std::deque memory usage - Visual C++, and comparison to others

Question

Follow up to What the heque is going on with the memory overhead of std::deque?

Visual C++ manages deque blocks according to the container element type using this:

#define _DEQUESIZ   (sizeof (value_type) <= 1 ? 16 \
    : sizeof (value_type) <= 2 ? 8 \
    : sizeof (value_type) <= 4 ? 4 \
    : sizeof (value_type) <= 8 ? 2 \
    : 1)    /* elements per block (a power of 2) */

This results in very large memory footprint for small elements. By changing the 16 in the first line to 128 I was able to drastically reduce the footprint required for a large deque<char>. Process Explorer Private Bytes dropped from 181MB -> 113MB after 100m push_back(const char& mychar) calls).

• Can anybody justify the values in that #define?
• How do other compilers handle deque block sizing?
• What would be their footprint (32-bit operation) for the simple test of 100m push_back calls to deque<char>?
• Does STL allow for overriding of this block size at compile-time, without modifying the <deque> code?

Answer 1

gcc has

return __size < 512 ? size_t(512 / __size) : size_t(1);

with a comment

/*  The '512' is
 *  tunable (and no other code needs to change), but no investigation has
 *  been done since inheriting the SGI code.
 */