The Paste HTTP Server Thread Pool

This document describes how the thread pool in paste.httpserver works, and how it can adapt to problems.

Note all of the configuration parameters listed here are prefixed with threadpool_ when running through a Paste Deploy configuration.

Error Cases

When a WSGI application is called, it’s possible that it will block indefinitely. There’s two basic ways you can manage threads:

  • Start a thread on every request, close it down when the thread stops
  • Start a pool of threads, and reuse those threads for subsequent requests

In both cases things go wrong – if you start a thread every request you will have an explosion of threads, and with it memory and a loss of performance. This can culminate in really high loads, swapping, and the whole site grinds to a halt.

If you are using a pool of threads, all the threads can simply be used up. New requests go into a queue to be processed, but since that queue never moves forward everyone will just block. The site basically freezes, though memory usage doesn’t generally get worse.

Paste Thread Pool

The thread pool in Paste has some options to walk the razor’s edge between the two techniques, and to try to respond usefully in most cases.

The pool tracks all workers threads. Threads can be in a few states:

  • Idle, waiting for a request (“idle”)
  • Working on a request
    • For a reasonable amount of time (“busy”)
    • For an unreasonably long amount of time (“hung”)
  • Thread that should die
    • An exception has been injected that should kill the thread, but it hasn’t happened yet (“dying”)
    • An exception has been injected, but the thread has persisted for an unreasonable amount of time (“zombie”)

When a request comes in, if there are no idle worker threads waiting then the server looks at the workers; all workers are busy or hung. If too many are hung, another thread is opened up. The limit is if there are less than spawn_if_under busy threads. So if you have 10 workers, spawn_if_under is 5, and there are 6 hung threads and 4 busy threads, another thread will be opened (bringing the number of busy threads back to 5). Later those threads may be collected again if some of the threads become un-hung. A thread is hung if it has been working for longer than hung_thread_limit (default 30 seconds).

Every so often, the server will check all the threads for error conditions. This happens every hung_check_period requests (default 100). At this time if there are more than enough threads (because of spawn_if_under) some threads may be collected. If any threads have been working for longer than kill_thread_limit (default 1800 seconds, i.e., 30 minutes) then the thread will be killed.

To kill a thread the ctypes module must be installed. This will raise an exception (SystemExit) in the thread, which should cause the thread to stop. It can take quite a while for this to actually take effect, sometimes on the order of several minutes. This uses a non-public API (hence the ctypes requirement), and so it might not work in all cases. I’ve tried it in pure Python code and with a hung socket, and in both cases it worked. As soon as the thread is killed (before it is actually dead) another worker is added to the pool.

If the killed thread lives longer than dying_thread_limit (default 300 seconds, 5 minutes) then it is considered a zombie.

Zombie threads are not handled specially unless you set max_zombies_before_die. If you set this and there are more than this many zombie threads, then the entire process will be killed. This is useful if you are running the server under some process monitor, such as start-stop-daemon, daemontools, runit, or with paster serve --monitor. To make the process die, it may run os._exit, which is considered an impolite way to exit a process (akin to kill -9). It will try to run the functions registered with atexit (except for the thread cleanup functions, which are the ones which will block so long as there are living threads).


If you set error_email (including setting it globally in a Paste Deploy [DEFAULT] section) then you will be notified of two error conditions: when hung threads are killed, and when the process is killed due to too many zombie threads.

Missed Cases

If you have a worker pool size of 10, and 11 slow or hung requests come in, the first 10 will get handed off but the server won’t know yet that they will hang. The last request will stay stuck in a queue until another request comes in. When a later request comes later (after hung_thread_limit seconds) the server will notice the problem and add more threads, and the 11th request will come through.

If a trickle of bad requests keeps coming in, the number of hung threads will keep increasing. At 100 the hung_check_period may not clean them up fast enough.

Killing threads is not something Python really supports. Corruption of the process, memory leaks, or who knows what might occur. For the most part the threads seem to be killed in a fairly simple manner – an exception is raised, and finally blocks do get executed. But this hasn’t been tried much in production, so there’s not much experience with it.


If you want to see what’s going on in your process, you can install the application egg:Paste#watch_threads (in the paste.debug.watchthreads module). This lets you see requests and how long they have been running. In Python 2.5 you can see tracebacks of the running requests; before that you can only see request data (URLs, User-Agent, etc). If you set allow_kill = true then you can also kill threads from the application. The thread pool is intended to run reliably without intervention, but this can help debug problems or give you some feeling of what causes problems in the site.

This does open up privacy problems, as it gives you access to all the request data in the site, including cookies, IP addresses, etc. It shouldn’t be left on in a public setting.


The HTTP server (not the thread pool) also accepts an argument socket_timeout. It is turned off by default. You might find it helpful to turn it on.