What is a "Logical CPU Core"

I am reading some Operating Systems materials. I read this phrase that confused me a little: "Multicore refers to a computer or processor that has more than one logical CPU core, and that can execute multiple instructions at the same time."

What is a "logical CPU core", is it a processor? Does it correspond to something physical, or is it the OS which sees logical CPU cores but in reality there is less physical processors than logical CPU cores?

Upvotes: 4

Answers (3)

Xiaoqi

Reputation: 21

It might be helpful to have some background knowledge:

Processor

A processor could describe either a single execution core or a single physical multi-core chip. The context of use will define the meaning of the term. e.g Normal PC computer should only have one processor

Chips

A chip refers to a physical integrated circuit (IC) on a computer. A chip is usually referred to an execution unit that can be single- or multi-core technology.

Sockets The socket refers to a physical connector on a computer motherboard that accepts a single physical chip. Many motherboards can have multiple sockets that can, in turn, accept multi-core chips.

Cores

Since the advent of multi-core technology, such as dual-cores and quad-cores. Essentially a core comprises a logical execution unit containing an L1 cache and functional units. Cores are able to independently execute programs or threads. Supercomputers are listed as having thousands of cores.

Hyper-threading

Hyper-threading is an Intel technology that originally preceded multi-core systems, and was used to make a single core appear logically as multiple cores on the same chip. Hyper-threading improves performance by sharing the computational workload between multiple cores whenever possible, allowing the operating system to schedule more than one process at a time. For more, see Intel Hyper-Threading Technology.

Physical/Logical Cores

sockets and cores

As shown in the picture, you have 2 sockets, and each socket has 4 cores, and each core can execute 4 threads currently(due to Hyper-threading). In this case, if you use command lscpu on Linux, you may see you have 32 CPUs. Actually, you have 1 chip, 2 sockets, 8 cores, and 32 CPUs (From Linux perspective)

Upvotes: 2

user2467198

Reputation:

A logical CPU core contains the complete architectural context of a uniprocessor. This is the unit for which the OS can do scheduling and control architectural state such as the address for exceptions (for an architecture that does not hardwire such).

There are two common cases where it will not correspond one-to-one with a physical core. First, a single physical core can implement multiple virtual processors, e.g., Intel's Hyper-Threading. In this case the OS scheduler should be aware that virtual processors may share various resources such as instruction fetch, instruction scheduling hardware, and execution units, which generally means that tasks should be scheduled to distinct physical cores to maximize performance. (This issue also applies to a lesser extent to distinct cores that share L2 cache. Such concerns are somewhat related to NUMA optimizations for multi-CPU computers.)

In the second case, a hypervisor's virtualization of the hardware can present an arbitrary number of cores to the OS. While a hypervisor would typically make visible to a guest OS no more logical processors than provided by hardware (i.e., including virtual processors associated with hardware multithreading), theoretically the hypervisor could present an arbitrary number of processors to the OS (just as an OS can present the impression of an arbitrary number of processors to the application layer by using time slicing). In such a software virtualization context, the hypervisor may not expose to the OS the nature of the processors, so the OS could only treat them as abstract units for scheduling.

Somewhat complicating this division, it is also possible for hardware to implement multithreading without providing a full virtual processor for each thread. E.g., the MIPS Multithreading Application Specific Extension makes a distinction between Virtual Processing Elements (which behave as distinct processors in terms of architectural state) and Thread Contexts (which share the system coprocessor among threads in the same VPE). As a further complication, it may be possible for Thread Contexts to be migrated among VPEs. E.g., a physical processor core might have two VPEs and five Thread Contexts and the OS might be allowed to assign a given TC to either VPE such that either VPE could have between one and four TCs. In addition, unprivileged software can FORK and YIELD threads without OS involvement if spare hardware threads are available (in the case of FORK) or at least one thread will still be active (in the case of YIELD).

For MIPS MT-ASE, the OS would generally only be concerned with Thread Contexts, but some optimizations are possible with a more complete knowledge of the actual hardware configuration and some correctness issues are possible if a Thread Context is treated as a Virtual Processing Element.

Upvotes: 6

apomene

Reputation: 14389

I guess it referes to ALU (Arithmetic Logical Unit) of CPU.

The ALU unit of any proccessor is the part pf the CPU reasponsible for performing all the arithmetic and logical operations.

Upvotes: 0

What is a &quot;Logical CPU Core&quot;