process_shared/README.md

process_shared

Concurrency primitives that may be used in a cross-process way to coordinate share memory between processes.

FFI is used to access POSIX semaphore on Linux or Mach semaphores on Mac. Atop these semaphores are implemented ProcessShared::Semaphore, ProcessShared::Mutex. POSIX shared memory is used to implement ProcessShared::SharedMemory.

On Linux, POSIX semaphores support sem_timedwait() which can wait on a semaphore but stop waiting after a timeout.

Mac OS X's implementation of POSIX semaphores does not support timeouts. But, the Mach layer in Mac OS X has its own semaphores that do support timeouts. Thus, process_shared implements a moderate subset of the Mach API, which is quite a bit different from POSIX. Namely, semaphores created in one process are not available in child processes created via fork(). Mach does provide the means to copy capabilities between tasks (Mach equivalent to processes). process_shared overrides Ruby's fork methods so that semaphores are copied from parent to child to emulate the POSIX behavior.

This is an incomplete work in progress.

License

MIT

Install

Install the gem with:

gem install process_shared

Usage

require 'process_shared'

mutex = ProcessShared::Mutex.new
mem = ProcessShared::SharedMemory.new(:int)  # extends FFI::Pointer
mem.put_int(0, 0)

pid1 = fork do
  puts "in process 1 (#{Process.pid})"
  10.times do
    sleep 0.01
    mutex.synchronize do
      value = mem.get_int(0)
      sleep 0.01
      puts "process 1 (#{Process.pid}) incrementing"
      mem.put_int(0, value + 1)
    end
  end
end

pid2 = fork do
  puts "in process 2 (#{Process.pid})"
  10.times do
    sleep 0.01
    mutex.synchronize do
      value = mem.get_int(0)
      sleep 0.01
      puts "process 2 (#{Process.pid}) decrementing"
      mem.put_int(0, value - 1)
    end
  end
end

Process.wait(pid1)
Process.wait(pid2)

puts "value should be zero: #{mem.get_int(0)}"

Transfer Objects Across Processes

# allocate a sufficient memory block
mem = ProcessShared::SharedMemory.new(1024)

# sub process can write (serialize) object to memory (with bounds checking)
pid = fork do
  mem.write_object(['a', 'b'])
end

Process.wait(pid)

# parent process can read the object back (synchronizing access
# with a Mutex left as an excercie to reader)

mem.read_object.must_equal ['a', 'b']

Todo

• Test ConditionVariable
• Implement optional override of core Thread/Mutex classes
• Extend to win32? (See Python's processing library)
• Add finalizer to Mutex? (finalizer on Semaphore objects may be enough) or a method to explicitly close and release resources?
• Test semantics of crashing processes who still hold locks, etc.
• Is SharedArray with Enumerable mixing sufficient Array-like interface?