3 ms·

It appears that you didn't enable optimizations. That's needed for SIMD, which can only be taken advantage of with contiguously packed data.

prirun 4y ago
Results with -O are below. x1 (AoS) vs x2 (SoA): no performance difference x3 (arrays not in structure, both arrays in loop): slower x4 (arrays not in structure, one array in loop): faster My advice is still not to assume that SoA is always faster than AoS without benchmarking. + cc -O -o x1 x1.c + ./x1 s=1808348672 real 0m11.775s user 0m3.540s sys 0m6.592s + ./x1 s=1808348672 real 0m5.427s user 0m2.727s sys 0m2.682s + cc -O -o x2 x2.c + ./x2 s=1808348672 real 0m5.185s user 0m2.296s sys 0m2.872s + ./x2 s=1808348672 real 0m5.140s user 0m2.273s sys 0m2.852s + cc -O -o x3 x3.c + ./x3 s=1808348672 real 0m6.423s user 0m3.745s sys 0m2.660s + ./x3 s=1808348672 real 0m6.485s user 0m3.741s sys 0m2.714s + cc -O -o x4 x4.c + ./x4 s=1808348672 real 0m4.875s user 0m2.205s sys 0m2.651s + ./x4 s=1808348672 real 0m4.894s user 0m2.189s sys 0m2.684s
- Narishma 4y ago
  The reason you're not seeing much difference is that your struct is very small, just 16 bytes. In the cases where you're using both x and y in the loop (x1 and x2), you can fit 4 of them in a cache line and you're not wasting space since you need to use both. In the case you're only using one of the values (x3), you're wasting half a cache line and that shows in the benchmark. If you had a bigger struct and/or where you're not using all the members in the calculation, you'd see a much bigger difference in performance between SoA and AoS.
- rpep 4y ago
  You need to add “-march=native” to turn on SIMD at the highest level your processor will support, otherwise it will just use SSE4.1 by default on most compilers (the lowest common denominator, as all x86-64 processors have it).

prirun 4y ago

Results with -O are below. x1 (AoS) vs x2 (SoA): no performance difference x3 (arrays not in structure, both arrays in loop): slower x4 (arrays not in structure, one array in loop): faster My advice is still not to assume that SoA is always faster than AoS without benchmarking. + cc -O -o x1 x1.c + ./x1 s=1808348672 real 0m11.775s user 0m3.540s sys 0m6.592s + ./x1 s=1808348672 real 0m5.427s user 0m2.727s sys 0m2.682s + cc -O -o x2 x2.c + ./x2 s=1808348672 real 0m5.185s user 0m2.296s sys 0m2.872s + ./x2 s=1808348672 real 0m5.140s user 0m2.273s sys 0m2.852s + cc -O -o x3 x3.c + ./x3 s=1808348672 real 0m6.423s user 0m3.745s sys 0m2.660s + ./x3 s=1808348672 real 0m6.485s user 0m3.741s sys 0m2.714s + cc -O -o x4 x4.c + ./x4 s=1808348672 real 0m4.875s user 0m2.205s sys 0m2.651s + ./x4 s=1808348672 real 0m4.894s user 0m2.189s sys 0m2.684s

Narishma 4y ago
The reason you're not seeing much difference is that your struct is very small, just 16 bytes. In the cases where you're using both x and y in the loop (x1 and x2), you can fit 4 of them in a cache line and you're not wasting space since you need to use both. In the case you're only using one of the values (x3), you're wasting half a cache line and that shows in the benchmark. If you had a bigger struct and/or where you're not using all the members in the calculation, you'd see a much bigger difference in performance between SoA and AoS.
rpep 4y ago
You need to add “-march=native” to turn on SIMD at the highest level your processor will support, otherwise it will just use SSE4.1 by default on most compilers (the lowest common denominator, as all x86-64 processors have it).