Flush CPU caches in C++ by iterating an array and disable compiler optimizations

Question

For benchmarking reasons, I need to run the same algorithm over the same data multiple times. Now, I want to explore the case where all the runs over the same data are on cold caches.

I was thinking of adding a for loop reading every element of an array (LOAD/MOV instruction on every element) so the cache fill with the array elements. Eg.

vector<size_t> vec(CACHE_SIZE/sizeof(size_t));
//...
//...
size_t element;
for (size_t i = 0; i < vec.size(); ++i) {
    element = vec[i];
}

But when compiled with optimization, all of this will probably be removed.

So how do I do this, possibly with minimal overhead.

Barmar · Accepted Answer · 2016-01-28 00:13:05Z

2

Declare element to be volatile. Accesses and assignments to a volatile variable are not allowed to be optimized away.

volatile size_t element;

answered Jan 28, 2016 at 0:13

Barmar

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2 Comments

dimm Over a year ago

This makes sense. It should probably generate additional LOAD/MOV to memory instruction but that seems fine. I'll check it out.

Barmar Over a year ago

vector<volatile size_t> vec might be better, but I'm not sure if this is allowed.

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