available resources on any of the physical cores. Multiple virtual cores can push threadlets into the reorder buffer of a single physical core, which can split partial instructions and data from multiple threadlets through the execution ports at the same time. Each virtual core keeps track of the position of the relative output. This form of multithreading can increase single threaded performance by allowing a single thread to use all resources of the CPU. The allocation of resources is dynamic on a near-single cycle latency level (1–4 cycles depending on the change in allocation depending on individual application needs. Therefore, if two virtual cores are competing for resources, there are appropriate algorithms in place to determine what resources are to be allocated where. Disadvantages Depending on the design and architecture of the processor, simultaneous multithreading can decrease performance if any of the shared resources are bottlenecks for performance. Critics argue that it is a considerable burden to put on software developers that they have to test whether simultaneous multithreading is good or bad for their application in various situations and insert extra logic to turn it off if it decreases performance. Current operating systems lack convenient API calls for this purpose and for preventing processes with different priority from taking resources from each other. There is also a security concern with certain simultaneous multithreading implementations. Intel's hyperthreading in NetBurst-based processors has a vulnerability through which it is possible for one application to steal a cryptographic key from another application running in the same processor by monitoring its cache use. There are also sophisticated machine learning exploits to HT implementation that were explained at Black Hat 2018. See also Hardware scout Speculative multithreading Symmetric multiprocessing References General External links SMT news articles and academic papers SMT research at the University of Washington Central processing unit Computer architecture Flynn's taxonomy Threads (computing)