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Multithreaded applications have the following drawbacks:
* ''[[#Synchronization|Synchronization]]'' complexity and related bugs: when using shared resources typical for threaded programs, the [[programmer]] must be careful to avoid [[Race condition#Computing|race conditions]] and other non-intuitive behaviors. In order for data to be correctly manipulated, threads will often need to [[Rendezvous problem|rendezvous]] in time in order to process the data in the correct order. Threads may also require [[mutual exclusion|mutually exclusive]] operations (often implemented using [[lock (computer science)|mutexes]]) to prevent common data from being read or overwritten in one thread while being modified by another. Careless use of such primitives can lead to [[deadlock (computer science)|deadlock]]s, livelocks or [[race condition|races]] over resources. As [[Edward A. Lee]] has written: "Although threads seem to be a small step from sequential computation, in fact, they represent a huge step. They discard the most essential and appealing properties of sequential computation: understandability, predictability, and determinism. Threads, as a model of computation, are wildly non-deterministic, and the job of the programmer becomes one of pruning that nondeterminism."<ref name="Lee" />
* ''Being untestable''. In general, multithreaded programs are non-deterministic, and as a result, are untestable. In other words, a multithreaded program can easily have bugs which never manifest on a test system, manifesting only in production.<ref>{{Cite journal |title=Multi-threading at Business-logic Level is Considered Harmful |url=https://accu.org/journals/overload/23/128/ignatchenko_2134/ |first=Sergey |last=Ignatchenko |journal=[[Overload (magazine)|Overload]] |issue=128 |pages=4–7 |date=August 2015 |publisher=[[ACCU (organisation)|ACCU]]}}</ref><ref name="Lee">{{Cite web |first=Edward |last=Lee |date=January 10, 2006 |title=The Problem with Threads |url=http://www.eecs.berkeley.edu/Pubs/TechRpts/2006/EECS-2006-1.html |publisher=UC Berkeley}}</ref> This can be alleviated by restricting inter-thread communications to certain well-defined patterns (such as message-passing).
* ''Synchronization costs''. As thread context switch on modern CPUs can cost up to 1 million CPU cycles,<ref>{{Cite web |author='No Bugs' Hare |date=12 September 2016 |title=Operation Costs in CPU Clock Cycles |url=http://ithare.com/infographics-operation-costs-in-cpu-clock-cycles/}}</ref> it makes writing efficient multithreading programs difficult. In particular, special attention has to be paid to avoid inter-thread synchronization from being too frequent.
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