Fork island

New in version 2.9.

#include <pagmo/islands/fork_island.hpp>

Note

The fork_island class is available only on POSIX platforms (e.g., Linux, OSX, the BSDs, but not Microsoft Windows, unless some POSIX compatibility layer has been installed on the system).

class fork_island

This user-defined island (UDI) will use the POSIX fork() system call to offload the evolution of a population to a child process.

Generally speaking, users are encouraged to use thread_island rather than fork_island: thread_island performs better, it works also with problems and algorithms which are not serialisable, and it is available on all platforms.

fork_island should however be preferred when dealing with problems and algorithms which do not offer the basic thread_safety guarantee. By offloading the optimisation to a separate process (rather than a separate thread), fork_island can ensure that thread-unsafe problems and algorithms are always run in only one thread at a time. This capability is particularly useful when wrapping in pagmo third-party code which does not support execution in multithreaded contexts (a notable example is the ipopt algorithm, which uses the thread-unsafe Ipopt optimiser).

fork_island is the UDI type automatically selected by the constructors of island on POSIX platforms when the island’s problem and/or algorithm do not provide the basic thread_safety guarantee.

Note

When using memory checking tools such as valgrind, or the address/memory sanitizers from GCC/clang, be aware that memory leaks in the child process may be flagged by such tools. These are spurious warnings due to the fact that the child process is exited via std::exit() (which does not invoke the destructors of objects with automatic storage duration). Thus, such warnings can be safely ignored.

Note

The ability of the forked process to handle errors raised during evolution is dependent on the specific fork() implementation in use. For instance, it has been reported that on recent OSX versions (i.e., since High Sierra), error handling in the forked process does not work, possibly because it employs code which is not asynchronous-safe.

fork_island()

fork_island(const fork_island&)

fork_island(fork_island&&) noexcept

fork_island is default, copy and move-constructible. The copy and move constructor are equivalent to the default constructor.

void run_evolve(island &isl) const

This method will fork the calling process, and, in the child process, the population of isl will be evolved using the algorithm of isl. At the end of the evolution, the evolved population and the algorithm used for the evolution will be sent back to the parent process, where they will replace, in isl, the original population and algorithm. The child process will then terminate via std::exit(0).

If any exception is raised during the evolution, the error message from the exception will be transferred back to the parent process, where a std::runtime_error containing the error message from the child will be raised.

Parameters

isl – the island that will be evolved.

Throws

• std::runtime_error – if any error arises from the use of POSIX primitives (fork(), pipes, etc.), or if any error is generated in the child process.

• unspecified –

  any exception raised by:

  • the serialisation of population or algorithm,

  • pagmo::island::set_population() or pagmo::island::set_algorithm().

std::string get_name() const

Returns

the string "Fork island".

std::string get_extra_info() const

Returns

if an evolution is ongoing, this method will return a string representation of the ID of the child process. Otherwise, the "No active child" string will be returned.

pid_t get_child_pid() const

Returns

a signed integral value representing the process ID of the child process, if an evolution is ongoing. Otherwise, 0 will be returned.