Python's Multiprocess Pool Memory Leak

Update : I investigated further to the problem, and found a better solution. I will write a topic on that.

Actually, I don't know how this happen, just can't narrow it down enough to a pin point.

The situation can be described here:

# no globals

def ...

def ...

def ...

def trigger():  
    pool = Pool()
    for each in pool.imap_unordered(memory_consuming_fn, some_list):

# run the trigger() function many times

Note : In my case the memory_cosuming_fn involves opening many large files, this could be the case (but I personally don't think so because the running context is separated from the main process, and is essentially freed up every time without fail).

Note2: For those who aren't familiar with multiprocess, it's essentially important to make sure that the global variables are not huge because it will be forked and duplicated ! esp. under Python 2. Moreover, it's much better if you keep the globals immutable.

The point is to run trigger() many times, may be in this fashion:

for i in range(10):  

After each run, not all memory are freed, more or less, so, I assume that memory leakage is encountered.

I strayed trying to make small changes to the code involving putting many gc.collect() to multiple places, still no luck.

Here are some of my unfruitful experiments:

  1. Making sure that a new process is spawn for each job, pool = Pool(maxtasksperchild=1).
  2. Put gc.collect() to the beginning of the memory_consuming_fn.
  3. Try deallocate large vars by <var> = None or del <var> and then gc.collect().

And, here are some working experiments:

A. Declaring an initializer:

def pool_init():  
    import gc

pool = Pool(initializer=pool_init)  

B. Put gc.collect() before declaring pool:

import gc  
pool = Pool()  

Note: both are not the same thing !

If initializer is not None then each worker process will call initializer(*initargs) when it starts.

That means the A. runs gc.collect() in the context of pool processes not the main process as the B. would.

And this makes the A. to be kinda same of the 2. and the 1. ? Don't know why both of those don't work.

I don't know specifically why all the memory leak is gone, in part, because I don't really know where the leakages are. But, it's likely that the memory leakage is somewhere (very tight) before spawning the pool processes. This might be some of the globals (if there is any) or the parameters of the memory_consuming_fn itself.

Konpat Preechakul

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