hyper-threading degrades performance?

Hey jdehls,

My company just got me a Dell Precision650 with single 3.06 zeon, 2 gig of ddr, nvida quadro4 900xgl video card, 320 scsi. It was ordered with Win 2000 pro, but I ordered it with the hyper threading on so it came shipped with xp pro instead (2000 didnt support hyperthreading.

Well with that said, I noticed a performance increase when I turned hyperthreading off.

When hyperthreading is on, my files open slower, and it hesitates when opening programs, or while rendering 3d drawings, its half a fast.

I thought it was me at first, but im glad I checked out the forums here. It looks like im not the only one with this issue

I hope dell has some answers for this issue, Its depressing to know that my company spent almost 6K (thats incncluding the 20" lcd screen) for this pc, and my home pc with a P4 2.53, 512 rambus, and a nvidia gforce4 and ide hard drive running xp pro as well benchmarks about 5% faster it it only cost about 1K to build.

Don

depending on the app Hyperthreading may degrade performance. It changes the way CPU time is qued up. Old or applications with no optimizations are the most common ones to have problems. Many vendors have P4/HT optimization updates (Discreet etc)

Wait a minute.  How is bringing down the CPU load from 50% to 25% not a performance improvement?  Depending on your operating system, hyper-threading can optimize your systems behavior when running either multi-threaded applications or simply running multiple applications simultaneously.  Windows XP takes full advantage of this by supporting the technology.  Windows 2000 does not have native support for it but it can be utilized in that environment as well.  However, when utilized under Windows 2000 Pro, realize that this operating system only supports a maximum of 2 CPU's.  So to enable it with that OS will likely not yield any benefits and may actually have a negative impact depending on the application(s).  In order to enable this feature in Windows 2000, simply apply the multiprocessor kernel by going into device manager, and under computer, change the processor type from UniProcessor to MultiProcessor and reboot.

Your not lowering the true CPU load, you are creating extra Ques for threads to request CPU time thru (overly simplied).

You still only have 1 CPU and it's associated resource (Cache, ALU etc)..

 

Add into this the P4's long pipe, and it's "Guess what I want done Next" branching, if it tries to predict the next action and fails the operation goes to the end of the line..

 

So, unoptimized code can slow down, Optimized code speeds up (since it understands the way the P4 "thinks" if you would)

Depending upon how W2K displays/calculates processor utilisation, 25% utilization with 2 HyperThreaded processors (="4" processors) could be exactly the same as 50% utilization with 2 processors.

You application is still only using one "real" processor, just that it represents a smaller % of the apparently available processors as W2K now thinks you have 4.

Hope this makes sense

Cheers

Steve

Yes windows hasn't realy got a way of reporting it correctly.

If you run hyperthreading you should see a small drop in performance on single apps but when running manny together it will be faster over all.

I write solvers and I clearly see a 40% improvement when running 4 or more solvers at the same time but if I run one it takes slightly longer.

 

Yes windows reports it incorrectly.  If I run one solver it runs almost as fast as when I run it on a machine with HT switched off.  Yet it shows 25% usage not 50%.

There are other problems with HyperThreading too.  If you run a 2 Processor machine and run 2 threads low priority it uses all left over cycles.  Now run something important in normal priority and you see it get a processor all to itself the two low priority thread being kicked to share the same processor.

With hyperthreading switched on you will run 4 low priority process's.  When you now run your app at normal priority it gets a whoel virtual processor to it's self.  Which in reality means it's still sharing the processor with 1 (or more) of the low priorty peices of work.

 

 

We have a Precision 450 system with dual processor 3.2 GHz. We have been running our computing-extensive application program on this machine (without using any multiple threads). We tried both enabling and disabling hyper-threading. We did find the system performace wtih hyper-threading on degraded. It was measured by calculating our application program performace, not by looing at the percentage of CPU usage.

--Yiqing

Most users running new apps will see a performance INCREASE when hyper threading is enabled.  Especially given the context of which these computers are sold for.  There are literally hundreds of articles on this very subject to be found on almost every tech forum out there.