Wednesday, September 24, 2014

CLR monitors - implemented by kernel objects

All sync objects in windows, across languages, are based on primitive kernel sync objects. (How about sync objects in jvm on windows? I guess the same.) Sync objects are not language constructs, but provided by the OS. I feel the stream construct is also something provided by the OS.

([[cpow]] means  [[concurrent programming on Windows]])


The CLR Mutex class is a "crude" wrapper over some kernel object(s). Crude because using this Mutex involves expensive kernel transition.

In contrast, The CLR monitor is Also based on kernel objects, but minimizes kernel transition and is more effcient/cheaper. See P188 [[cpow]]. I feel this efficiency is achieved at the expense of features. For eg, CLR monitors aren't cross-process.

CLR monitor offers 1) mutual exclusion and 2) condVar features. Entire CLR monitor including the cond var is based on kernel objects, according to [[cpow]]. The CLR monitor is considered a "higher level of abstraction from the basic kernel objects".

From the above analysis, the CLR offers 2 categories of sync objects -- 1) wrappers over kernel objects, and 2) CLR-specific constructs.

1) Examples in the wrapper category -- anything based on WaitHandle including Mutex/Semaphore
2) Examples in the CLR category --
- interlocked
- monitor class including Wait/Pulse
- the *Slim classes

Key differences between the two --
$ slow - kernel mode "transition" required in the wrapper objects. Therefore much slower.
$ IPC - only kernel sync objects are usable across processes. If no IPC required, then the non-kernel constructs are better (much faster).
$ P/Invoke - you could use P/Invoke to simulate many WaitHandle-based constructs
$ predate - there are win32 constructs to access the same kernel objects. They predate CLR. I think the wrappers are similar to the win32 constructs.

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