CPU Physical/Logical Confusion

VermilionNeko

Distinguished
Apr 24, 2014
268
7
18,795
Hi

I'm a bit confused with the difference between physical and logical cores, particularly with the i5 4670K. I've read it has 4 cores and 4 threads, but does that mean it has 4 physical cores or just 2 physical and 2 logical?

Using a power supply calculator just out of curiosity - to see how much power is being used - there's a counter option, which says 'CPU count applies only if the system has more than 1 physical CPU'. Thinking that meant it would need to be set to 4 (4 physical cores/CPUs?) it bumped the overall wattage, along with everything else selected, to over 800W+, which can't be right otherwise my whole system would have locked up by now. :p

Thanks
 
Solution
The physical/logical cores concept was introduced due to Intel's hyperthreading technology. Hyperthreading allows the unused parts of a CPU core to be used for other calculations, theoretically allowing two threads to be run on one physical core at the same time So a core with hyperthreading has one physical core, and two logical cores. (The effectiveness of the logical core depends on whether the second thread needs the same parts of the CPU as the first thread. If they need the same parts, then the second thread can't get anything done and overall performance is the same as one physical core. If they need completely different parts, then the second thread is nearly as effective as a separate full physical core. It's not quite as...
The physical/logical cores concept was introduced due to Intel's hyperthreading technology. Hyperthreading allows the unused parts of a CPU core to be used for other calculations, theoretically allowing two threads to be run on one physical core at the same time So a core with hyperthreading has one physical core, and two logical cores. (The effectiveness of the logical core depends on whether the second thread needs the same parts of the CPU as the first thread. If they need the same parts, then the second thread can't get anything done and overall performance is the same as one physical core. If they need completely different parts, then the second thread is nearly as effective as a separate full physical core. It's not quite as effective because cache is shared.)

For desktop processors:

- the i3 has 2 physical cores, 4 logical cores
- the i5 has 4 physical cores, 4 logical cores (no hyperthreading)
- the i7 has 4 physical cores, 8 logical cores

The "CPU count" refers to actual number of CPUs. Many server motherboards can take 2 or more CPUs (each with 2/4 cores and 4/8 logical cores). For the vast majority of desktop builds, the CPU count is 1.
 
Solution

VermilionNeko

Distinguished
Apr 24, 2014
268
7
18,795
Thanks guys for your explanations. Looks like what I thought was right, though I didn't realise the CPU count meant a system with more CPUs; that's what threw me off. To be honest, I didn't even know that some systems could have more than one CPU, so I've learnt something new there. :)