Function estimate_memory_size

pub fn estimate_memory_size<T>(
    num_elements: usize,
    fixed_size: usize,
) -> Result<usize>

Estimates the memory size required for a hash table prior to allocation.

Parameters

• num_elements: The number of elements expected in the hash table.
• fixed_size: A fixed overhead size associated with the collection (e.g., HashSet or HashTable).
• T: The type of elements stored in the hash table.

Details

This function calculates the estimated memory size by considering:

• An overestimation of buckets to keep approximately 1/8 of them empty.
• The total memory size is computed as:
  • The size of each entry (T) multiplied by the estimated number of buckets.
  • One byte overhead for each bucket.
  • The fixed size overhead of the collection.
• If the estimation overflows, we return a DataFusionError

Examples

---

From within a struct

struct MyStruct<T> {
    values: Vec<T>,
    other_data: usize,
}

impl<T> MyStruct<T> {
    fn size(&self) -> Result<usize> {
        let num_elements = self.values.len();
        let fixed_size = std::mem::size_of_val(self) +
          std::mem::size_of_val(&self.values);

        estimate_memory_size::<T>(num_elements, fixed_size)
    }
}

---

With a simple collection

let num_rows = 100;
let fixed_size = std::mem::size_of::<HashMap<u64, u64>>();
let estimated_hashtable_size =
  estimate_memory_size::<(u64, u64)>(num_rows,fixed_size)
    .expect("Size estimation failed");