CODEWITHSUNDEEP
cudagpu
Farzad
Farzad

Reputation: 53

Cuda global memory load and store

So I am trying to hide global memory latency. Take the following code:

for(int i = 0; i < N; i++){
     x = global_memory[i];

     ... do some computation on x ...

     global_memory[i] = x;
}

I wanted to know whether load and store from global memory is blocking, i.e, it doesn't run next line until load or store is finished. For example take the following code:

x_next = global_memory[0];
for(int i = 0; i < N; i++){
     x = x_next;
     x_next = global_memory[i+1];

     ... do some computation on x ...

     global_memory[i] = x;
}

In this code, x_next is not used until next iteration, so does loading x_next overlap with the computation? In other words, which of the following figures will happen?

enter image description here

Upvotes: 2

Views: 387

Answers (1)

Robert Crovella
Robert Crovella

Reputation: 151869

I wanted to know whether load and store from global memory is blocking, i.e, it doesn't run next line until load or store is finished.

It is not blocking. A load operation does not stall a thread.

Note that the compiler will often seek to unroll loops (and reorder activity) to enable what you are proposing to do "manually".

But in any event your 2nd realization should allow the load of gm[1] to be issued and proceed while the computation being done on gm[0] is proceeding.

Global memory stores are also "fire and forget" -- nonblocking.

Upvotes: 3

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