Even when we put a regular processor under 100% load, we're never fully utilizing 100% of the execution units. With a HyperThreading enabled processor those spare execution units can used towards computing other things now.

As you saw from the previous slides from last years Intel Developer Forum, the single Superscalar processor is busy computing information however about half the processor remains unused.

In the Multiprocessing portion of the demonstration we see a dual CPU system working on two separate threads, however again about 50% of both CPU's remain unutilized. In the lone HyperThreading enabled processor, both threads are simultaneously being computed and the CPU usage efficiency has gone up from around 50% to over 90%!

The last example illustrated that dual HyperThreading enabled processors can naturally work on four independent threads at the same time. Again CPU efficiency is around 90%.

While this sounds all very well and good, it's also about the most ideal situation ever so let's get back to the real world. In those examples there is a of overlapping when computing threads.

For the execution units that have been "overlapped", twice as much work is required now, so the end result is not necessarily any faster than a regular superscalar processor that worked on the same information separately. If you were to run two pieces of software that are completely different, and use different execution units, your system performance should get a noticeable boost!

The operating system will also play a key role in how well HyperThreading works. The OS assigns operations to the independent logical processors, and so if it's determined that one of the logical CPU's is to remain idle, the OS will issue a HALT command to the free logical processor thus devoting all of the other system resources to the working logical processor.

Please understand this is an extremely simplified explanation on how HyperThreading works. For more detailed information, you can read through these sections from from Intel's website: HyperThreading and Xeon processors , eBusiness Xeon and desktop HyperThreading .

pcstats test system specs
computer hardware:	server test system	client system #3	client system #2
processor:	dual xeon 2.0 ghz	pentium 3 800eb	pentium 4 2.8
clock speed:	20 x 100 mhz = 2.0 ghz	6 x 133 mhz = 800 mhz	21 x 133 mhz = 2.8 ghz
motherboards:	tyan s2722	asus cuv4x-e	msi 648 max
chipset:	e7500	via 694x	sis648
videocard:	ati rage xl	nvidia geforce ddr	msi g4ti4600-td
memory:	2x 512mb registered samsung pc2100	512mb apacer pc133	256mb corsair xms3500
hard drive:	2x 40gb maxtor d740x	40gb maxtor diamondmax+	20gb maxtor diamondmax+
cdrom:	notebook cd-rom	lg 40x cd-rom	nec 52x cd-rom
floppy:	panasonic 1.44mb floppy drive	panasonic 1.44mb floppy drive	panasonic 1.44mb floppy drive
heatsink:	appro custom heatsinks	retail intel hsf	avc sunflower
powersupply:	appro 350w custom psu	sparkle 300w	vantec 470w stealth psu
benchmarks:	webmark 2001 netbench 7.0 webbench 7.0 sisoft sandra
software setup:	windows 2000 server + sp2 intel inf 4.04.1012 windows 2000 default driver	windows 2000 pro + sp3 via 4in1 4.39v detonator 40.72	windows 2000 pro + sp2 intel inf 4.04.1012 detonator 40.72