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Temporal multithreading is one of the two main forms of multithreading that can be implemented on computer processor hardware, the other being
simultaneous multithreading Simultaneous multithreading (SMT) is a technique for improving the overall efficiency of superscalar CPUs with hardware multithreading. SMT permits multiple independent threads of execution to better use the resources provided by modern proces ...
. The distinguishing difference between the two forms is the maximum number of concurrent threads that can execute in any given
pipeline A pipeline is a system of Pipe (fluid conveyance), pipes for long-distance transportation of a liquid or gas, typically to a market area for consumption. The latest data from 2014 gives a total of slightly less than of pipeline in 120 countries ...
stage in a given cycle. In temporal multithreading the number is one, while in simultaneous multithreading the number is greater than one. Some authors use the term super-threading synonymously.


Variations

There are many possible variations of temporal multithreading, but most can be classified into two sub-forms: ; Coarse-grained: The main processor pipeline contains only one thread at a time. The processor must effectively perform a rapid
context switch In computing, a context switch is the process of storing the state of a process or thread, so that it can be restored and resume execution at a later point, and then restoring a different, previously saved, state. This allows multiple processes ...
before executing a different thread. This fast context switch is sometimes referred to as a ''thread switch''. There may or may not be additional penalty cycles when switching. :There are many possible variations of coarse-grained temporal multithreading, mainly concerning the algorithm that determines when thread switching occurs. This algorithm may be based on one or more of many different factors, including cycle counts, cache misses, and fairness. ;
Fine-grained Granularity (also called graininess) is the degree to which a material or system is composed of distinguishable pieces, "granules" or "grains" (metaphorically). It can either refer to the extent to which a larger entity is subdivided, or the ...
(or interleaved): The main processor pipeline may contain multiple threads, with context switches effectively occurring between pipe stages (e.g., in the barrel processor). This form of multithreading can be more expensive than the coarse-grained forms because execution resources that span multiple pipe stages may have to deal with multiple threads. Also contributing to cost is the fact that this design cannot be optimized around the concept of a "background" thread — any of the concurrent threads implemented by the hardware might require its
state State most commonly refers to: * State (polity), a centralized political organization that regulates law and society within a territory **Sovereign state, a sovereign polity in international law, commonly referred to as a country **Nation state, a ...
to be read or written on any cycle.


Comparison to simultaneous multithreading

In any of its forms, temporal multithreading is similar in many ways to simultaneous multithreading. As in the simultaneous process, the hardware must store a complete set of states per concurrent thread implemented. The hardware must also preserve the illusion that a given thread has the processor resources to itself. Fairness algorithms must be included in both types of multithreading situations to prevent one thread from dominating processor time and/or resources. Temporal multithreading has an advantage over simultaneous multithreading in that it causes lower processor heat output; however, it allows only one thread to be executed at a time.


See also

* Barrel processor *
Symmetric multiprocessing Symmetric multiprocessing or shared-memory multiprocessing (SMP) involves a multiprocessor computer hardware and software architecture where two or more identical processors are connected to a single, shared main memory, have full access to all ...


References

{{DEFAULTSORT:Temporal Multithreading Flynn's taxonomy Threads (computing) Central processing unit Computer architecture