What is a CPU thread and how is it related to logical threads in code?

multithreading cpu-architecture hyperthreading
MikeJ picture MikeJ · May 27, 2009 · Viewed 31k times · Source

I have been seeing in the literature for some of the newer CPU's such as the Intel Xeon "Nehalem-EX" as having 8 cores and 16 threads. What are they talking about here? I saw mention of this in reference so SPARCS too, surely this isn't the kind of logical threads spawned by code ? Is this hyperthreading re-named?

Answer

coobird picture coobird · May 27, 2009

Yes, Nehalem-based processors implement Hyper-threading.

The new Nehalem-EX which you refer to has 8 physical cores where each core can be seen as 2 logical cores for a total of 16 logical cores, allowing for the execution of 16 application threads on a single processor.

This is the same technology used in the Hyper-threading-enabled Pentium 4 processors, and more recently, on the Atom processors. My Eee PC has a single-core Atom processor which has two logical cores -- the Windows Task Manager will show two CPU graphs; one for each logical core.

Sun's UltraSPARC T2 (and the T1) also allow for simultaneous multithreading (of which Intel's implementation is called Hyper-Threading -- an trademark of Intel) which allows a single core to appear as multiple logical cores to execute multiple threads on a single core.

The rough idea behind simultaneous multithreading is to have multiple registers to store the processor state, so it appears that there actually are multiple cores in a single core, because it has multiple full-sets of hardware registers.

While the physical facilities such as the ALU and FPU may not increase, having more sets of registers to run more threads on a physical core can lead to better utilization of the available processor resources. The core may have not been saturated when executing a single thread, but executing multiple could saturate all the units to its fullest potential.

So what does it mean for programmers?

It means that we still will need to write multi-threaded software -- having a program that only has a single thread will only be able to utilize a single logical core. Only by having well-written multi-threaded code are we able to take advantage of the massive number of logical cores these processors offer.

Even with simultaneous multithreading, the code is executed at one thread per logical core.

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