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Probably a dumb question about HT technology....

G

Grathrax

Limp Gawd
Joined
Apr 9, 2005
Messages
333
You all probably get this a lot, but so far google has proven unhelpful. If one has a 3.4Ghz P4 with HT in it, does that mean it is thought of (when HT is on) as 2 1.7Ghz Cpus or 2 3.4Ghz cpus? Im assuming the former.
 
think of it this way...if you tell the processor to do one thing that uses 25% of the processors power, then the remaining 75% is set up to emulate a second processor so that the operating system can assign the remaining cpu cycles another task. its just a more efficient way of using the cpu's speed.

thats how i understand it, someone correct me if im wrong.
 
It's 2 logical processors, one physical.
So it appears as two processors to the OS (both with the same clockspeed, because there is only one clockspeed ofcourse, so in theory it appears as 2 3.4 GHz processors... not that the clock frequency matters to the OS at all). Which means the OS can run two threads at a time...
But since these two logical processors share the physical one, you can't really speak of 2 processors of a certain frequency. The point is that a single CPU would not be using all of its execution units fully... by making it into two logical CPUs the second thread can use the execution units that are left idle by the first one.
So you should look at it as "1 3.4 GHz processor + whatever extra instructions happen to execute in the second thread". Depending on the software you run, you can get somewhere between 0-25% boost with HT.
 
Scali said:
It's 2 logical processors, one physical.
So it appears as two processors to the OS (both with the same clockspeed, because there is only one clockspeed ofcourse, so in theory it appears as 2 3.4 GHz processors... not that the clock frequency matters to the OS at all). Which means the OS can run two threads at a time...
But since these two logical processors share the physical one, you can't really speak of 2 processors of a certain frequency. The point is that a single CPU would not be using all of its execution units fully... by making it into two logical CPUs the second thread can use the execution units that are left idle by the first one.
So you should look at it as "1 3.4 GHz processor + whatever extra instructions happen to execute in the second thread". Depending on the software you run, you can get somewhere between 0-25% boost with HT.
Click to expand...

Exactly.
 
Scali said:
It's 2 logical processors, one physical.
So it appears as two processors to the OS (both with the same clockspeed, because there is only one clockspeed ofcourse, so in theory it appears as 2 3.4 GHz processors... not that the clock frequency matters to the OS at all). Which means the OS can run two threads at a time...
But since these two logical processors share the physical one, you can't really speak of 2 processors of a certain frequency. The point is that a single CPU would not be using all of its execution units fully... by making it into two logical CPUs the second thread can use the execution units that are left idle by the first one.
So you should look at it as "1 3.4 GHz processor + whatever extra instructions happen to execute in the second thread". Depending on the software you run, you can get somewhere between 0-25% boost with HT.
Click to expand...
thanks for putting it in words that i couldnt find :)
 
Perfect explanation. I remember reading about it a while back in magazines and on the net but couldn't quite grasp what it was.
 
Scali said:
So you should look at it as "1 3.4 GHz processor + whatever extra instructions happen to execute in the second thread". Depending on the software you run, you can get somewhere between 0-25% boost with HT.
Click to expand...

For the win!
 
xonik said:
More like -10% to +25%.
Click to expand...

If you're running a non-HT aware OS perhaps, like Windows 2000.
With Windows XP you should not encounter any situations where HT will be slower than no HT, because the OS scheduler is aware of HT, and avoids such situations.
 
I only have anecdotal evidence to back this up, but I suspect that Windows 2000 had a post-SP4 update to the scheduler. On my 640, my Windows 2000 install became much more responsive (and even gained 200 points in 3DMark05) after I updated past SP4...

Other than the minor differences between the UIs between 2000 and XP (with the Windows Classic UI), I wouldn't notice if I was in 2000 or XP because they tend to perform the same (on my system).
 
Scali said:
If you're running a non-HT aware OS perhaps, like Windows 2000.
With Windows XP you should not encounter any situations where HT will be slower than no HT, because the OS scheduler is aware of HT, and avoids such situations.
Click to expand...
Yes, but still, nearly 40% of Internet-connected computers aren't HT-aware. I think it's a point worth noting.

Source:
http://www.w3schools.com/browsers/browsers_stats.asp

edit: I realize that most of the owners of computers with HT technology use Windows XP, by the way ;)
 
I have a ViewSonic 17" and it looks best at 800x600. Plus, my old eyes can finally see the enemy when I am playing Desert Combat :p

EDIT: Okay, thanks xonik. I can't find the option to delete my comment so that I can stop thread-crapping. :(
 
Please, out of respect to the thread starter, no more comments about the W3C stats. I know some of them are really interesting, but let's not get off track.
 
xonik said:
Yes, but still, nearly 40% of Internet-connected computers aren't HT-aware. I think it's a point worth noting.

Source:
http://www.w3schools.com/browsers/browsers_stats.asp

edit: I realize that most of the owners of computers with HT technology use Windows XP, by the way ;)
Click to expand...

True, but let's not judge HT's performance by deliberately using the wrong software on it. I thought it was more fair to consider the performance on an OS that is HT-aware.
People who own a P4, especially one with HT, should be ashamed of themselves anyway if they run anything older than Windows XP on it :)
 
Yes, but wouldn't you consider a 0% performance increase to also be a misuse of the technology? I don't draw a line when it comes to performance of a given technology. If there's a reasonable situation where the performance could suffer, I think it's worthwhile to mention it, for the sake of full exposition of the performance advantages and drawbacks.

Of course, this conversation is quickly coming down to hair-splitting, so unless you have a retort, I'll leave this be :)
 
xonik said:
Yes, but wouldn't you consider a 0% performance increase to also be a misuse of the technology?
Click to expand...

Not really. Some technologies just don't work in all situations.
As long as they work in some situations, and don't adversely affect others, I don't see why you shouldn't use them. I wouldn't call it misuse unless it would actually adversely affects your daily situations, while you would rarely encounter any situations where it would have a positive effect, in daily usage.
I think it's safe to say that most users with Windows XP are generally better off with HT enabled.

I don't draw a line when it comes to performance of a given technology. If there's a reasonable situation where the performance could suffer, I think it's worthwhile to mention it, for the sake of full exposition of the performance advantages and drawbacks.
Click to expand...

True, but as you see, I pretty much take Windows XP for granted on HT-enabled PCs... I mean, to me it's no different than pointing out that a P4 processor won't fit on your 286 motherboard :)
 
xonik said:
Yes, but wouldn't you consider a 0% performance increase to also be a misuse of the technology? I don't draw a line when it comes to performance of a given technology. If there's a reasonable situation where the performance could suffer, I think it's worthwhile to mention it, for the sake of full exposition of the performance advantages and drawbacks.

Of course, this conversation is quickly coming down to hair-splitting, so unless you have a retort, I'll leave this be :)
Click to expand...

Even disregarding the extra performance increase (which should be seen as a bonus in any case), one huge advantage it has given me is control over my system when an application hangs with 100% cpu usage. Since hyper threading gives me control over unused pipeline cycles, I can smoothly terminate those programs :cool:
 
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