Understanding the ray.get() method

Ray Core
1

Hi everyone, I am fairly new to Ray and I am struggling to understand what the ray.get() function actually does. I found a small example online here that can help.

If I run the code below I get:

duration = 15.932732820510864 
sum_prime =  9914236193

If I change the result = ray.get(prime.SumPrime.remote(num)) to result = prime.SumPrime.remote(num)

I get:

duration = 0.0009586811065673828 
sum_prime =  ObjectRef(76385d6aec5a2d681682b671b29cf0241608aada0100000001000000)

I am unsure as to why it takes so long when I us the ray.get() method. I use this in my code and so I am seeing no improvement from using ray. I guess I am using it incorrectly?
I would appreciate any advise.

@ray.remote
class Prime:
    # Constructor
    def __init__(self,number) :
       self.num = number

    def SumPrime(self,num) :
       tot = 0
       for i in range(3,num):
           c = 0
           for j in range(2, int(i**0.5)+1):
                if i%j == 0:
                    c = c + 1
           if c == 0:
              tot = tot + i
       return tot
            
num = 500000
prime = Prime.remote(num)

start = time.time()
result = ray.get(prime.SumPrime.remote(num))
print("duration =", time.time() - start, "\nsum_prime = ",result)
2

prime.SumPrime.remote(num) only launches tasks and return future immediately. ray.get actually waits until computation is finished

3

I am unsure as to why it takes so long when I us the ray.get() method. I use this in my code and so I am seeing no improvement from using ray

Also @ray.remote just runs your method on a remote process, it does not speed up execution by itself. If you use @ray.remote to distribute a number of methods to different processes, you will get speed up due to parallelism.