Installation guide

IBM Eserver xSeries 366 Technical Introduction 7
򐂰 Hyper-Threading
Hyper-Threading Technology enables a single physical processor to execute two separate
code streams (threads) concurrently. To the operating system, a processor with
Hyper-Threading appears as two
logical processors, each of which has its own
architectural state (that is, its own data, segment, and control registers) and its own
advanced programmable interrupt controller (APIC).
Each logical processor can be individually halted, interrupted, or directed to execute a
specified thread, independent of the other logical processor on the chip. Unlike a
traditional two-way SMP configuration that uses two separate physical processors, the
logical processors share the execution resources of the processor core, which include the
execution engine, the caches, the system bus interface, and the firmware.
Figure 7 outlines the basic layout of a Hyper-Threading-enabled CPU, where you can see
that only the components for the architectural state of the CPU have doubled.
Figure 7 The architectural differences of a Hyper-Threading enabled processor versus a
non-Hyper-Threading processor
Hyper-Threading Technology is designed to improve server performance by exploiting the
multi-threading capability of operating systems, such as Windows® 2003 and Linux®, and
server applications, in such a way as to increase the use of the on-chip execution
resources available on these processors.
Fewer or slower processors usually achieve the best gains from Hyper-Threading
because there is a greater likelihood that the software can spawn sufficient numbers of
threads to keep both paths busy. The following performance gains are likely:
– Two physical processors: up to approximately 25% performance gain
– Four physical processors: up to approximately 15% performance gain
– Eight physical processors: up to approximately 10% performance gain
Tests have shown that software often limits SMP scalability, but customers should expect
improved results as software matures. Best-case applications today are:
– Databases
–Java™
Note: Hyper-Threading is enabled by default on the x366, but can be easily disabled in
the BIOS.
Architectural state
Cache
Processing
resources
Cache
Processing
resources
Architectural
state
Architectural
state
Physical processor
Logical processor
Processor
without
Hyper-Threading
Processor with
Hyper-Threading