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The implementation of a general purpose spin lock is a more difficult task than you might imagine, however There are many trivia-like details to ensure spin waiting works properly on Windows and the kinds of proces sors on which Windows runs; these have to do with the thread scheduler, Intel HyperThreading (HT), and caches In addition, most spin locks really should fall back to true waiting in worst case situations, such as when the cost of a context switch has already been exceeded at some implementation complexity Even when the worst cases seem statistically improbable, they can occur if a thread is interrupted by a context switch while in a critical section or when the arrival rate at a lock becomes unusually high In this section, we'll look at two spin lock approaches The first spins on a shared variable, and doesn't fall back to true waiting, although it does explicitly yield the thread's timeslice after some time The second is a lock called a Mellor-Crummey-Scott (MCS) lock, which reduces contention on shared memory locations It has been proven to exhibit higher degrees of scalability on large multiprocessor machines with nonuniform memory access (Both are shown in C# code The transformation to C++ is typically much easier than the reverse because C# needs to deal with the possibility of asynchronous thread aborts This fact can complicate matters, particu larly when we look at MCS locks)
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How to Properly Spin on Windows Before moving on to the lock specifics, there are some basic rules you should consider when using spin waits on Windows
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Issue calls to Yie l d P ro c e s s o r (in Win32) or T h r e a d Y i e l d (in NET) on each iteration of your spin wait loop These emit YI E LD or PAU S E instructions on relevant processors-which is only Intel's Hyper Threading (HT) enabled processors-and NOPs on other processors where HT isn't present (Th read Y i e l d in NET takes a numeric argu ment and emits that number of these instructions in a loop) This ensures the processor is made aware that the code currently running is performing spin waits and will make the execution unit available to other logical processors so they can make true forward progress
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C h a pter l it : P erfo r m a n c e a n d S c a l a b i l i ty
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In most spin wait circumstances, shared state will be read during each iteration This can lead to memory traffic and cache contention Therefore, it is wise to introduce a growing delay-called exponen
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tial backoff-on each spin iteration It also sometimes makes sense to introduce randomization to avoid multiple threads from execut ing in a lock step fashion, which would possibly lead to a severe case of livelock
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When pure spin waiting is being used (versus two phase), it is some times worth issuing explicit context switches with one of the appro priate platform APIs The reason is that if a thread has already consumed a full context switch of spinning, it may be more appropri ate for it to allow others to make forward progress than continuing to use processing resources (possibly interfering with the very thread that is being waited for)
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When issuing explicit context switches, the Win32 function Swit c h ToTh r e a d i s most appropriate to use (The equivalent i s not available in NET unless you P I Invoke) It relinquishes the calling thread's timeslice and runs another runnable thread in its place This is in effect for a single timeslice It returns T R U E to indicate that a switch occurred, and F A L S E otherwise As of Windows Vista and Server 2008, this function may not consider all threads on the system Because Swi t c hToTh r e a d may not consider all threads on the system for execution, it is wise to occasionally call S l e e p or S l e e p E x (in Win32) or T h r e a d S l e e p (in NET) Passing a value of e as the argu ment is best because it does not result in a context switch if there are no threads of equal priority ready run However, passing a value of 1 occasionally is also wise: if you ever get into a situation where a higher priority thread is spin waiting on a lower priority thread, this can help avoid a nasty starvation problem that would require get ting the balance set manager involved to fix
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Because of the tricky rules, we can create a reusable S p i nWa i t data struc ture that encapsulates all of this logic Replicating it repeatedly in a program's code base would create a maintenance problem Determining the ratio of calls to Swi t c hToT h r e a d , S l e e p ( e ) , and S l e e p ( l ) is left as a performance
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