Hyperthreading?

Yeti · 03-04-2005, 04:06 AM

I was wondering what cases it would be advantageous to not have hyperthreading. For example, at work I use a P4 2.26 GHz w/o hyperthreading and at home I use a P4 3.4 GHz w/ hyperthreading. My work computer runs my simulations quite fast, with CPU usage going to 100%. I run the same simulations on my home computer and it actually seems slower, and the CPU usage stays right around 65%. I noticed the same thing on my other Celeron D processor as well. Is this a CPU issue or a software issue?

**Praetor** · 03-04-2005, 04:19 AM

While there are situations where HT is not ideal the performance drop is usually not earth shattering. For general every day stuff, leave it enabled however for most games and stuff (with some exceptions for games that are coded for HT), you'll prolly want it off (essentially disable it for anything that depends on the BUS as the individual execution units will be negotiation for those resources). Even then (worst case) you wont notice something like a million fps drop or anything

Yeti · 03-04-2005, 04:37 AM

Yeah, I don't think I'd ever dissable it; I do way too much multitasking to do that. I guess my question is why doesn't my simulation (basically numerical modeling) use the CPU to its max? I know my encoding programs due.

**Cromewell** · 03-04-2005, 04:41 AM

hyperthreaded CPUs always show 50% load when they are under full load for a single threaded application, only when both logical cores are in use will you ever see usage over 50%

Yeti · 03-04-2005, 05:02 AM

Quote:

hyperthreaded CPUs always show 50% load when they are under full load for a single threaded application, only when both logical cores are in use will you ever see usage over 50%

Okay, that's one of the things I was wondering about. I wasn't sure if it was able to allocate different amounts of processing power (ie 60-40, 70-30). So in that respect, hyperthreading brings with it the downfalls of a multiprocessor system along with the advantages.

**Praetor** · 03-04-2005, 06:11 AM

Quote:

hyperthreading brings with it the downfalls of a multiprocessor system along with the advantages.

Dunno if id go that far ... SMP is generally good. HT attempts to simulate that however the processing bottleneck is now access to memory and other BUS related ops

Yeti · 03-04-2005, 01:14 PM

So if a CPU is running with hyperthreading enabled its speed won't be equal to the speed without hyperthreading enabled due to the speed using multiple processors not being additive? Or am I just using screwed up logic?

Sorry for all the questions and no actual problem. I figured I'd just directly leech some knowledge from you guys

... and this hyperthreading thing has been bugging me for some time.

**Praetor** · 03-04-2005, 01:27 PM

Quote:

So if a CPU is running with hyperthreading enabled its speed won't be equal to the speed without hyperthreading enabled due to the speed using multiple processors not being additive? Or am I just using screwed up logic?

Im slightly confused and since ive not slept in 42 hours im not gonna try and think too hard ... but if you're only dealing with one thread there is somewhat of an overhead invovled (think of it as a hardware listener for new threads but since there is never a new thread, that listening acts as a minor overhead). Now if you're doing multithreaded ops than it should work as intended.

Yeti · 03-04-2005, 02:22 PM

Okay, I think I can make sense of that. Thanks for the responses... and Praetor get some sleep

**Praetor** · 03-04-2005, 02:28 PM

Quote:

Thanks for the responses... and Praetor get some sleep

No.

03-04-2005, 04:06 AM	#1 (permalink)
Yeti VIP Member Join Date: Aug 2004 Posts: 1,238	Hyperthreading? I was wondering what cases it would be advantageous to not have hyperthreading. For example, at work I use a P4 2.26 GHz w/o hyperthreading and at home I use a P4 3.4 GHz w/ hyperthreading. My work computer runs my simulations quite fast, with CPU usage going to 100%. I run the same simulations on my home computer and it actually seems slower, and the CPU usage stays right around 65%. I noticed the same thing on my other Celeron D processor as well. Is this a CPU issue or a software issue?

03-04-2005, 04:19 AM	#2 (permalink)
Praetor Administrator Join Date: Jul 2004 Location: Canada Age: 24 Posts: 19,900	While there are situations where HT is not ideal the performance drop is usually not earth shattering. For general every day stuff, leave it enabled however for most games and stuff (with some exceptions for games that are coded for HT), you'll prolly want it off (essentially disable it for anything that depends on the BUS as the individual execution units will be negotiation for those resources). Even then (worst case) you wont notice something like a million fps drop or anything __________________ ASUS P5K Premium WiFi-AP, Q6600@3.7 / ASUS P5ND, E6400@3.8 4GB OCz Platinum XTC 8500 / 4GB CorsairXMS2 6400 5x500GB Seagate 7200.10 / 2x500 Seagate 7200.10 OCz 8800GTX 768MB @ 630/800 / 2x Galaxy 8800GT SLI

03-04-2005, 04:41 AM	#4 (permalink)
Cromewell Moderator Join Date: Dec 2004 Location: Canada Age: 24 Posts: 9,786	hyperthreaded CPUs always show 50% load when they are under full load for a single threaded application, only when both logical cores are in use will you ever see usage over 50% __________________ Then, when you have found the shrubbery, you must cut down the mightiest tree in the forest with a herring!! Tiax rules all! Know this and be judged. I must plug a couple comics because they are good :D: www.ctrlaltdel-online.com www.userfriendly.org

Thread Tools
Show Printable Version Email this Page
Display Modes
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode

03-04-2005, 04:37 AM	#3 (permalink)
Yeti VIP Member Join Date: Aug 2004 Posts: 1,238	Yeah, I don't think I'd ever dissable it; I do way too much multitasking to do that. I guess my question is why doesn't my simulation (basically numerical modeling) use the CPU to its max? I know my encoding programs due.

03-04-2005, 01:14 PM	#7 (permalink)
Yeti VIP Member Join Date: Aug 2004 Posts: 1,238	So if a CPU is running with hyperthreading enabled its speed won't be equal to the speed without hyperthreading enabled due to the speed using multiple processors not being additive? Or am I just using screwed up logic? Sorry for all the questions and no actual problem. I figured I'd just directly leech some knowledge from you guys ... and this hyperthreading thing has been bugging me for some time.

03-04-2005, 02:22 PM	#9 (permalink)
Yeti VIP Member Join Date: Aug 2004 Posts: 1,238	Okay, I think I can make sense of that. Thanks for the responses... and Praetor get some sleep