I recently found out that bits in DRAMs can be randomly flipped by the decay of particles in it or by cosmic rays. And I wondered how often this errors occur.
Unfortunately, the most recent statistic I found is from the year 1990 (source), which states that an error should occur every month per 128MB of memory.
Since I couldn't find any recent statistic of soft error rates in modern RAMs, I tried to write a program in java to measure the soft error frequency on 4GB of my RAM. I expect the program to work to detect every soft error in the allocated 4GB of RAM, if it weren't optimized in any way.
The problem is, I have no idea how to check if the program works (I assume it doesn't because of optimization), and I don't know how to change it to work as expected.
Based on the 1990's statistic I should expect to detect an error every 22 hours, thus I would need to run the program for almost a week to state with a 99% confidence that it works. Assuming that modern hardware doesn't have a better soft error rate than in the 90s.
The following loop is the most important part of my program:
int[] memory = new int[1_073_741_824]; // 4GB array, each value initialized to 0
while (true) {
for (int i = 0; i < memory.length; i++) {
if (memory[i] != 0) {
// soft error found
memory[i] = 0;
// print information about the error in a log file
}
}
// Sleep for a minute
}
What can I do to avoid optimization to break the intended use of the program?
P.S. If you think that my code wouldn't even work without optimization, please explain why.
Simple: as soon as your loop body does something (like just printing the index it failed on) , optimizing away the loop body would be invalid!
And if at all, it would be the JIT optimizing away things, javac is not doing much more than constant folding in regard of optimizations.
But of course, you would be on the safe side using languages where you have full control over such things.
Beyond that: I rather doubt that you will ever hit an error with such code.