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  #1 (permalink)  
Old 30th September, 2003, 08:31 PM
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Hyper-Threading, does it mean anything in the real world?

Folks, my ignorance is deep about Hyper-Threading. I'm lead to believe that processor's so equipped can process more than one thread at a time. Is this a genuine replacement for a multi-processor rig? If so, do P4's capable of H-T really have more than one "core," or is this not needed because of impressive mumbo-jumbo? Or genuine mojo? Thank you very much for your time and effort!
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Old 30th September, 2003, 09:08 PM
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Yes, no and maybe, depending on your applications!

The H-T enabled processors don't have more than one core. Instead, they have some extra logic...

You see, a modern processor can actually execute more than one instruction at a time. Each bit of the CPU that can execute an instruction is known as an execution unit. By running several execution units in parallel, you can speed up the processor, as it usually takes more than one clock cycle to execute a single instruction. Hence, by having several execution units, you can make the processor exectute instructions faster in the same time. Of course, this ties in with "out of order" execution, where the execution units on the CPU can execute instructions in a different order to the way they're lined up in the program. This is to help keep the CPU running flat out.

That's cool, but there's different types of execution unit. For instance, there's integer executions units, there's floating point execution units, and more, depending on the CPU in question.

This is where H-T comes in. Most programs don't contain an optimised mix of instructions, meaning that quite often, there's execution units on the CPU sitting idle, even with the out of order execution going on.

What H-T does, it take those idle execution units, and makes a virtual CPU out of the idle ones. Of course, there's a slight overhead in doing this, but it's usually outweighed by the fact that programs are not optimised very well.

The times there's problems is when two programs both want the same execution units (like two copies of F@H for instance). Then, obviously, one program is stalled whilst the other proceeds.

The other issue is the fact that there's still the same amount of cache onboard the CPU, but now it's feeding two programs. Effectively, you've just halved the amount of cache on the CPU. Worse still is when two programs go heavily after the main memory, as you've just split the FSB between these two programs. The likelihood is that the two programs will be using different pages in RAM, and hence involve a lot of shuttling between pages. This can seriously slow down a machine! Fortunately for Intel, this doesn't happen too often, otherwise H-T would cause more of a slowdown than a speed up.

There, nice and simple.

Of course, if applications were better optimised, then H-T would bring much less benefit to users...
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Old 3rd October, 2003, 04:00 PM
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Thank you Aedan, you da man! Ahh. So programs don't need to be "optimized" to benefit from Hyper-Threading? If so, then H-T is really a boon to the PCing public? Thanks for taking the trouble to explain H-T!
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Old 3rd October, 2003, 05:26 PM
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Old 4th October, 2003, 01:05 PM
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Hyper threading is one of those things like raid, once you use it you dont want to go back. Benchmarks and such might not be boosted that much but the overall feel of the system is much better it makes multitasking so much more fluid.

With respect to the cache issue that is most likely why the P4 EE is so quick it has 4x stock cache so even when its halved by ht its got twice as much as a normal p4 would have.
Again with the fsb thing the 800mhz bus is nice and quick (although mines running at 1000mhz )

:Thumbs Up: to intel :-)

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Old 5th October, 2003, 06:34 AM
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I have been using HT for a little over a week and I am pretty impressed so far. Haven't been able to do much with it yet but have played some games and of course a lot of folding. My new chip is a 2.6C not overclocked which is 200 Mhz slower than the overclocked P4 it replaced. One instance of F@H runs slower of course but running a second instance produces more points overall than the faster chip it replaced. The games seem to run somewhat faster and of course normal surfing seems about the same.

One other thing is Windows XP Pro and Mother Board Monitor CPU load reports two CPU's just the same as a dual CPU system.
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Old 5th October, 2003, 10:56 AM
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Glad to see ya still floating around S_Wilson.
Thanks for the 411 Aedan!!
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Old 11th October, 2003, 03:25 PM
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Quote:
Originally Posted by Áedán
What H-T does, it take those idle execution units, and makes a virtual CPU out of the idle ones. Of course, there's a slight overhead in doing this, but it's usually outweighed by the fact that programs are not optimised very well.
Aedan, when you say programs aren't optimised, what do you mean exactly? Is the code just sloppy and not as efficient as it could be in performing operations? Or do you mean that it's not optimised for a particular processor etc?

great article
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Old 11th October, 2003, 03:56 PM
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Writing in C++ or similar high level languages is all very well... However, the compiler has to turn it back into machine code that the processor can actually understand! This is where things can go wrong.

A lot of code isn't optimised specifically for any particular processor. This means it'll run on any machine, just suboptimally.

The problem is, to write fully optimised code means having a different version of code for each type of processor the code will run on. That means at least Athlon, Pentium IV and generic (IE, anything else!)
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Old 11th October, 2003, 04:31 PM
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If "non-optimised" code runs better on a processor equipped with Hyper-Threading, Intel has created one hell of a CPU. Argh. Can AMD steal an eighteen month lead on Intel again? If 64-bit capability wasn't of very little value to the majority of PC owner's IMHO, Chimpzilla could be ahead right now.
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Old 11th October, 2003, 07:04 PM
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The non-optimised code doesn't necessarily run better. HT just allows another process access to those execution units that aren't being used.

HT does have an overhead, so if the code were to be better optimised, then HT would have less of an impact. I suspect this is why some programs demonstrate little performance increase, and perhaps a performance decrease. In the case of the decrease, the code is already heavily optimised for the processor, and HT can do nothing but get in the way.

Conversely, a huge speedup tends to indicate that the program's pretty badly optimised...

Of course, if you run Linux, you can change the level of optimisations. That's where distributions like Gentoo come in handy, as you can optimise the whole system!
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Old 18th October, 2003, 04:07 PM
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Quote:
Originally Posted by Áedán
The non-optimised code doesn't necessarily run better. HT just allows another process access to those execution units that aren't being used.

HT does have an overhead, so if the code were to be better optimised, then HT would have less of an impact. I suspect this is why some programs demonstrate little performance increase, and perhaps a performance decrease. In the case of the decrease, the code is already heavily optimised for the processor, and HT can do nothing but get in the way.

Conversely, a huge speedup tends to indicate that the program's pretty badly optimised...

Of course, if you run Linux, you can change the level of optimisations. That's where distributions like Gentoo come in handy, as you can optimise the whole system!

Is it just me or does this sound like the whole idea with the "quantispeed architecture" on the Ahtlon XP`s the whole point with that is that they do more than 1 instruction per cycle? If so (and if I was intel advertising dept) id start calling chips with HT P4 5000+ etc
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Old 18th October, 2003, 05:38 PM
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Quote:
Originally Posted by MercutioUK
Is it just me or does this sound like the whole idea with the "quantispeed architecture" on the Ahtlon XP`s the whole point with that is that they do more than 1 instruction per cycle? If so (and if I was intel advertising dept) id start calling chips with HT P4 5000+ etc
Actually, the more than 1 instruction per cycle has been around for a long time, at least as long as the first Pentium. What Aedan is talking about is something a little different. H-T takes unused execution units and creates a virtual processor. This allows H-T to take advantage of programs written for multiple processors.
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Old 12th July, 2004, 08:55 AM
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I have had a HT P4 3.06GHz for a while now, and have been impressed with the multitasking abilities, it does everything I need.
Here is an example, I run prime95 torture, it takes 50% of CPU, leaving me with the other 50% for all my stuff, I run 4instances of IE, and 3 of mozilla and my music with winamp and visuals, and the PC does not slow hardly..this is why HT is wonderful....MULTITASKING
I only have the 533MHz fsb version, but I have yet to see a CPU that can compete when it is at 3.4Ghz.. here is bench.. http://alexrushing.com/miscpictures/megaownage2.jpg

The bench shows it whomping a FX53, 3200XP, and a couple of P4C, so in my judgement, the P4 HT is perfect for me, because I like extreme performance.
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Old 12th July, 2004, 02:22 PM
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HT doesn't give you an extra CPU however. It simply uses any unused execution units in another 'instance' of a CPU. If I run code that uses nearly all the execution units on an HT processor, the second 'virtual' processor doesn't have any execution units free to run code on. So, effectively, it's halted until my program finishes with those units. That's not 50% of a CPU. That's any remaining dregs, and it also gives you a really good idea has to how badly optimised the code you're running is.
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Old 12th July, 2004, 06:01 PM
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Well, prime95 torture uses no more than 50%
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Old 12th July, 2004, 06:02 PM
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That's due to limitations within Windows (and other OSes), in that they cannot tell exactly how busy the execution units are. They can only tell how long a process was running on the CPU.

If I have a program that will use just one execution unit, but will use it every time it gets the chance, Windows will show 50% usage. If I have a program that uses 18 execution units at once, and will use them every time it gets the chance, Windows will STILL show 50% usage.

However, the first program will leave the other 17 execution units free for another program to use. The second program will use all of the execution units, so anything waiting on the virtual processor will have to wait until the second program is no longer running.
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Old 12th July, 2004, 06:05 PM
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That's because it is only using 1 of the two virtual cores. It doesn't mean that it is only using half of the CPU's total horsepower.

Think of it this way. There are some vehicles that have two differentials and four wheel drive. Does this mean that they are able to pull twice the load as a vehicle with the same size engine, but only one differential? Sometimes, yes. Usually, though, the limit is the amount of power the engine can put out, not the ability to deliver it to the wheels.
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Old 12th July, 2004, 06:25 PM
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Well, I run prime 95 on my AMD and it is slow as horse shet, and I run it on my p4 and it is soo fast, even with p95 working torture in the background, this is what I am saying guys, do not get huffy and puffy about it.
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Old 12th July, 2004, 07:52 PM
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Not getting huffy and puffy, sorry if it seemed that way. Just want to make sure you understand the difference between P4 HT and true multiprocessing with two independent CPUs. There's no doubt that, for a lot of things, the hyper-threading in the P4 makes a lot of difference in the end user experience. It's not as good as a true dual processor setup (and under certain conditions may actually be worse) but for most of the things that people do with it, HT is nice. However, don't believe that when task manager tells you it is only using 50% of the CPU, it really is. It might be using all of the available CPU resources, but only doing so in one HT virtual CPU. You would still show 50% cpu usage, but the system would be dog slow.
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