Monday, June 17, 2013

atomic operations offered in c++11 ^ pthreads libraries

P341 [[c++ concurrency in action]] has a nice table showing about 7 to 10 most important concurrency features across pthreads vs c++11 [2]. It's obvious to me the 2 fundamental[1] and universally important features are locks and condVars. These are thoroughly and adequately supported everywhere -- pthreads, c++11, java, c#. Beyond these 2 features, other features aren't universally supported across the board. Let's look at some of them.

-- Atomic operations --
pthreads? no support
c++11? yes atomic types
java? yes atomic variables
boost? no support
c#? yes interlocked

Notice in the C#, c++11, java thread libraries there are specific constructs (classes and methods) for atomic INT and atomic BOOLEAN (but not atomic float), because in practice most atomic algorithms use primitive int and boolean types.

Atomic operations often rely on specific CPU instructions. They are often "offered" on (no more than a few) specific atomic data types. Can we apply atomic operations on a random data type, like some method in a regular class? I doubt it. I feel in each thread library, there are no more than a few specific atomic Operations, tied to a handful of atomic data types.

-- thread pool --
java? yes
c#? yes
c++11? no
pthreads? no
boost? To my surprise, No, according to the maintainer of boost::thread

I think it's technically very feasible to implement thread pool using locks and condVars, so this feature is left out of the c/c++ base libraries.

[1] "Fundamental" is a imprecise term that I would not spend too much time debating. In c#, locks and condVars aren't really rock-bottom fundamental. They are based on more fundamental constructs namely primitive kernel objects. In other languages, I'm not sure. Locks and condVars are often implemented in thread libraries not syscalls. It's a bit obscure, arcane and even irrelevant to many developers.
[2] (and java and boost too, but this blog is about c++)

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