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Parallelism and Multithreading

Seq supports parallelism and multithreading via OpenMP out of the box. Here’s an example:

@par
for i in range(10):
    import threading as thr
    print('hello from thread', thr.get_ident())

By default, parallel loops will use all available threads, or use the number of threads specified by the OMP_NUM_THREADS environment variable. A specific thread number can be given directly on the @par line as well:

@par(num_threads=5)
for i in range(10):
    import threading as thr
    print('hello from thread', thr.get_ident())

@par supports several OpenMP parameters, including:

  • num_threads (int): the number of threads to use when running the loop

  • schedule (str): either static, dynamic, guided, auto or runtime

  • chunk_size (int): chunk size when partitioning loop iterations

  • ordered (bool): whether the loop iterations should be executed in the same order

Other OpenMP parameters like private, shared or reduction, are inferred automatically by the compiler. For example, the following loop

total = 0
@par
for i in range(N):
    a += foo(i)

will automatically generate a reduction for variable a.

Here is an example that finds the sum of prime numbers up to a user-defined limit, using a parallel loop on 16 threads with a dynamic schedule and chunk size of 100:

from sys import argv

def is_prime(n):
    factors = 0
    for i in range(2, n):
        if n % i == 0:
            factors += 1
    return factors == 0

limit = int(argv[1])
total = 0

@par(schedule='dynamic', chunk_size=100, num_threads=16)
for i in range(2, limit):
    if is_prime(i):
        total += 1

print(total)

Static schedules work best when each loop iteration takes roughly the same amount of time, whereas dynamic schedules are superior when each iteration varies in duration. Since counting the factors of an integer takes more time for larger integers, we use a dynamic schedule here.

@par also supports C/C++ OpenMP pragma strings. For example, the @par line in the above example can also be written as:

# same as: @par(schedule='dynamic', chunk_size=100, num_threads=16)
@par('schedule(dynamic, 100) num_threads(16)')

Different kinds of loops

for-loops can iterate over arbitrary generators, but OpenMP’s parallel loop construct only applies to imperative for-loops of the form for i in range(a, b, c) (where c is constant). For general parallel for-loops of the form for i in some_generator(), a task-based approach is used instead, where each loop iteration is executed as an independent task.

The Seq compiler also converts iterations over lists (for a in some_list) to imperative for-loops, meaning these loops can be executed using OpenMP’s loop parallelism.

Custom reductions

Seq can automatically generate efficient reductions for int and float values. For other data types, user-defined reductions can be specified. A class that supports reductions must include:

  • A default constructor that represents the zero value

  • An __add__ method (assuming + is used as the reduction operator)

Here is an example for reducing a new Vector type:

@tuple
class Vector:
    x: int
    y: int

    def __new__():
        return Vector(0, 0)

    def __add__(self, other: Vector):
        return Vector(self.x + other.x, self.y + other.y)

v = Vector()
@par
for i in range(100):
    v += Vector(i,i)
print(v)  # (x: 4950, y: 4950)

OpenMP constructs

All of OpenMP’s API functions are accessible directly in Seq. For example:

import openmp as omp
print(omp.get_num_threads())
omp.set_num_threads(32)

OpenMP’s critical, master, single and ordered constructs can be applied via the corresponding decorators:

import openmp as omp

@omp.critical
def only_run_by_one_thread_at_a_time():
    print('critical!', omp.get_thread_num())

@omp.master
def only_run_by_master_thread():
    print('master!', omp.get_thread_num())

@omp.single
def only_run_by_single_thread():
    print('single!', omp.get_thread_num())

@omp.ordered
def run_ordered_by_iteration(i):
    print('ordered!', i)

@par(ordered=True)
for i in range(100):
    only_run_by_one_thread_at_a_time()
    only_run_by_master_thread()
    only_run_by_single_thread()
    run_ordered_by_iteration(i)

For finer-grained locking, consider using the locks from the threading module:

from threading import Lock
lock = Lock()  # or RLock for re-entrant lock

@par
for i in range(100):
    with lock:
        print('only one thread at a time allowed here')