CODEWITHSUNDEEP
c++parallel-processingstlc++17
atb
atb

Reputation: 1462

How is scheduling handled in C++17 STL parallel algorithms?

Is there a standard scheduler specification for the C++17 STL parallel algorithms or is it entirely implementation dependant? The serial algorithms have complexity guarantees but the scheduler implementation is critical for performance with non uniform task loads, does the specification address this? It seems like it would be hard to guarantee cross-platform performance without a standardized scheduler.

Upvotes: 2

Views: 296

Answers (2)

capatober
capatober

Reputation: 461

Scheduling for C++17 STL algorithms is implementation-defined. Moreover, C++17 doesn't guarantee parallel execution. It just allows parallelism.

The class execution::parallel_policy is an execution policy type used as a unique type to disambiguate parallel algorithm overloading and indicate that a parallel algorithm’s execution may be parallelized

Upvotes: 0

Lightness Races in Orbit
Lightness Races in Orbit

Reputation: 385144

As far as I can tell from the wording, such details are completely within the domain of implementation specification, as one would expect. The standard generally makes no effort to guarantee absolute performance of any kind, only complexity requirements, as you're seeing in this case.

Ultimately, though your source code can now take advantage of parallelism while being completely standard-defined, the actual practical outcome of running your program is up to your implementation, and I think that still makes sense. The goal of standardising features is not cross-platform performance, but portable code that can be proven correct in a vacuum.

I'd expect your toolchain to give further information on how this sort of thing works, and that may even influence your choice of toolchain! But it does make sense for them to have freedom in that regard, as they do in other areas. After all, there is a multitude of target platforms out there (theoretically infinite), all with their own potential and quirks.

It could be that a future standard emplaces further constraints on scheduling in order to kick implementers up the backside a little, but personally I wouldn't count on it.

Upvotes: 6

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