gipc: child processes and IPC for gevent
Table of contents:
gipc (pronunciation “gipsy”) is a Python package tested on CPython 2.6 and 2.7 on Linux as well as on Windows.
What is gipc good for?
The usage of multiple processes in the context of gevent in principal can be a decent solution whenever a generally I/O-limited Python application needs to distribute tasks among multiple CPUs in parallel.
However, naive usage of Python's multiprocessing package within a gevent-powered application may raise various problems and most likely breaks the application in many ways. gipc is developed with the motivation to solve these issues transparently and make using gevent in combination with multiprocessing-based child processes and inter-process communication (IPC) a no-brainer again:
- With gipc, multiprocessing.Process-based child processes can safely be created and monitored anywhere within your gevent-powered application. Malicious side-effects of child process creation in the context of gevent are prevented.
- The API of multiprocessing.Process objects is provided in a gevent-cooperative fashion.
- gevent natively works in children.
- gipc comes up with a pipe-based transport layer for gevent-cooperative IPC.
- gipc is lightweight and simple to integrate.
In the following code snippet, a Python object is sent from a greenlet in the main process through a pipe to a child process:
import gevent import gipc def child(reader): assert reader.get() == 0 if __name__ == "__main__": with gipc.pipe() as (reader, writer): writelet = gevent.spawn(lambda w: w.put(0), writer) readchild = gipc.start_process(target=child, args=(reader,)) writelet.join() readchild.join()
Although quite simple, this code would have various negative side-effects if used with the canonical multiprocessing API instead of gipc.start_process() and gipc.pipe(), as outlined in the next paragrah.
What are the challenges and what is gipc's approach?
Depending on the operating system, child process creation via Python's multiprocessing in the context of gevent requires special treatment. Most care is needed on POSIX-compliant systems. There, a fork might yield a faulty libev event loop state in the child. Most noticeable, greenlets spawned before forking are cloned and haunt in the child upon context switch. Consider this code running on Unix (tested with Python 2.7 & gevent 1.0rc2):
import gevent import multiprocessing def child(c): gevent.sleep(0) assert c.recv() == 0 assert c.recv() == 0 if __name__ == "__main__": def writelet(c): c.send(0) c1, c2 = multiprocessing.Pipe() writelet = gevent.spawn(writelet, c1) readchild = multiprocessing.Process(target=child, args=(c2,)) readchild.start() writelet.join() readchild.join()
It runs without error. Although the code intends to send only one message to the child through a multiprocessing Pipe, the two assert statements verify that the child actually receives two times the same message. One message is sent -- as intended -- from the writelet in the parent through the c1 end of the pipe. It is retrieved at the c2 end of the pipe in the child. The other message is sent from the spooky writelet clone in the child. It is also written to the c1 end of the pipe which has implicitly been duplicated during forking. Greenlet clones in the child of course only run when a context switch is triggered; in this case via gevent.sleep(0). As you can imagine, this behavior in general might lead to a wide range of side-effects and tedious debugging sessions.
In addition, the code above contains several non-cooperatively blocking method calls: readchild.join() as well as the send()/recv() calls (of multiprocessing.Connection objects in general) block the calling thread and do not allow for context switches.
gipc overcomes these and other challenges for you transparently and in a straight-forward fashion. It provides high performing gevent-cooperative pipe-based message transport channels. Dispensable gipc pipe handles are closed in the child. Greenlet clones are cleanly killed in the child before being harmful. The libev event loop state is fixed in time. Basically, children start off with a fresh gevent state before entering the user-given target function. On POSIX-compliant systems, gipc entirely avoids multiprocessing's child monitoring capabilities and uses libev's wonderful child watcher system.
gipc allows for integration of child processes in your application via a simple API -- on POSIX-compliant systems as well as on Windows.
gipc's usage is pretty simple. Its interface is clear and slim. Make yourself comfortable with gipc.start_process() and gipc.pipe() by going through the :ref:`examples <examples>` and the :ref:`API <api>` section.
- gipc uses classical anonymous pipes as transport layer for gevent-cooperative communication between greenlets and/or processes. A binary pickle protocol is used for transmitting arbitrary objects. Reading and writing on pipes is done with gevent's cooperative versions of os.read() and os.write() (on POSIX-compliant systems they use non-blocking I/O, on Windows a threadpool is used). On Linux, my test system (Xeon E5630) achieved a payload transfer rate of 1200 MB/s and a message transmission rate of 100.000 messages/s through one pipe between two processes.
- Child process creation and invocation is done via a thin wrapper around multiprocessing.Process. On Unix, gevent's state and the libev event loop are re-initialized in the child before execution of the target function.
- On POSIX-compliant systems, gevent-aware child process monitoring is based on libev child watchers (this affects is_alive() and join()).
- Convenience features such as a context manager for pipe handles or timeout controls based on gevent.Timeout are available.
- Any read/write operation on a pipe is gevent.lock.Semaphore-protected and therefore greenlet-/threadsafe and atomic.
- gipc obeys semantic versioning 2.
- Although gipc is in an early development phase, I found it to work very stable already. The unit test suite aims to cover all of gipc's features within a clean gevent environment. More complex application scenarios, however, are not covered so far. Please let me know in which cases gipc + gevent fails for you.
Code, requirements, download, installation
gipc's Mercurial repository is hosted at Bitbucket.
Download & install via pip
The latest official gipc release from PyPI can be pulled and installed via pip:
$ pip install gipc
pip can also install the current development version of gipc:
$ pip install hg+https://bitbucket.org/jgehrcke/gipc
pip is recommended over easy_install. pip installation instructions can be found here.
Notes for Windows users
- The _GIPCReader.get() timeout feature is not available.
- "Non-blocking I/O" is imitated by outsourcing blocking I/O calls to threads in a gevent thread pool. Compared to native non-blocking I/O as is available on POSIX-compliant systems, this leads to a significant messaging performance drop.
Windows I/O Completion Ports (IOCP) could solve both issues in an elegant way. Currently, gevent is built on top of libev which does not support IOCP. In the future, however, gevent might become libuv-backed. libuv supports IOCP and would allow for running the same gevent code on Windows as on POSIX-compliant systems. Furthermore, if gevent went with libuv, the strengths of both, the node.js and the gevent worlds would be merged. Denis Bilenko, the maintainer of gevent, seems to be open to such a transition and the first steps are already done.
- :ref:`gipc.pipe()-based IPC <exampleipc>`
- :ref:`Serving multiple clients (in child) from one server (in parent) <exampleserverclient>`
- :ref:`Time-synchronization between processes <examplesync>`
Note that these examples are invented with the motivation to demonstrate the API and capabilities of gipc rather than showing interesting use cases.
gipc.pipe()-based messaging from greenlet in parent to child
Some basic concepts are explained by means of this simple messaging example:
import gevent import gipc def main(): with gipc.pipe() as (r, w): p = gipc.start_process(target=child_process, args=(r, )) wg = gevent.spawn(writegreenlet, w) try: p.join() except KeyboardInterrupt: wg.kill(block=True) p.terminate() p.join() def writegreenlet(writer): while True: writer.put("written to pipe from a greenlet running in the main process") gevent.sleep(1) def child_process(reader): while True: print "Child process got message from pipe:\n\t'%s'" % reader.get() if __name__ == "__main__": main()
The context manager with gipc.pipe() as (r, w) creates a pipe with read handle r and write handle w. On context exit (latest) the pipe ends will be closed properly.
Within the context, a child process is spawned via gipc.start_process(). The read handle r is provided to the child process. The child invokes child_process(r) where an endless loop waits for objects on the read end of the pipe. Upon retrieval, it immediately writes them to stdout.
While child process p is running, a greenlet wg is started in the main process. It executes the function writegreenlet and passes the write handle w as an argument. Within this greenlet, one string per second is written to the write end of the pipe.
After spawning wg, p.join() is called immediately, i.e. the write greenlet is running while p.join() waits for the child process to terminate. In this state, one message per second is passed between parent and child until a KeyboardInterrupt exception is raised in the parent.
Upon KeyboardInterrupt, the parent first kills the write greenlet and blocks cooperatively until it has stopped. Then it terminates the child process (via SIGTER on Unix) and waits for it to exit via p.join().
Serving multiple clients (in child) from one server (in parent)
For pure API and reliability demonstration purposes, this example implements TCP communication between a server in the parent process and multiple clients in one child process:
- gevent's StreamServer is started in a greenlet within the initial (parent) process. For each connecting client, it receives one newline-terminated message and echoes it back.
- A child process is started using gipc. Its starting point is the function clientprocess. There, N TCP clients are started concurrently from N greenlets.
- Each client sends one message, validates the echo response and terminates.
- The child process terminates.
- After the child process is joined in the parent, the server is killed.
- The server greenlet is joined.
import gevent from gevent.server import StreamServer from gevent import socket import gipc import time PORT = 1337 N_CLIENTS = 1000 MSG = "HELLO\n" def serve(sock, addr): f = sock.makefile() f.write(f.readline()) f.flush() f.close() def server(): ss = StreamServer(('localhost', PORT), serve).serve_forever() def clientprocess(): t1 = time.time() clients = [gevent.spawn(client) for _ in xrange(N_CLIENTS)] gevent.joinall(clients) duration = time.time()-t1 print "%s clients served within %.2f s." % (N_CLIENTS, duration) def client(): sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM) sock.connect(('localhost', PORT)) f = sock.makefile() f.write(MSG) f.flush() assert f.readline() == MSG f.close() if __name__ == "__main__": s = gevent.spawn(server) c = gipc.start_process(clientprocess) c.join() s.kill() s.join()
Output on my test machine: 1000 clients served within 0.54 s.
Time-synchronization between processes
Child process creation may take a significant amount of time, especially on Windows. This time is not predictable.
When code in the parent should only proceed in the moment the code in the child has reached a certain state, the proper way to tackle this is a bidirectional synchronization handshake:
- Process A sends a synchronization request to process B and waits for an acknowledgement response. It proceeds upon retrieval.
- Process B sends the acknowledgement in the moment it retrieves the sync request and proceeds.
This concept can easily be implemented using a bidirectional gipc.pipe():
import gevent import gipc import time def main(): with gipc.pipe(duplex=True) as (cend, pend): # `cend` is the channel end for the child, `pend` for the parent. p = gipc.start_process(writer_process, args=(cend,)) # Synchronize with child process. pend.put("SYN") assert pend.get() == "ACK" # Now in sync with child. ptime = time.time() ctime = pend.get() p.join() print "Time delta: %.8f s." % abs(ptime - ctime) def writer_process(cend): with cend: assert cend.get() == "SYN" cend.put("ACK") # Now in sync with parent. cend.put(time.time()) if __name__ == "__main__": main()
The marked code blocks in parent and child are entered quasi-simultaneously. Example output on my test machine (Linux): Time delta: 0.00005388 s. On Windows, time.time()'s precision is not sufficient to resolve the time delta (and time.clock() is not applicable for comparing times across processes).
- :ref:`Spawning child processes <api_spawn>`
- :ref:`Creating a pipe and its handle-pair <api_pipe_create>`
- :ref:`Handling handles <api_handles>`
- :ref:`Controlling child processes <api_control_childs>`
- :ref:`Exception types <api_exceptions>`