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Disabling hyperthreading in i7 920


Myatu
09-09-2009, 16:15
Quote Originally Posted by bago
My application is not so concurrent. I tested that on real 8 core it only uses 4 cores. I never tested on 4x2 HT cpu like the i7, so I don't know how they perform when you simply use 4 of them (is it *half* like real 8 cores or does HT understand and handle this well?).
When you tested it on a PC with 8 real cores, would it be possible that the 2nd CPU (4 cores) was not used because of the OS (ie., how many CPUs opposed to cores is the OS capable of using?). As for the HT, it's also a matter of OS support. If it does, then a "thread is a thread".

wil
09-09-2009, 13:05
I can't speak for the new Core I7s but what a lot of you are saying is not true if looking at Pentium 4s (the hyper threading version).

For example here, I had one core constantly running at 90+% for my app, and the other around 5% for everything else.

Whilst if you have a busy system, having more "cores (virtual)" makes the system faster, on systems where the workload is very specific and you know you only need a set amount of cores, having hyper threading off increases the speed in the primary "core" by around 20-30%.

I am sure if I was doing something really heavy and having both cores at 90+%, I would see the benefit, but with HT off, my pc only displayed one core and had a average CPU of 75% - and it completed its tasks a lot quicker (compiling took a few minutes off of a 5-6 hour job).

Again, this was for P4s, but I think that Core i7 would be the same.

Andy
09-09-2009, 11:50
HT just allows for 2 threads to run on the same core. It shouldn't affect single thread speeds unless a 2nd thread is also running. It should just split up the core for each thread, so if thread 1 is using 2% then thread 2 will get 98%, but don't hold me to it. Its a very efficient technology.

Something you could try is set the affinity on the processes, and tell everything to only use the cores you selected. But I doubt HT will make the speed drop. The only way to find out would be to try it.

wackomoo
09-09-2009, 11:48
You can always just tell the process which core(s) to use in the Task Manager in Windows.

bago
09-09-2009, 11:44
Quote Originally Posted by Andy
HT shouldn't make much difference, but if you're sure you can do it via kvm over IP.
My application is not so concurrent. I tested that on real 8 core it only uses 4 cores. I never tested on 4x2 HT cpu like the i7, so I don't know how they perform when you simply use 4 of them (is it *half* like real 8 cores or does HT understand and handle this well?).

Andy
09-09-2009, 11:28
It cannot be done unless you purchase kvm over IP (expensive). The free vKVM system does not support this.

HT shouldn't make much difference, but if you're sure you can do it via kvm over IP.

bago
09-09-2009, 10:55
My application works better with 4 cores than 8. Is it possible to disable HT in the new i7 kimsufi? I guess this is a BIOS operation: do we have to pay for this?