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I'm using MySQLdb under Python32 on Windows 7:

Python 3.2.3 (default, Apr 11 2012, 07:12:16) [MSC v.1500 64 bit (AMD64)] on win32 >>> import MySQLdb as My >>> My.version_info (1, 2, 3, 'final', 0)

I'm running service which calls this many times over and over and over again:

cursor = self._connection._conn.cursor() cursor.execute(sql) for i in cursor.fetchall(): pass # Operation that is not important cursor.close() gc.collect() return set() # Will be filled with DB data

And memory usage just goes up and up and up, I've already tried diagnosing it and end up with this:

83 23.129 MB 0.000 MB cursor = self._connection._conn.cursor() 84 23.129 MB 0.000 MB cursor.execute(sql) 85 23.137 MB 0.008 MB for i in cursor.fetchall(): pass 86 23.137 MB 0.000 MB cursor.close() 87 88 23.137 MB 0.000 MB gc.collect() 89 90 23.137 MB 0.000 MB return set()

Neither __iter__ API seems to be better:

84 23.145 MB 0.000 MB cursor.execute(sql) 85 23.145 MB 0.000 MB for i in cursor: pass 86 23.152 MB 0.008 MB cursor.close() 87 88 23.152 MB 0.000 MB gc.collect() 89 90 23.152 MB 0.000 MB return set()

And neither looping manually with fetchone():

84 23.141 MB 0.000 MB cursor.execute(sql) 85 23.141 MB 0.000 MB while True: 86 23.141 MB 0.000 MB row = cursor.fetchone() 87 23.141 MB 0.000 MB if not row: 88 23.141 MB 0.000 MB break 89 23.148 MB 0.008 MB cursor.close() 90 91 23.148 MB 0.000 MB gc.collect() 92 93 23.148 MB 0.000 MB return set()

So why is not memory cleaning back to 23.129MB (why it always uses new 8KB)? Is the cursor buggy? Am I doing something wrong?

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  • The GC module has a debug interface for looking into memory leaks, does that give you any more insight? Commented Oct 9, 2013 at 9:25
  • @dwxw It gives me that much info I'm unable to interpret it, if you can provide any good resource for diagnosing this using gc, I'll be glad to read it. Commented Oct 9, 2013 at 9:36

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IIRC cursor.fetchall() builds an in-memory list of rows, and since memory allocation is costly Python tends to retains memory already allocated. Try iterating over your cursor instead, ie for row in cursor: do_something_with(row).

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4 Comments

I was already writing down edit with different methods. Always the same result... +8KB
Plus gc.collect() should take care of that, shouldn't it?
I notice that in the two methods not using cursor.fetchall() the memory usage increases on the call to cursor.close()... You should perhaps 1/ set the pass statement on it's own line and 2/ add some noop statement just before the call to cursor.close(). You may also read stackoverflow.com/questions/9617001/… and stackoverflow.com/questions/1316767/… for more on the topic. Finally note that virtual memory management is a complex topic... But I assume you already know this
Ho and yes: if the memory is used by the cursor object itself, gc.collect() won't collect it as you still have a reference to it in the current namespace. Adding a del cursor between cursor.close() and gc.collect() might be a good idea here ;)

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