Multithreading

In computer architecture, multithreading is the ability of a central processing unit (CPU) (or a single core in a multi-core processor) to provide multiple threads of execution concurrently, which differs from multiprocessing. Threads use the memory of the process they belong to. Inter-process communication is slow as processes have different memory addresses. Inter-thread communication can be faster than inter-process communication because threads of the same process share memory with the process they belong to.

 Where multiprocessing systems include multiple complete processing units in one or more cores, multithreading aims to increase utilization of a single core by using thread-level parallelism, as well as instruction-level parallelism. As the two techniques are complementary, they are combined in nearly all modern systems architectures with multiple multithreading CPUs and with CPUs with multiple multithreading cores.

If a thread gets a lot of cache misses, the other threads can continue taking advantage of the unused computing resources, which may lead to faster overall execution, as these resources would have been idle if only a single thread were executed. Also, if a thread cannot use all the computing resources of the CPU (because instructions depend on each other's result), running another thread may prevent those resources from becoming idle.

Downside is that multiple threads can interfere with each other when sharing hardware resources such as caches or translation lookaside buffers (TLBs). As a result, execution times of a single thread are not improved and can be degraded, even when only one thread is executing, due to lower frequencies or additional pipeline stages that are necessary to accommodate thread-switching hardware.

shared resources (Tanenbaum):

among processes	among threads
Address space; global variables; open files; child processes; pending alarms; signals and signal handlers; accounting info;	programme counter; registers; stack; state;

From the software standpoint, hardware support for multithreading is more visible to software, requiring more changes to both application programs and operating systems than multiprocessing. Hardware techniques used to support multithreading often parallel the software techniques used for computer multitasking. Thread scheduling is also a major problem in multithreading. 

Threads use the memory of the process they belong to. Inter-process communication is slow as processes have different memory addresses. Inter-thread communication can be faster than inter-process communication because threads of the same process share memory with the process they belong to.