Measuring memory bandwidth from the dot product of two arrays

Question

The dot product of two arrays

for(int i=0; i

does not reuse data so it should be a memory bound opera

Answer 1

I made my own memory benchmark code https://github.com/zboson/bandwidth

Here are the current results for eight threads:

write:    0.5 GB, time 2.96e-01 s, 18.11 GB/s
copy:       1 GB, time 4.50e-01 s, 23.85 GB/s
scale:      1 GB, time 4.50e-01 s, 23.85 GB/s
add:      1.5 GB, time 6.59e-01 s, 24.45 GB/s
mul:      1.5 GB, time 6.56e-01 s, 24.57 GB/s
triad:    1.5 GB, time 6.61e-01 s, 24.37 GB/s
vsum:     0.5 GB, time 1.49e-01 s, 36.09 GB/s, sum -8.986818e+03
vmul:     0.5 GB, time 9.00e-05 s, 59635.10 GB/s, sum 0.000000e+00
vmul_sum:   1 GB, time 3.25e-01 s, 33.06 GB/s, sum 1.910421e+04

Here are the currents results for 1 thread:

write:    0.5 GB, time 4.65e-01 s, 11.54 GB/s
copy:       1 GB, time 7.51e-01 s, 14.30 GB/s
scale:      1 GB, time 7.45e-01 s, 14.41 GB/s
add:      1.5 GB, time 1.02e+00 s, 15.80 GB/s
mul:      1.5 GB, time 1.07e+00 s, 15.08 GB/s
triad:    1.5 GB, time 1.02e+00 s, 15.76 GB/s
vsum:     0.5 GB, time 2.78e-01 s, 19.29 GB/s, sum -8.990941e+03
vmul:     0.5 GB, time 1.15e-05 s, 468719.08 GB/s, sum 0.000000e+00
vmul_sum:   1 GB, time 5.72e-01 s, 18.78 GB/s, sum 1.910549e+04

1. write: writes a constant (3.14159) to an array. This should be like memset.
2. copy, scale, add, and triad are defined the same as in STREAM
3. mul: a(i) = b(i) * c(i)
4. vsum: sum += a(i)
5. vmul: sum *= a(i)
6. vmul_sum: sum += a(i)*b(i) // the dot product

My results are consistent with STREAM. I get the highest bandwidth for vsum. The vmul method does not work currently (once the value is zero it finishes early). I can get slightly better results (by about 10%) using intrinsics and unrolling the loop which I will add later.