Intels Hyper Threading

[buddhika]

What is hyper threading?
what are pros and cons of it?

Im not crossposting this, I post this because I havent got any answers to this in highly technical forum.

 

[Fullmetal Chocobo]

HyperThreading is used to create two logical processors on one physical CPU. For example, I have two actual processors in my computer (Xeon 2.66), but when I hit ALT-CTRL-DEL and look at performance, four processors come up. The pros are that multitasking goes much smoother, although the performance gain isn't twice of one processor, by any means. Also, some programs don't take advantage of multiprocessor capabilities. If you have it, enable it. See what happens.
--IC2 (SW) Sander

 

[dguy6789]

What he said. Hyper threading lets you do more things at once(multi tasking) smoother and faster then you could with it off.

 

[sunase]

Intel's HT just means the processor has extra storage locations to hold a second thread context (the instruction pointer, etc) when running processes and some other modifications to make use of it in parallel with the first. This works out to only like a 5% increase in die size, whereas a second cpu say would be 100%. ^^ Normally a processor can only work on one and has to switch out that information to work on another, but a HT one can work on instructions from both at once. This let's it make more efficient use of all the other parts of the processor some of which get stuck sitting idle at times (modern processors, even non-HT work on multiple instructions at once and don't even execute them in order when they aren't dependent on each other).

I'm not sure why you're even bothering to ask on a forum, there are plenty of good articles about it. Arstechnica had a particularlly good write up (was something like 5-6 pages if you include introductory material) although probably a bit dated by now. Re more real world considerations, it involves a rather small increase in die size and ups the efficieny of using the rest of the processor when running multithreaded stuff. So it seems like a good deal for both Intel and us. ;p

 

[dullard]

This has benchmarks for you to decide the pros and cons.

Multitasking: (see top graph on that link)
Look at the number of dots above 1.00 (pros) vs. number of dots below 1.00 (cons). Obviously if you run more than one program at once, HT is definately good and had a positive impact on almost all tests. Expect ~30% speed boost on average when multitasking and have HT enabled.

Single tasking: (see bottom graph on that link)
When running just one program at a time, HT was less impressive. HT did nothing (good or bad) on most programs, had a noticible good impact on some programs, and had a slight negative impact on very few programs.

 

[Cerb]

The Pentium 4, with its long pipelines, takes a long time to get anything done. Each stage of the pipeline takes a cycle. With a longer pipeline, however, not having the right information in cache seriously hurts the performance.
Athlons get around this by simply having shorter pipelines, and the A64 having a short pipeline and direct RAM access from the CPU.
However, lengthening the pipeline lets you get to higher speeds (less work per cycle, but you can increase the work being done all totalled by hitting high speeds), and in theory, you can get the number of instructions completed at a rate equal to the clock cycle (every stage is filled with an instruction). So there are benefits t this.

The Pentium 4 gets around the performance hit by using HyperThreading. It looks to the OS like two CPUs. So if one thread gets stuck and has to wait on getting info from the RAM, other threads can be completed as normal, instead of stalling the CPU until that one thread finally gets the information it needs. Note that it gets bogged down on an OS, like NT4 or Win2k, that won't see it as HT, but think it really is two complete physical CPUs. XP can tell the difference, and will schedule tasks accordingly.

Athlons are more responsive than P4s even in heavy multitasking situations, and Athlon64s even more so.
However, you get that responsiveness at a serious performance penalty. I've expereinced it myself w/ my AXP. Duvie did quite a few heavy multitasking benchmarks in the CPU section. A P4 with HT can do bandwidth-hogging tasks, like encoding video, and do just about any other heavy task at the same time, and both tasks perform reasonably well. Not like if you only ran one, but you don't get 50% performance on both, typically from 50-75% normal performance. On an Athlon, you'd egt 50% of both at best, but often less than that on both tasks.

So, if you want to push your machine to the limit, but don't want to spend the cash for a dually, a P4C is the answer.
If you want to click on something and get the action taken care of RIGHT THEN AND THERE, no matter what else is going on, get an Athlon64.