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    <p>You did not understood the basic problem of synchronization. What is a 'monitor', what does it mean that a thread sleeps and what does it mean that it is about to be woken up?</p> <p>A monitor is a mid-level synchronization structure. This is not a low-level petty volatile boolean flag with bus-halting XCHG operation, and this is not high-level thread pool handler that requires dozens of other special mechanisms..</p> <p>On a monitor, <strong>MANY</strong> threads may sleep. There are <strong>logical queues</strong> out there that i.e. preserver order of being put to sleep/woken up, or mechanisms that guarantee proper time scheduling and fairnees. I will not get into details, all of it is out there on the web, even on wiki.</p> <p>Add to that that the operation is PULSE. Pulse is instantenous. It does not "stick". Pulse will wake those now sleeping. If after the pulse another one check the monitor, it will go to sleep.</p> <p>Now imagine: you have a queue of 5 sleeping threads. One thread (6th) wants now to pulse them, and yet another (7th) wants to check the monitor.</p> <p>6th and 7th are running in parallel, truly simultaneously, since you have quad-core CPU.</p> <p>So, tell me, what would happen to the queue's implementtion if the 6th starts pulsing andwaking and removing woken threads from the queue, and in the same time the 7th one starts adding itself there?</p> <p>To solve that, the internal queues would have to be internally synchronized and locked, so only one thread at time modifies them.</p> <p>Um wait. We just stumbled upon a case where we wanted to SYNCHRONIZE something, and to do it properly we need to SYNCHRONIZE on another thing? Not good.</p> <p>Therefore, the actual LOCK is done EXTERNALLY before you talk to the monitor itself. This is to achieve SINGLE LOCKING, instead of introduce several layers of hierarchical locks.</p> <p>That way it is simplier, faster, and more resource-friendly.</p>
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    1. POMonitor.Pulse in C# appears suboptimal : must be in lock scope
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    1. USquetzalcoatl
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    1. COI think I get synchronization good enough, I was using boost mutex and condition_variable a lot in the past with, I think, correct understanding. I also used the Win32 equivalent primitives. And now in C# the same "concept" is turned upside-down. So you must understand my puzzling. And all of your explanation only makes it more tangled. You want to say that Monitors are between XCHG and poor handler, but now you just made it look exaclty like a poor handler. Thanks though :)
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      1. USv.oddou
    2. COIIRC, when using condition variables, to raise/change them you also had to lock them first. Even if it was done completely out of your sight, it was still performed within the API call. Here it is just on the eyes so that you remember about the fact that the API call may instantly put you to sleep, just because the sync is temporarily blocked.
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    3. COWell, that is covered by the two links I put in the question. The first is explaining clearly why notification has to be made after the lock : "prevent useless context switches". Which is also mentioned in the "Linked Question". The MSDN sample also clearly notifies (Wake..) after the lock.
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