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submitting cpu-bound tasks to processes and io-bound tasks to threads

Project description

Convert a classic sequential program into a parallel one.

Why?

It runs faster.

What if not?

Don’t use it.

How?

for image in images:
    create_thumbnail(image)       # original

for image in images:
    fork(create_thumbnail, image) # parallelized explicitly

for image in images:
    create_thumbnail(image)       # parallelized implicitly (read below)

What about return values?

result = fork(my_func, *args, **kwargs)

That is a proxy object that behaves almost exactly as if it were the real return value of my_func.

Furthermore, result proxies evaluate lazily, e.g. operators (like +, - etc.) evaluate until really needed.

What about exceptions?

Relax.

In case of an error, you will receive the normal traceback that you would see in the sequential case.

Speaking of threads …

and processes? fork will take care of that for you.

You can assist fork by decorating your functions (not decorating defaults to cpu_bound):

@io_bound
def call_remote_webservice():
    # implementation

@cpu_bound
def fib(n):
    # naive implementation of Fibonacci numbers

@unsafe # don't fork; run sequentially
def weird_side_effects(*args, **kwargs):
    # implementation

Parallelize implicitly?

If you don’t like the fork calling syntax, you can convert certain functions into forks.

Use with caution.

@io_bound_fork
def create_thumbnail_by_webservice(image):
    # implementation

@cpu_bound_fork
def create_thumbnail_by_bare_processing_power(image):
    # implementation

# the following two lines spawn two forks
create_thumbnail_by_webservice(image1)
create_thumbnail_by_bare_processing_power(image2)

Conclusion

Good

  • easy way back and forth (from sequential to parallel and vice versa)
  • results evaluate lazily
  • tracebacks are preserved
  • cascading possible (thread-safe)
  • compatible with Python 2 and 3

Bad

  • weird calling syntax (no syntax support)
  • type(result) == ResultProxy
  • not working with lambdas due to PickleError
  • needs fix:
    • “maximum recursion depth exceeded” due to encapsulating all operations into proxies
    • not working with coroutines (asyncio) yet

Project details


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