Saturday, April 16, 2005

Introduction to Hyper-Threading

Intel has a web-based tutorial on their new Hyper-Threading Technology (HT Technology) entitled "Introduction to Hyper-Threading", which tells you how the technology works and how you can exploit it in your software.
Hyper-threading is an advanced processor design technique that allows a single processor to run more than one code thread simultaneously.  It takes advantage of the fact that not all "units" of the processor are simultaneously in use when executing a single thread.
Please note that HT is rather different than the new "dual-core" chips that are coming out.  The latter actually have two completely distinct processors ("cores").  It's not clear to me whether each of Intel's dual-core cores also support HT.


At 11:31 PM MDT , Anonymous Anonymous said...

so with all things being equal, which cpu is faster a P4 3.0Ghz with HT or a P4 3.0Ghz with dual core.

At 11:14 AM MDT , Anonymous Jack Krupansky said...

In general, an HT core is faster than a non-HT core (asuuming that the hardware is otherwise identical), although there are reports that for some application test cases it is slower.

Two cores are certainly going to be faster than one core even if it is an HT core (for hardware that is otherwise identical).

It is debatable whether a dual-core "chip" is truly a single "CPU" or is in fact two CPU's. The general public may not be able to discern the difference, but the technical term central processing unit should have a clear technical meaning. The essence of "dual-core" is that each "core" has the vast bulk of a distinct processor. So, I'd lean toward saying that a dual-core chip is really a dual-CPU chip. The other criteria is that a processor or CPU is the hardware that is capable of processing the instruction stream for a single process thread. HT confuses things a little, but it's more the exception that proves the rule. Intel's marketing literature does seem to refer to their chip as a "processor", but that may be confusion on the part of the marketing staff than a technical judgment as to the meaning of the term processor. In other words, maybe we have to cope with two distinct senses for "processor": 1) hardware that can execute a process thread, and 2) integrated hardware (chip) that contains one or more sense #1 processors.

My point is that each of the cores of a dual-core "processor chip" functions AS IF it were a distinct processor.

BTW, Intel says they are working on supporting HT for each core of a dual-core processor, but its not clear when that will be happening.


-- Jack Krupansky


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