[RPC][REFACTOR] Use PopenWorker to handle RPC Server.

Previously the rpc server relies multiprocessing to start a new process and does not work under jupyter.
It also have a popen mode that does ensure the socket start listening before returning the port number.

This PR switches the implementations use PopenWorker. The port number is returned after the socket
get binded, which resolves some of the RPC flaky issues(need sleep to wait the server to start).
It also makes the RPC server jupyter friendly.

python/tvm/autotvm/measure/measure_methods.py

@@ -404,7 +404,6 @@ def set_task(self, task):
             port=9000,
             port_end=10000,
             key=device_key,
-            use_popen=True,
             silent=True,
             tracker_addr=(tracker.host, tracker.port),
         )

python/tvm/contrib/popen_pool.py

@@ -44,10 +44,11 @@ def kill_child_processes(pid):
 
     try:
         parent = psutil.Process(pid)
+        children = parent.children(recursive=True)
     except psutil.NoSuchProcess:
         return
 
-    for process in parent.children(recursive=True):
+    for process in children:
         try:
             process.kill()
         except psutil.NoSuchProcess:

python/tvm/exec/rpc_server.py

@@ -16,11 +16,7 @@
 # under the License.
 # pylint: disable=redefined-outer-name, invalid-name
 """Start an RPC server"""
-from __future__ import absolute_import
-
 import argparse
-import multiprocessing
-import sys
 import logging
 from .. import rpc
 
@@ -51,6 +47,7 @@ def main(args):
         load_library=args.load_library,
         custom_addr=args.custom_addr,
         silent=args.silent,
+        no_fork=not args.fork,
     )
     server.proc.join()
 
@@ -85,14 +82,9 @@ def main(args):
     parser.set_defaults(fork=True)
     args = parser.parse_args()
     logging.basicConfig(level=logging.INFO)
-    if args.fork is False:
-        if sys.version_info[0] < 3:
-            raise RuntimeError("Python3 is required for spawn mode.")
-        multiprocessing.set_start_method("spawn")
-    else:
-        if not args.silent:
-            logging.info(
-                "If you are running ROCM/Metal, fork will cause "
-                "compiler internal error. Try to launch with arg ```--no-fork```"
-            )
+    if not args.fork is False and not args.silent:
+        logging.info(
+            "If you are running ROCM/Metal, fork will cause "
+            "compiler internal error. Try to launch with arg ```--no-fork```"
+        )
     main(args)

python/tvm/rpc/proxy.py

@@ -537,7 +537,7 @@ def __init__(
         self.thread.start()
 
 
-def _popen_start_server(
+def _popen_start_proxy_server(
     host,
     port=9091,
     port_end=9199,
@@ -570,7 +570,7 @@ def _popen_start_server(
 class Proxy(object):
     """Start RPC proxy server on a seperate process.
 
-    Python implementation based on multi-processing.
+    Python implementation based on PopenWorker.
 
     Parameters
     ----------
@@ -618,7 +618,7 @@ def __init__(
         self.proc = PopenWorker()
         # send the function
         self.proc.send(
-            _popen_start_server,
+            _popen_start_proxy_server,
             [
                 host,
                 port,

python/tvm/rpc/server.py

@@ -31,18 +31,16 @@
 import select
 import struct
 import logging
+import threading
 import multiprocessing
-import subprocess
 import time
-import sys
-import signal
-import platform
 import tvm._ffi
 
 from tvm._ffi.base import py_str
 from tvm._ffi.libinfo import find_lib_path
 from tvm.runtime.module import load_module as _load_module
 from tvm.contrib import utils
+from tvm.contrib.popen_pool import PopenWorker
 from . import _ffi_api
 from . import base
 from .base import TrackerCode
@@ -296,13 +294,85 @@ def _connect_proxy_loop(addr, key, load_library):
             time.sleep(retry_period)
 
 
-def _popen(cmd):
-    proc = subprocess.Popen(cmd, stdout=subprocess.PIPE, stderr=subprocess.STDOUT, env=os.environ)
-    (out, _) = proc.communicate()
-    if proc.returncode != 0:
-        msg = "Server invoke error:\n"
-        msg += out
-        raise RuntimeError(msg)
+class PopenRPCServerState(object):
+    """Internal PopenRPCServer State"""
+
+    current = None
+
+    def __init__(
+        self,
+        host,
+        port=9091,
+        port_end=9199,
+        is_proxy=False,
+        tracker_addr=None,
+        key="",
+        load_library=None,
+        custom_addr=None,
+        silent=False,
+    ):
+
+        # start update
+        self.host = host
+        self.port = port
+        self.libs = []
+        self.custom_addr = custom_addr
+
+        if silent:
+            logger.setLevel(logging.ERROR)
+
+        if not is_proxy:
+            sock = socket.socket(base.get_addr_family((host, port)), socket.SOCK_STREAM)
+            self.port = None
+            for my_port in range(port, port_end):
+                try:
+                    sock.bind((host, my_port))
+                    self.port = my_port
+                    break
+                except socket.error as sock_err:
+                    if sock_err.errno in [98, 48]:
+                        continue
+                    raise sock_err
+            if not self.port:
+                raise ValueError("cannot bind to any port in [%d, %d)" % (port, port_end))
+            logger.info("bind to %s:%d", host, self.port)
+            sock.listen(1)
+            self.sock = sock
+            self.thread = threading.Thread(
+                target=_listen_loop,
+                args=(self.sock, self.port, key, tracker_addr, load_library, self.custom_addr),
+            )
+            self.thread.start()
+        else:
+            self.thread = threading.Thread(
+                target=_connect_proxy_loop, args=((host, port), key, load_library)
+            )
+            self.thread.start()
+
+
+def _popen_start_rpc_server(
+    host,
+    port=9091,
+    port_end=9199,
+    is_proxy=False,
+    tracker_addr=None,
+    key="",
+    load_library=None,
+    custom_addr=None,
+    silent=False,
+    no_fork=False,
+):
+    if no_fork:
+        multiprocessing.set_start_method("spawn")
+    # This is a function that will be sent to the
+    # Popen worker to run on a separate process.
+    # Create and start the server in a different thread
+    state = PopenRPCServerState(
+        host, port, port_end, is_proxy, tracker_addr, key, load_library, custom_addr, silent
+    )
+    PopenRPCServerState.current = state
+    # returns the port so that the main can get the port number.
+    return state.port
 
 
 class Server(object):
@@ -328,11 +398,6 @@ class Server(object):
         If this is true, the host and port actually corresponds to the
         address of the proxy server.
 
-    use_popen : bool, optional
-        Whether to use Popen to start a fresh new process instead of fork.
-        This is recommended to switch on if we want to do local RPC demonstration
-        for GPU devices to avoid fork safety issues.
-
     tracker_addr: Tuple (str, int) , optional
         The address of RPC Tracker in tuple(host, ip) format.
         If is not None, the server will register itself to the tracker.
@@ -348,6 +413,9 @@ class Server(object):
 
     silent: bool, optional
         Whether run this server in silent mode.
+
+    no_fork: bool, optional
+        Whether forbid fork in multiprocessing.
     """
 
     def __init__(
@@ -356,101 +424,44 @@ def __init__(
         port=9091,
         port_end=9199,
         is_proxy=False,
-        use_popen=False,
         tracker_addr=None,
         key="",
         load_library=None,
         custom_addr=None,
         silent=False,
+        no_fork=False,
     ):
         try:
             if _ffi_api.ServerLoop is None:
                 raise RuntimeError("Please compile with USE_RPC=1")
         except NameError:
             raise RuntimeError("Please compile with USE_RPC=1")
+        self.proc = PopenWorker()
+        # send the function
+        self.proc.send(
+            _popen_start_rpc_server,
+            [
+                host,
+                port,
+                port_end,
+                is_proxy,
+                tracker_addr,
+                key,
+                load_library,
+                custom_addr,
+                silent,
+                no_fork,
+            ],
+        )
+        # receive the port
+        self.port = self.proc.recv()
         self.host = host
-        self.port = port
-        self.libs = []
-        self.custom_addr = custom_addr
-        self.use_popen = use_popen
-
-        if silent:
-            logger.setLevel(logging.ERROR)
-
-        if use_popen:
-            cmd = [
-                sys.executable,
-                "-m",
-                "tvm.exec.rpc_server",
-                "--host=%s" % host,
-                "--port=%s" % port,
-                "--port-end=%s" % port_end,
-            ]
-            if tracker_addr:
-                assert key
-                cmd += ["--tracker=%s:%d" % tracker_addr, "--key=%s" % key]
-            if load_library:
-                cmd += ["--load-library", load_library]
-            if custom_addr:
-                cmd += ["--custom-addr", custom_addr]
-            if silent:
-                cmd += ["--silent"]
-
-            # prexec_fn is not thread safe and may result in deadlock.
-            # python 3.2 introduced the start_new_session parameter as
-            # an alternative to the common use case of
-            # prexec_fn=os.setsid.  Once the minimum version of python
-            # supported by TVM reaches python 3.2 this code can be
-            # rewritten in favour of start_new_session.  In the
-            # interim, stop the pylint diagnostic.
-            #
-            # pylint: disable=subprocess-popen-preexec-fn
-            if platform.system() == "Windows":
-                self.proc = subprocess.Popen(cmd, creationflags=subprocess.CREATE_NEW_PROCESS_GROUP)
-            else:
-                self.proc = subprocess.Popen(cmd, preexec_fn=os.setsid)
-            time.sleep(0.5)
-        elif not is_proxy:
-            sock = socket.socket(base.get_addr_family((host, port)), socket.SOCK_STREAM)
-            self.port = None
-            for my_port in range(port, port_end):
-                try:
-                    sock.bind((host, my_port))
-                    self.port = my_port
-                    break
-                except socket.error as sock_err:
-                    if sock_err.errno in [98, 48]:
-                        continue
-                    raise sock_err
-            if not self.port:
-                raise ValueError("cannot bind to any port in [%d, %d)" % (port, port_end))
-            logger.info("bind to %s:%d", host, self.port)
-            sock.listen(1)
-            self.sock = sock
-            self.proc = multiprocessing.Process(
-                target=_listen_loop,
-                args=(self.sock, self.port, key, tracker_addr, load_library, self.custom_addr),
-            )
-            self.proc.start()
-        else:
-            self.proc = multiprocessing.Process(
-                target=_connect_proxy_loop, args=((host, port), key, load_library)
-            )
-            self.proc.start()
 
     def terminate(self):
         """Terminate the server process"""
-        if self.use_popen:
-            if self.proc:
-                if platform.system() == "Windows":
-                    os.kill(self.proc.pid, signal.CTRL_C_EVENT)
-                else:
-                    os.killpg(self.proc.pid, signal.SIGTERM)
-                self.proc = None
-        else:
-            if self.proc:
-                self.proc.terminate()
-                self.proc = None
+        if self.proc:
+            self.proc.kill()
+            self.proc = None
 
     def __del__(self):
         self.terminate()