What happens when a GPU writes memory

In one line

Following an STG.E SASS instruction through the hardware units in an RTX 4090.

Opening excerpt

In our previous post we followed one LDG.E down through all the hardware units on an RTX 4090 — through its L1, translation, through the crossbar to its L2 slice, and thence to DRAM. The request retrieved its result, and then came back up through its waystations, returning its result to its warp, which then continued to execute its part in the kernel.

The part it was playing was in a kernel that performed a vector add. The same kernel, once it has loaded elements of both vectors, adds them together, and then stores the result.

/00c0/ IMAD.WIDE R6 , R6 , R7 , c [0x0][0x170] ; // &c[i] /00d0/ FADD R9 , R4 , R3 ; // a[i] + b[i] /00e0/ STG.E [R6.64] , R9 ; // c[i] = … /00f0/ EXIT ; STG.E is the instruction that’s responsible for writing the calculated sum back to global memory. In this post we’re going to follow STG.E through the same waystations, figuring out what happens at each step. As before, this information is not all publicly available; where it isn’t, we’ll run new experiments.

(Excerpted from the original; full article via the source link below.)

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