Estimating hashmap occupied size

[spektom]

Is there a way to estimate how much memory is occupied by a hashmap instance?

[Amanieu]

Not really. capacity only reports an estimate of how many more elements you can insert before having to resize, but it doesn't tell you how much memory is actually used by the table.

[tgross35]

Is there a good formula to estimate this? I would assume something like:

// HashMap
size_of::<HashMap>() + (size_of::<KeyType>() + size_of::<ValueType>() + size_of::<u64>()) * my_map.capacity()
// HashSet
size_of::<HashSet>() + (size_of::<ValueType>() + size_of::<u64>()) * my_set.capacity()

But I am unsure if there is any extra per-element overhead, if the hash size is stored as u64, or if any of those items are dependent on .len() rather than .capacity().

Also curious how this formula compares with std::collections::HashMap/std::collections::HashSet

[Imberflur]

There is a RawTable::allocation_info method that seems like it would work for this: https://docs.rs/hashbrown/0.14.5/hashbrown/raw/struct.RawTable.html#method.allocation_info