Set the segment size in the segmented sieve of Eratosthenes

segmented_sieve1.cpp

/// Set your CPU's L1 data cache size (in bytes) here
const int64_t L1D_CACHE_SIZE = 32768;

/// Generate primes using the segmented sieve of Eratosthenes.
/// This algorithm uses O(n log log n) operations and O(sqrt(n)) space.
/// @param limit  Sieve primes <= limit.
///
void segmented_sieve(int64_t limit)
{
  int64_t sqrt = (int64_t) std::sqrt(limit);
  int64_t segment_size = std::max(sqrt, L1D_CACHE_SIZE);