Douglas Gregor ha escrito:

Hello,

I'm working on a project where we're watching memory usage very closely. We have several multi_index_containers around (they are *very* useful), and we would like to measure the memory overhead of these containers. Is there a way to determine the per-item overhead of a multi_index_container?

Hi Doug, I'm glad you're finding the lib useful. Memory consumption of multi_index_containers is briefly explained at http://boost.org/libs/multi_index/doc/performance.html#spatial_efficiency while the program http://boost.org/libs/multi_index/perf/test_perf.cpp performs some actual calculations on 1- to 3-index containers (look for multi_index_node_size.) Basically, a node of an N-index multi_index_container holds the value_type object plus N headers, one for each index, whose contents are as follows: * Ordered indices: three pointers when http://tinyurl.com/yp8h29 applies, three pointers plus a 2-valued enum otherwise. * Hashed indices: a pointer. * Sequenced indices: two pointers. * Random access indices: one pointer. Aditionally, some types of indices maintain their own bookkeeping data structures which add to the memory consumption of the container: * Hashed indices: A bucket table containing bucket_count() pointers. * Random access indices: An internal array of capacity() pointers. So, the per-item overhead of each index of a multi_index_container with n elements is (in sizeof(pointer) units): * Ordered: 3 * Hashed: 1+1,33/max_load_factor() (assuming the typical occupation of a hash table is 0.5(1+1/GF) where GF is the table growth factor, in our case ~2) * Sequenced: 2 * Random access: 1+0,83 (again assuming an occupation with respect to capacity of 0.5(1+1/GF), but thes indices use GF=1.5) In real life, you could in principle have padding adding to the size of a multi_index_container node, though I haven't seen them myself as all the members involved are pointers with the same alignment requirements. You can measure yourself the way multi_index_node_size --referred to above-- does. HTH, Joaquín M López Muñoz Telefónica, Investigación y Desarrollo