Starting n number of threads from a loop-CodePudding

So basically, I've this function th() which counts till certain number and then prints "done". I'd want to start n number of such threads at the same time, running simultaneously. So I wrote:

thread_num = 3 #here n is 3, but I'd normally want something way higher

thrds = []

i = 0
while i < thread_num:
    thr = Thread(target=th, args=())
    thrds.append(thr)
    i  = 1
    print("thread", str(i), "added")

for t in thrds:
    t.start()
    t.join()

I'd want all the threads to print "done" at the same time, but they have a noticeable lag in between of them. They print "thread i started" at seemingly the same time, but print "done" with quite a bit of time lag. Why is this happening?

CodePudding user response：

This is happening because of the t.join() method that you are calling on each thread before start the next one. t.join() blocks the execution of the current thread until the thread t has completed execution. So, each thread is starting after the previous one has finished.

CodePudding user response：

because you are launching them sequentially

CodePudding user response：

Because of memory management and object reference count issues, python only lets a single thread execute byte code at a time. Periodically, each thread will release and reacquire the Global Interpreter Lock (GIL) to let other threads run. Exactly which thread runs at any given time is up to the operating system and you may find one gets more slices than another, causing staggered results.

To get them all to print "done" at the same time, you could use a control structure like a barrier for threads to wait until all are done. With a barrier, all threads must call wait before any can continue.

thread_num = 3 #here n is 3, but I'd normally want something way higher
wait_done = threading.Barrier(thread_num)

def th(waiter):
    x = 1 # to what you want
    waiter.wait()
    print("done")

thrds = []

i = 0
while i < thread_num:
    thr = Thread(target=th, args=(wait_done,))
    thrds.append(thr)
    i  = 1
    print("thread", str(i), "added")

for t in thrds:
    t.start()
    t.join()