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and the second is a function that contains the code to be protected. This function should take unit as its parameter, and it can return any value. The following example demonstrates the subtle issues involved in locking. It needs to be quite long and has been deliberately written to exaggerate the problem of context switching. The idea is that two threads will run at the same time, both trying to write the console. The aim of the sample is to write the string "One ... Two ... Three ... " to the console atomically; that is, one thread should be able to finish writing its message before the next one starts. The example has a function, called makeUnsafeThread, that creates a thread that will not be able to write to the console atomically and a second one, makeSafeThread, that writes to the console atomically by using a lock. #light open System open System.Threading // function to print to the console character by character // this increases the chance of there being a context switch // between threads. let printSlowly (s : string) = s.ToCharArray() |> Array.iter print_char
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// create a thread that prints to the console in an unsafe way let makeUnsafeThread() = new Thread(fun () -> for x = 1 to 100 do printSlowly "One ... Two ... Three ... " print_newline() done) // the object that will be used as a lock let lockObj = new Object() // create a thread that prints to the console in a safe way let makeSafeThread() = new Thread(fun () -> for x = 1 to 100 do // use lock to ensure operation is atomic lock lockObj (fun () -> printSlowly "One ... Two ... Three ... " print_newline() ) done)
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// helper function to run the test to let runTest (f : unit -> Thread) message = print_endline message; let t1 = f() in let t2 = f() in t1.Start() t2.Start() t1.Join() t2.Join() // runs the demonstrations let main() = runTest makeUnsafeThread "Running test without locking ..." runTest makeSafeThread "Running test with locking ..." main () The part of the example that actually uses the lock is repeated next to highlight the important points. You should note a couple of important factors. First, the declaration of the lockObj will be used to create the critical section. Second, the use of the lock function is embedded in the makeSafeThread function. The most important thing to notice is how the printing functions you want to be atomic are placed inside the function that is passed to lock. // the object that will be used as a lock let lockObj = new Object() // create a thread that prints to the console in a safe way let makeSafeThread() = new Thread(fun () -> for x = 1 to 100 do // use lock to ensure operation is atomic lock lockObj (fun () -> printSlowly "One ... Two ... Three ... " print_newline() ) done) The results of the first part of the test will vary each time it is run, since it depends on when a thread context switches. It might also vary based on the number of processors, because if a machine has two or more processors, then threads can run at the same time, and therefore the messages will be more tightly interspersed. On a single-processor machine, things will look less messy, because the messages will go wrong only when a content switch takes place. The results of the first part of the sample, run on a single-processor machine, are as follows:
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Running ... One ... One One One ... ...
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test without locking ... Two ... Three ... ... Two ... Three ... Two ... Three ...
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The results of the second half of the example will not vary at all, because of the lock, so it will always look like this: Running One ... One ... One ... ... test with locking ... Two ... Three ... Two ... Three ... Two ... Three ...
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Locking is an important aspect of concurrency. Any resource that will be written to and shared between threads should be locked. A resource is often a variable, but it could also be a file or even the console, as shown in this example. You should not think of locks as a magical solution to concurrency. Although they work, they also can also create problems of their own since they can create a deadlock, where different threads lock resources that each other needs and neither can advance. The simplest solution to concurrency is often to simply avoid sharing a resource that can be written to between threads.
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I Note You can find more information about concurrency at http://www.strangelights.com/fsharp/
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