product is, may benefit substantially from using SMT.

Additionally, newer versions of the Linux kernel than the version in SUSE Linux Enterprise Server SP1 contain enhancements that enhance SMT behavior for POWER7 processors. These enhancements migrate work from tertiary threads to secondary or primary threads and from secondary threads to primary threads when possible. These enhancements can help to provide better processor utilization and decrease or eliminate, the effect shown in Table 1. These enhancements are not yet available in the SUSE Linux Enterprise 11 product. Currently, these are available in the Red Hat Enterprise Linux 6 product.