8 logical threads at 4 cores will at a maximum run 4 times faster in parallel?

Question

I\'m benchmarking software which executes 4x faster on Intel 2670QM then my serial version using all 8 of my \'logical\' threads. I would like some community feedback on my

Answer 1

This is a quick summary of Hyperthreading

Thread switching is slow, having to stop execution, copy a bunch of values into memory, copy a bunch of values out of memory into the CPU, then start things going again with the new thread.

This is where your 4 virtual cores come in. You have 4 cores, that is it, but what hyperthreading allows the CPU to do is have 2 threads on a single core.

Only 1 thread can execute at a time, however when 1 thread needs to stop to do a memory access, disk access or anything else that is going to take some time, it can switch in the other thread and run it for a bit. On old processors, they basically had a bit of a sleep in this time.

So your quad core has 4 cores, which can do 1 thing at a time each, but can have a 2nd job on standby as soon as they need to wait on another part of the computer.

If your task has a lot of memory usage and a lot of CPU usage, you should see a slight decrease in total execution time, but if you are almost entirely CPU bound you will be better off sticking with just 4 threads