python, how to incrementally create Threads

Question

I have a list of items aprox 60,000 items - i would like to send queries to the database to check if they exist and if they do return some computed results. I run an ordinary query, while iterating through the list one-by-one, the query has been running for the last 4 days. I thought i could use the threading module to improve on this. I did something like this

if __name__ == '__main__':
  for ra, dec in candidates:
    t = threading.Thread(target=search_sl, args=(ra,dec, q))
    t.start()
  t.join()

I tested with only 10 items and it worked fine - when i submitted the whole list of 60k items, i run into errors i.e, "maximum number of sessions exceeded". What I want to do is to create maybe 10 thread at a time. When the 1st bunch of thread have finished excuting, i send another request and so on.

Answer 1

You could try using a process pool, which is available in the multiprocessing module. Here is the example from the python docs:

from multiprocessing import Pool

def f(x):
    return x*x

if __name__ == '__main__':
    pool = Pool(processes=4)              # start 4 worker processes
    result = pool.apply_async(f, [10])    # evaluate "f(10)" asynchronously
    print result.get(timeout=1)           # prints "100" unless your computer is *very* slow
    print pool.map(f, range(10))          # prints "[0, 1, 4,..., 81]"

http://docs.python.org/library/multiprocessing.html#using-a-pool-of-workers

Try increasing the number of processes until you reach the maximum your system can support.