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New Concurrency Features in C++14

Wednesday, 08 July 2015

It might have been out for 7 months already, but the C++14 standard is still pretty fresh. The changes include a couple of enhancements to the thread library, so I thought it was about time I wrote about them here.

Shared locking

The biggest of the changes is the addition of std::shared_timed_mutex. This is a multiple-reader, single-writer mutex. This means that in addition to the single-ownership mode supported by the other standard mutexes, you can also lock it in shared ownership mode, in which case multiple threads may hold a shared ownership lock at the same time.

This is commonly used for data structures that are read frequently but modified only rarely. When the data structure is stable, all threads that want to read the data structure are free to do so concurrently, but when the data structure is being modified then only the thread doing the modification is permitted to access the data structure.

The timed part of the name is analagous to std::timed_mutex and std::recursive_timed_mutex: timeouts can be specified for any attempt to acquire a lock, whether a shared-ownership lock or a single-ownership lock. There is a proposal to add a plain std::shared_mutex to the next standard, since this can have lower overhead on some platforms.

To manage the new shared ownership mode there are a new set of member functions: lock_shared(), try_lock_shared(), unlock_shared(), try_lock_shared_for() and try_lock_shared_until(). Obtaining and releasing the single-ownership lock is performed with the same set of operations as for std::timed_mutex.

However, it is generally considered bad practise to use these functions directly in C++, since that leaves the possibility of dangling locks if a code path doesn't release the lock correctly. It is better to use std::lock_guard and std::unique_lock to perform lock management in the single-ownership case, and the shared-ownership case is no different: the C++14 standard also provides a new std::shared_lock class template for managing shared ownership locks. It works just the same as std::unique_lock; for common use cases it acquires the lock in the constructor, and releases it in the destructor, but there are member functions to allow alternative access patterns.

Typical uses will thus look like the following:

std::shared_timed_mutex m;
my_data_structure data;

void reader(){
    std::shared_lock<std::shared_timed_mutex> lk(m);
    do_something_with(data);
}

void writer(){
    std::lock_guard<std::shared_timed_mutex> lk(m);
    update(data);
}

Performance warning

The implementation of a mutex that supports shared ownership is inherently more complex than a mutex that only supports exclusive ownership, and all the shared-ownership locks still need to modify the mutex internals. This means that the mutex itself can become a point of contention, and sap performance. Whether using a std::shared_timed_mutex instead of a std::mutex provides better or worse performance overall is therefore strongly dependent on the work load and access patterns.

As ever, I therefore strongly recommend profiling your application with std::mutex and std::shared_timed_mutex in order to ascertain which performs best for your use case.

std::chrono enhancements

The other concurrency enhancements in the C++14 standard are all in the <chrono> header. Though this isn't strictly about concurrency, it is used for all the timing-related functions in the concurrency library, and is therefore important for any threaded code that has timeouts.

constexpr all round

The first change is that the library has been given a hefty dose of constexpr goodness. Instances of std::chrono::duration and std::chrono::time_point now have constexpr constructors and simple arithmetic operations are also constexpr. This means you can now create durations and time points which are compile-time constants. It also means they are literal types, which is important for the other enhancement to <chrono>: user-defined literals for durations.

User-defined literals

C++11 introduced the idea of user-defined literals, so you could provide a suffix to numeric and string literals in your code to construct a user-defined type of object, much as 3.2f creates a float rather than the default double, however there were no new types of literals provided by the standard library.

C++14 changes that. We now have user-defined literals for std::chrono::duration, so you can write 30s instead of std::chrono::seconds(30). To get this user-defined literal goodness you need to explicitly enable it in your code with a using directive — you might have other code that wants to use these suffixes for a different set of types, so the standard let's you choose.

That using directive is:

using namespace std::literals::chrono_literals;

The supported suffixes are:

  • hstd::chrono::hours
  • minstd::chrono::minutes
  • sstd::chrono::seconds
  • msstd::chrono::milliseconds
  • usstd::chrono::microseconds
  • nsstd::chrono::nanoseconds

You can therefore wait for a shared ownership lock with a 50 millisecond timeout like this:

void foo(){
    using namespace std::literals::chrono_literals;
    std::shared_lock<std::shared_timed_mutex> lk(m,50ms);
    if(lk.owns_lock()){
        do_something_with_lock_held();
    }
    else {
        do_something_without_lock_held();
    }
}

Just::Thread support

As you might expect, Just::Thread provides an implementation of all of this. std::shared_timed_mutex is available on all supported platforms, but the constexpr and user-defined literal enhancements are only available for those compilers that support the new language features: gcc 4.6 or later for constexpr and gcc 4.7 or later for user-defined literals, with the -std=c++11 or std=c++14 switch enabled in either case.

Get your copy of Just::Thread while our 10th anniversary sale is on for a 50% discount.

Posted by Anthony Williams
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