highfestiva / volatile_test

A tiny test for checking volatile int contention between CPUs

volatile_test

A tiny and simple test (written in C++) for volatile int contention between CPUs. An atomic_int is used as a synchronization primitive.

Two threads simultaneously loop ten times and try to increase the volatile counter, and race conditions ensue. Sometimes the final count will be 20, but often it will be less.

ARM results are a lot worse. I assume that has to do with snooping without fences works pretty well on x64, but must be extremely complicated to develop. ARM probably opted for a simpler solution.

Example output on Intel i7-12850HX, 2100 Mhz, 16 cores, 24 CPUs

$ ./build-and-run.sh
result  hits
0       0
1       0
2       0
3       1
4       2
5       6
6       10
7       8
8       9
9       14
10      3049181
11      176292
12      438441
13      2261718
14      2438862
15      43057
16      23614
17      296018
18      477901
19      351906
20      442960

Example output on Amazon Graviton 2 ARM Neoverse-N1 aarch64 2.5 GHz, 4 cores, 4 CPUs:

$ ./build-and-run.sh
result  hits
0       0
1       0
2       0
3       0
4       0
5       0
6       0
7       0
8       0
9       0
10      9999258
11      0
12      0
13      0
14      1
15      0
16      0
17      0
18      1
19      1
20      739