Stealth cpu's

EdR · 10-14-2004, 08:14 AM

Hardware-
PE 2650 Dual CPU 533 BUS 4G RAM
OS-
RH ES 3.0

Question-
When running top, it shows 4 cpu's instead of the 2 that I have. I thought that they might be mirrors (CPU 0, CPU 2 /CPU 1, CPU3)but the numbers do not support that. Is this just a quirky function of top?

druuna · 10-14-2004, 08:20 AM

I don't think this is a bug, rather hypethreading at work.

In short, hyperthreading is a way to virtualy 'double' your real cpu('s). There is a 'downside': Speed. Instead of 2 cpu's running at speed X, you have 4 running at X/2.

Hope this clears things up a bit.

EdR · 10-14-2004, 10:18 AM

Thanks. I turned it off in the bios.

foo_bar_foo · 10-14-2004, 01:15 PM

Quote:

Originally posted by druuna
There is a 'downside': Speed. Instead of 2 cpu's running at speed X, you have 4 running at X/2.

Hope this clears things up a bit.

This isn't true

while i don't disagree that in some heavy load instances it might be best to turn it off in bios
the cpus in hyperthreading mode cycle at the stated clock speed not half as fast !

the hyperthreading simply uses the dead time the processor spends waiting for memory access
to do work on other threads...... results -- increased efficiency -- increased performance

druuna · 10-14-2004, 01:35 PM

You are right, ofcourse :-)

But, when running a 2.4 kernel, the increase isn't worth mentioning (in my case that is). Roughly a 5% overall speed increase. I've been told that the 2.6 kernel does a (much?) better job.

I do use hyperthreading, but am not that impressed with the speed increase (ok, 5% is an increase

). I do like the virtual cpu mechanism, though.

You have any experience with hyperthreading and 2.6 kernels?