I have a specific problem.
Main content of program starts with creating Process with dbus loop, where I listen for signals.
Content of signals I store in queues. In next part of main I have a threadpool.
When some thread takes item from queue, it use specific function(detection) to handle request - based on content of item from queue. (There is operation on database, from where I take data and make some operations depends on request)
Every thread in thread pool starts one more thread, which should handle signals (current status and interrupt).
For example: I receive signal, which means I have to handle something on numbers. Any thread from threadpool takes this item from queue and starts function which handle something on numbers - it can take long time. So after any time, I receive signal for current status and I need to send current status of detection - that's why I use threads (for shared memory). Also I can receive interrupt signal from D-Bus ("it takes too long time, so stop this detection and be free for another request"). And the interrupt is the main problem...
So my main questions are:
Is there any way, I can raise exception on interrupt signal and stop function (detection)? (I just found solution, but only for catch in main... but I need to catch it in thread which is in threadpool and raise in thread which is in thread in threadpool)
Second question is about GIL... does my thread with signal receiving receive all signals? I think it doesn't... (Yes, I use threads_init())
program:
SERVICE = multiprocessing.Process(target=dbus_signal_receiver, args=(...))
SERVICE.daemon = True
SERVICE.start()
class worker(threading.Thread):
def __init__(self,...):
threading.Thread.__init__(self)
def run(self):
while True:
#get item from queue
s = threading.Thread(target=curr_and_interr_signal_handle, args=(ID of item from queue,...))
s.daemon = True
s.start()
#start specific detection based on request
for i in range(number of threads):
t = worker(...)
t.daemon = True
t.start()
and I hoped, something like this will work... (but it doesn't)
...
class worker(threading.Thread):
def __init__(self,...):
threading.Thread.__init__(self)
def run(self):
while True:
try:
#get item from queue
s = threading.Thread(target=curr_and_interr_signal_handle, args=(ID of item from queue,...))
s.daemon = True
s.start()
#start specific detection based on request
except raised_interrupt_exception:
#continue - wait for another request from queue
...
Read about 18.8.1.2. Signals and threads
Python signal handlers are always executed in the main Python thread,
even if the signal was received in another thread.
This means that signals can’t be used as a means of inter-thread communication.
You can use the synchronization primitives from the threading module instead.
Besides, only the main thread is allowed to set a new signal handler.
Read about 17.1.7. Event Objects
This is one of the simplest mechanisms for communication between threads: one thread signals an event and other threads wait for it
Isn't clear why you have to use thread in thread.
Why could your worker thread not handle detection?
For instance, the following should be do it:
def run(self):
while self.running.is_set():
#get item from queue
#start specific detection based on request
Related
I have two threads in a producer consumer pattern. When the consumer receives data it calls an time consuming function expensive() and then enters in a for loop.
But if while the consumer is working new data arrives, it should abort the current work, (exit the loop) and start with the new data.
I tried with a queue.Queue something like this:
q = queue.Queue()
def producer():
while True:
...
q.put(d)
def consumer():
while True:
d = q.get()
expensive(d)
for i in range(10000):
...
if not q.empty():
break
But the problem with this code is that if the producer put data too too fast, and the queue get to have many items, the consumer will do the expensive(d) call plus one loop iteration and then abort for each item, which is time consuming. The code should work, but is not optimized.
Without modifying the code in expensive one solution could be to run it as a separate process which will provide you the ability to terminateit prematurely. Since there's no mention to how long expensive runs this may or may not be more time efficient, however.
import multiprocessing as mp
q = queue.Queue()
def producer():
while True:
...
q.put(d)
def consumer():
while True:
d = q.get()
exp = mp.Thread(target=expensive, args=(d,))
for i in range(10000):
...
if not q.empty():
exp.terminate() # or exp.kill()
break
Well, one way is to use a queue design that can keep an internal lists of waiting and working threads. You can then create several consumer threads to wait on the queue and, when work arrives, set a known consumer thread to do the work. When the thread has finished, it calls into the queue to remove itself from the working list and add itself to the waiting list.
The consumer threads each have an 'abort' atomic that can signal the thread to finish early. There will be some latency while the thread performs inner loops, but that will not matter....
If new work arrives at the queue from the producer, and the working queue is not empty, the 'abort' bool of the working thread/s can be set and their priority set to the minimum possible. The new work can then be dispatched onto one of the waiting threads from the pool, so setting it working.
The waiting threads will need a 'start' function that signals an event/sema/condvar that the wait thread..well..waits on. That allows the producer that supplied work to set that specific thread running, rather than the 'usual' practice where any thread from a pool may pick up work.
Such a design allows new work to be started 'immediately', makes the previous work thread irrelevant by de-prioritizing it and avoids the overheads of thread/process termination.
I don't manage to understand why my SIGINT is never caught by the piece of code below.
#!/usr/bin/env python
from threading import Thread
from time import sleep
import signal
class MyThread(Thread):
def __init__(self):
Thread.__init__(self)
self.running = True
def stop(self):
self.running = False
def run(self):
while self.running:
for i in range(500):
col = i**i
print col
sleep(0.01)
global threads
threads = []
for w in range(150):
threads.append(MyThread())
def stop(s, f):
for t in threads:
t.stop()
signal.signal(signal.SIGINT, stop)
for t in threads:
t.start()
for t in threads:
t.join()
To clean this code I would prefer to try/except the join() and closing all threads in case of exception, would that work?
One of the problems with multithreading in python is that join() more or less disables signals.
This is because the signal can only be delivered to the main thread, but the main thread is already busy with performing the join() and the join is not interruptible.
You can deduce this from the documentation of the signal module
Some care must be taken if both signals and threads are used in the same program. The fundamental thing to remember in using signals and threads simultaneously is: always perform signal() operations in the main thread of execution. Any thread can perform an alarm(), getsignal(), pause(), setitimer() or getitimer(); only the main thread can set a new signal handler, and the main thread will be the only one to receive signals (this is enforced by the Python signal module, even if the underlying thread implementation supports sending signals to individual threads). This means that signals can’t be used as a means of inter-thread communication. Use locks instead.
You can work your way around it, by busy-looping over the join operation:
for t in threads:
while t.isAlive():
t.join(timeout=1)
This is, however, none to efficient:
The workaround of calling join() with a timeout has a drawback:
Python's threading wait routine polls 20 times a second when
given any timeout. All this polling can mean lots of CPU
interrupts/wakeups on an otherwise idle laptop and drain the
battery faster.
Some more details are provided here:
Python program with thread can't catch CTRL+C
Bug reports for this problem with a discussion of the underlying issue can be found here:
https://bugs.python.org/issue1167930
https://bugs.python.org/issue1171023
I have this thread running :
def run(self):
while 1:
msg = self.connection.recv(1024).decode()
I wish I could end this thread when I close the Tkinter Window like this :
self.window.protocol('WM_DELETE_WINDOW', self.closeThreads)
def closeThreads(self):
self.game.destroy()
#End the thread
Can't use thread._close() because it is deprecated and python 3.4 does not allow it.
The only really satisfactory solution I've seen for this problem is not to allow your thread to block inside recv(). Instead, set the socket to non-blocking and have the thread block inside select() instead. The advantage of blocking inside select() is that you can tell select() to return when any one of several sockets becomes ready-for-read, which brings us to the next part: as part of setting up your thread, create a second socket (either a locally-connected TCP socket e.g. as provided by socketpair, or a UDP socket listening on a port for packets from localhost). When your main thread wants your networking thread to go away, your main thread should send a byte to that socket (or in the TCP case, the main thread could just close its end of the socket-pair). That will cause select() to return ready-for-read on that socket, and when your network thread realizes that the socket is marked ready-for-read, it should respond by exiting immediately.
The advantages of doing it that way are that it works well on all OS's, always reacts immediately (unlike a polling/timeout solution), takes up zero extra CPU cycles when the network is idle, and doesn't have any nasty side effects in multithreaded environments. The downside is that it uses up a couple of extra sockets, but that's usually not a big deal.
Two solutions:
1) Don't stop the thread, just allow it to die when the process exits with sys.exit()
2) Start the thread with a "die now" flag. The Event class is specifically designed to signal one thread from another.
The following example starts a thread, which connects to a server. Any data is handled, and if the parent signals the thread to exit, it will. As an additional safety feature we have an alarm signal to kill everything, just it case something gets out of hand.
source
import signal, socket, threading
class MyThread(threading.Thread):
def __init__(self, conn, event):
super(MyThread,self).__init__()
self.conn = conn
self.event = event
def handle_data(self):
"process data if any"
try:
data = self.conn.recv(4096)
if data:
print 'data:',data,len(data)
except socket.timeout:
print '(timeout)'
def run(self):
self.conn.settimeout(1.0)
# exit on signal from caller
while not self.event.is_set():
# handle any data; continue loop after 1 second
self.handle_data()
print 'got event; returning to caller'
sock = socket.create_connection( ('example.com', 80) )
event = threading.Event()
# connect to server and start connection handler
th = MyThread(conn=sock, event=event)
# watchdog: kill everything in 3 seconds
signal.alarm(3)
# after 2 seconds, tell data thread to exit
threading.Timer(2.0, event.set).start()
# start data thread and wait for it
th.start()
th.join()
output
(timeout)
(timeout)
got event; returning to caller
I start a bunch of threads working on a queue and I want to kill them when sending the SIGINT (Ctrl+C). What is the best way to handle this?
targets = Queue.Queue()
threads_num = 10
threads = []
for i in threads_num:
t = MyThread()
t.setDaemon(True)
threads.append(t)
t.start()
targets.join()
If you are not interested in letting the other threads shut down gracefully, simply start them in daemon mode and wrap the join of the queue in a terminator thread.
That way, you can make use of the join method of the thread -- which supports a timeout and does not block off exceptions -- instead of having to wait on the queue's join method.
In other words, do something like this:
term = Thread(target=someQueueVar.join)
term.daemon = True
term.start()
while (term.isAlive()):
term.join(3600)
Now, Ctrl+C will terminate the MainThread whereupon the Python Interpreter hard-kills all threads marked as "daemons". Do note that this means that you have to set "Thread.daemon" for all the other threads or shut them down gracefully by catching the correct exception (KeyboardInterrupt or SystemExit) and doing whatever needs to be done for them to quit.
Do also note that you absolutely need to pass a number to term.join(), as otherwise it will, too, ignore all exceptions. You can select an arbitrarily high number, though.
Isn't Ctrl+C SIGINT?
Anyway, you can install a handler for the appropriate signal, and in the handler:
set a global flag that instructs the workers to exit, and make sure they check it periodically
or put 10 shutdown tokens on the queue, and have the workers exit when they pop this magic token
or set a flag which instructs the main thread to push those tokens, make sure the main thread checks that flag
etc. Mostly it depends on the structure of the application you're interrupting.
One way to do it is to install a signal handler for SIGTERM that directly calls os._exit(signal.SIGTERM). However unless you specify the optional timeout argument to Queue.get the signal handler function will not run until after the get method returns. (That's completely undocumented; I discovered that on my own.) So you can specify sys.maxint as the timeout and put your Queue.get call in a retry loop for purity to get around that.
Why don't you set timeouts for any operation on the queue? Then your threads can regular check if they have to finish by checking if an Event is raised.
This is how I tackled this.
class Worker(threading.Thread):
def __init__(self):
self.shutdown_flag = threading.Event()
def run(self):
logging.info('Worker started')
while not self.shutdown_flag.is_set():
try:
task = self.get_task_from_queue()
except queue.Empty:
continue
self.process_task(task)
def get_task_from_queue(self) -> Task:
return self.task_queue.get(block=True, timeout=10)
def shutdown(self):
logging.info('Shutdown received')
self.shutdown_flag.set()
Upon receiving a signal the main thread sets the shutdown event on workers. The workers wait on a blocking queue, but keep checking every 10 seconds if they have received a shutdown signal.
I managed to solve the problem by emptying the queue on KeyboardInterrupt and letting threads to gracefully stop themselves.
I don't know if it's the best way to handle this but is simple and quite clean.
targets = Queue.Queue()
threads_num = 10
threads = []
for i in threads_num:
t = MyThread()
t.setDaemon(True)
threads.append(t)
t.start()
while True:
try:
# If the queue is empty exit loop
if self.targets.empty() is True:
break
# KeyboardInterrupt handler
except KeyboardInterrupt:
print "[X] Interrupt! Killing threads..."
# Substitute the old queue with a new empty one and exit loop
targets = Queue.Queue()
break
# Join every thread on the queue normally
targets.join()
I have a queue that always needs to be ready to process items when they are added to it. The function that runs on each item in the queue creates and starts thread to execute the operation in the background so the program can go do other things.
However, the function I am calling on each item in the queue simply starts the thread and then completes execution, regardless of whether or not the thread it started completed. Because of this, the loop will move on to the next item in the queue before the program is done processing the last item.
Here is code to better demonstrate what I am trying to do:
queue = Queue.Queue()
t = threading.Thread(target=worker)
t.start()
def addTask():
queue.put(SomeObject())
def worker():
while True:
try:
# If an item is put onto the queue, immediately execute it (unless
# an item on the queue is still being processed, in which case wait
# for it to complete before moving on to the next item in the queue)
item = queue.get()
runTests(item)
# I want to wait for 'runTests' to complete before moving past this point
except Queue.Empty, err:
# If the queue is empty, just keep running the loop until something
# is put on top of it.
pass
def runTests(args):
op_thread = SomeThread(args)
op_thread.start()
# My problem is once this last line 't.start()' starts the thread,
# the 'runTests' function completes operation, but the operation executed
# by some thread is not yet done executing because it is still running in
# the background. I do not want the 'runTests' function to actually complete
# execution until the operation in thread t is done executing.
"""t.join()"""
# I tried putting this line after 't.start()', but that did not solve anything.
# I have commented it out because it is not necessary to demonstrate what
# I am trying to do, but I just wanted to show that I tried it.
Some notes:
This is all running in a PyGTK application. Once the 'SomeThread' operation is complete, it sends a callback to the GUI to display the results of the operation.
I do not know how much this affects the issue I am having, but I thought it might be important.
A fundamental issue with Python threads is that you can't just kill them - they have to agree to die.
What you should do is:
Implement the thread as a class
Add a threading.Event member which the join method clears and the thread's main loop occasionally checks. If it sees it's cleared, it returns. For this override threading.Thread.join to check the event and then call Thread.join on itself
To allow (2), make the read from Queue block with some small timeout. This way your thread's "response time" to the kill request will be the timeout, and OTOH no CPU choking is done
Here's some code from a socket client thread I have that has the same issue with blocking on a queue:
class SocketClientThread(threading.Thread):
""" Implements the threading.Thread interface (start, join, etc.) and
can be controlled via the cmd_q Queue attribute. Replies are placed in
the reply_q Queue attribute.
"""
def __init__(self, cmd_q=Queue.Queue(), reply_q=Queue.Queue()):
super(SocketClientThread, self).__init__()
self.cmd_q = cmd_q
self.reply_q = reply_q
self.alive = threading.Event()
self.alive.set()
self.socket = None
self.handlers = {
ClientCommand.CONNECT: self._handle_CONNECT,
ClientCommand.CLOSE: self._handle_CLOSE,
ClientCommand.SEND: self._handle_SEND,
ClientCommand.RECEIVE: self._handle_RECEIVE,
}
def run(self):
while self.alive.isSet():
try:
# Queue.get with timeout to allow checking self.alive
cmd = self.cmd_q.get(True, 0.1)
self.handlers[cmd.type](cmd)
except Queue.Empty as e:
continue
def join(self, timeout=None):
self.alive.clear()
threading.Thread.join(self, timeout)
Note self.alive and the loop in run.