error: can't start new thread

Question

Hello my friends.

I have a site that runs with follow configuration:

Django + mod-wsgi + apache

In one of user's request, I send another HTTP request to another service, and solve this by httplib library of python.

But sometimes this service don't get answer too long, and timeout for httplib doesn't work. So I creating thread, in this thread I send request to service, and join it after 20 sec (20 sec - is a timeout of request). This is how it works:

class HttpGetTimeOut(threading.Thread):
    def __init__(self,**kwargs):
        self.config = kwargs
        self.resp_data = None
        self.exception = None
        super(HttpGetTimeOut,self).__init__()
    def run(self):

        h = httplib.HTTPSConnection(self.config['server'])
        h.connect()
        sended_data = self.config['sended_data']
        h.putrequest("POST", self.config['path'])
        h.putheader("Content-Length", str(len(sended_data)))
        h.putheader("Content-Type", 'text/xml; charset="utf-8"')
        if 'base_auth' in self.config:
            base64string = base64.encodestring('%s:%s' % self.config['base_auth'])[:-1]
            h.putheader("Authorization", "Basic %s" % base64string)
        h.endheaders()

        try:
            h.send(sended_data)
            self.resp_data = h.getresponse()
        except httplib.HTTPException,e:
            self.exception = e
        except Exception,e:
            self.exception = e

something like this...

And use it by this function:

getting = HttpGetTimeOut(**req_config)
getting.start()
getting.join(COOPERATION_TIMEOUT)
if getting.isAlive(): #maybe need some block
    getting._Thread__stop()
    raise ValueError('Timeout')
else:
    if getting.resp_data:
        r = getting.resp_data
    else:
        if getting.exception:
            raise ValueError('REquest Exception')
        else:
            raise ValueError('Undefined exception')

And all works fine, but sometime I start catching this exception:

error: can't start new thread

at the line of starting new thread:

getting.start()

and the next and the final line of traceback is

File "/usr/lib/python2.5/threading.py", line 440, in start
    _start_new_thread(self.__bootstrap, ())

And the answer is: What's happen?

Thank's for all, and sorry for my pure English. :)

Answer 1

You are starting more threads than can be handled by your system. There is a limit to the number of threads that can be active for one process.

Your application is starting threads faster than the threads are running to completion. If you need to start many threads you need to do it in a more controlled manner I would suggest using a thread pool.

Answer 2

The "can't start new thread" error almost certainly due to the fact that you have already have too many threads running within your python process, and due to a resource limit of some kind the request to create a new thread is refused.

You should probably look at the number of threads you're creating; the maximum number you will be able to create will be determined by your environment, but it should be in the order of hundreds at least.

It would probably be a good idea to re-think your architecture here; seeing as this is running asynchronously anyhow, perhaps you could use a pool of threads to fetch resources from another site instead of always starting up a thread for every request.

Another improvement to consider is your use of Thread.join and Thread.stop; this would probably be better accomplished by providing a timeout value to the constructor of HTTPSConnection.

Answer 3

I think the best way in your case is to set socket timeout instead of spawning thread:

h = httplib.HTTPSConnection(self.config['server'], 
                            timeout=self.config['timeout'])

Also you can set global default timeout with socket.setdefaulttimeout() function.

Update: See answers to Is there any way to kill a Thread in Python? question (there are several quite informative) to understand why. Thread.__stop() doesn't terminate thread, but rather set internal flag so that it's considered already stopped.

Answer 4

If you are tying to set timeout why don't you use urllib2.

Answer 5

I completely rewrite code from httplib to pycurl.

c = pycurl.Curl()
c.setopt(pycurl.FOLLOWLOCATION, 1)
c.setopt(pycurl.MAXREDIRS, 5)
c.setopt(pycurl.CONNECTTIMEOUT, CONNECTION_TIMEOUT)
c.setopt(pycurl.TIMEOUT, COOPERATION_TIMEOUT)
c.setopt(pycurl.NOSIGNAL, 1)
c.setopt(pycurl.POST, 1)
c.setopt(pycurl.SSL_VERIFYHOST, 0)
c.setopt(pycurl.SSL_VERIFYPEER, 0)
c.setopt(pycurl.URL, "https://"+server+path)
c.setopt(pycurl.POSTFIELDS,sended_data)

b = StringIO.StringIO()
c.setopt(pycurl.WRITEFUNCTION, b.write)

c.perform()

something like that.

And I testing it now. Thanks all of you for help.