Specifications

5102ch02.fm Draft Document for Review May 12, 2014 12:46 pm
28 IBM Power System S822 Technical Overview and Introduction
Figure 2-8 on page 32 shows a picture of the POWER8 core, with some of the functional units
highlighted.
Figure 2-5 POWER8 processor core
2.1.3 Simultaneous multithreading
POWER8 processor advancements in multi-core and multi-thread scaling are remarkable. A
significant performance opportunity comes from parallelizing workloads to enable the full
potential of the microprocessor, as well as the large memory bandwidth. Application scaling is
influenced by both multi-core and multi-thread technology.
Simultaneous Multithreading (SMT) allows a single physical processor core to simultaneously
dispatch instructions from more than one hardware thread context. With SMT, each POWER8
core can present eight hardware threads. Because there are multiple hardware threads per
physical processor core, additional instructions can run at the same time. SMT is primarily
beneficial in commercial environments where the speed of an individual transaction is not as
critical as the total number of transactions performed. SMT typically increases the throughput
of workloads with large or frequently changing working sets, such as database servers and
web servers.
Table 2-2 shows a comparison between the different POWER processors in terms of SMT
capabilities supported by each processor architecture.
Table 2-2 SMT levels supported by POWER processors
The architecture of the POWER8 processor, with its larger caches, larger cache bandwidth
and faster memory allows threads to have faster access to memory resources, that translates
in a more efficient use of threads. Because of that, POWER8 allows more threads per core to
run concurrently, increasing the total throughput of the processor and of the system.
Technology Cores/system Maximum SMT mode Maximum hardware threads
per partition
IBM POWER4 32 Single Thread (ST) 32
IBM POWER5 64 SMT2 128
IBM POWER6 64 SMT2 128
IBM POWER7 256 SMT4 1024
IBM POWER8 192 SMT8 1536