| If I may be so bold, I may have discovered two disadvantages to Hyperthreading. The first is very iffy and should be tested more to come to a conclusion. A processor with Hyperthreading enabled puts out more heat for a given load. The 2nd is so compelling that I no longer bother to test for the first one. It's a subtle problem which may not ever show up for anyone else in general use. One needs to obtain a program called "crashme" by some guy named Carrette. It spawns processes and hands the processor garbage data to execute in them. An OS crash is the intended result. Running Win2003 Server on a dual Opteron system, the program could not cause a crash. When I transferred the system to reside atop a 950, it crashed within an hour. After I turned off Hyperthreading, it never crashes even after running all night. I have not kept conditions to be absolutely identical between all of these runs. I like to use my computer at least occasionally. :-) So this must be tested by others in case I am mistaken.
(The OS in question is Win2003 Server 32bit, Enterprise, with all the patches up to the time of testing, including SP2. The first machine was a dual Opteron 248 on an Iwill dk8n board. The 2nd machine is a ASUS P6T SE and an Intel Core i7 950, stepping D0. Both mainboard BIOSes regularly updated to latest versions.)
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How good is hyperthreading? new - Agner Fog - 2009-09-29
How good is hyperthreading? new - Agner Fog - 2009-11-03