Python/Django polling of database has memory leak

Question

I've got a Python script running Django for database and memcache, but it's notably runnning as a standalone daemon (i.e. not responding to webserver requests). The daemon checks a Django model Requisition for objects with a status=STATUS_NEW, then marks them STATUS_WORKING and puts them into a queue.

A number of processes (created using the multiprocess package) will pull things out of the Queue and do work on the Requisition with the pr.id that was passed to the Queue. I believe the memory leak is probably in the following code (but it could be in the 'Worker' code on the other side of the Queue though this is unlikely because because the memory size is growing even when no Requisitions are coming up -- i.e. when the workers are all blocking on Queue.get()).

from requisitions.models import Requisition # our Django model
from multiprocessing import Queue

while True:
    # Wait for "N"ew requisitions, then pop them into the queue.
    for pr in Requisition.objects.all().filter(status=Requisition.STATUS_NEW):
        pr.set_status(pr.STATUS_WORKING)
        pr.save()
        queue.put(pr.id)

    time.sleep(settings.DAEMON_POLL_WAIT)

Where settings.DAEMON_POLL_WAIT=0.01.

It seems if I leave this running for a period of time (i.e. a couple days) the Python process will grow to infinite size and eventually the system will run out of memory.

What's going on here (or how can I find out), and more importantly - how can you run a daemon that does this?

My first thought is to change the dynamic of the function, notably by putting the check for new Requisition objects into a django.core.cache cache, i.e.

from django.core.cache import cache

while True:
    time.sleep(settings.DAEMON_POLL_WAIT)
    if cache.get('new_requisitions'):
       # Possible race condition
       cache.clear()
       process_new_requisitions(queue)

 def process_new_requisitions(queue):
    for pr in Requisition.objects.all().filter(status=Requisition.STATUS_NEW):
        pr.set_status(pr.STATUS_WORKING)
        pr.save()
        queue.put(pr.id)

The process that's creating Requisitions with status=STATUS_NEW can do a cache.set('new_requisitions', 1) (or alternatively we could catch a signal or Requisition.save() event where a new Requisition is being created and then set the flag in the cache from there).

However I'm not sure that the solution I've proposed here addresses the memory issues (which are probably related to garbage collection - so the scoping by way of the process_new_requisitions may solve the problem).

I'm grateful for any thoughts and feedback.

Answer 1

You need to regularly reset a list of queries that Django keeps for debugging purposes. Normally it is cleared after every request, but since your application is not request based, you need to do this manually:

from django import db

db.reset_queries()

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See also:

• "Debugging Django memory leak with TrackRefs and Guppy" by Mikko Ohtamaa:

  Django keeps track of all queries for debugging purposes (connection.queries). This list is reseted at the end of HTTP request. But in standalone mode, there are no requests. So you need to manually reset to queries list after each working cycle

• "Why is Django leaking memory?" in Django FAQ - it talks both about setting DEBUG to False, which is always important, and about clearing the list of queries using db.reset_queries(), important in applications like yours.