How to calculate the size of the hashbrown::HashMap at runtime?

[iddm]

I need to calculate how much memory the hashmap object with all the contents occupies. How could I do that?

[cuviper]

I doubt it will ever be exposed, but for a given version you can inspect the source. The immediate size is just std::mem::size_of::<HashMap<K, V>>(), and the heap size comes from the computed layout here:

hashbrown/src/raw/mod.rs

Lines 233 to 276 in 3741813

	/// Helper which allows the max calculation for ctrl_align to be statically computed for each T
	/// while keeping the rest of `calculate_layout_for` independent of `T`
	#[derive(Copy, Clone)]
	struct TableLayout {
	size: usize,
	ctrl_align: usize,
	}
	impl TableLayout {
	#[inline]
	const fn new<T>() -> Self {
	let layout = Layout::new::<T>();
	Self {
	size: layout.size(),
	ctrl_align: if layout.align() > Group::WIDTH {
	layout.align()
	} else {
	Group::WIDTH
	},
	}
	}
	#[inline]
	fn calculate_layout_for(self, buckets: usize) -> Option<(Layout, usize)> {
	debug_assert!(buckets.is_power_of_two());
	let TableLayout { size, ctrl_align } = self;
	// Manual layout calculation since Layout methods are not yet stable.
	let ctrl_offset =
	size.checked_mul(buckets)?.checked_add(ctrl_align - 1)? & !(ctrl_align - 1);
	let len = ctrl_offset.checked_add(buckets + Group::WIDTH)?;
	// We need an additional check to ensure that the allocation doesn't
	// exceed `isize::MAX` (https://github.com/rust-lang/rust/pull/95295).
	if len > isize::MAX as usize - (ctrl_align - 1) {
	return None;
	}
	Some((
	unsafe { Layout::from_size_align_unchecked(len, ctrl_align) },
	ctrl_offset,
	))
	}
	}

... where T = (K, V) for a HashMap<K, V>, and buckets can be determined from the reported capacity like:

hashbrown/src/raw/mod.rs

Lines 188 to 217 in 3741813

	/// Returns the number of buckets needed to hold the given number of items,
	/// taking the maximum load factor into account.
	///
	/// Returns `None` if an overflow occurs.
	// Workaround for emscripten bug emscripten-core/emscripten-fastcomp#258
	#[cfg_attr(target_os = "emscripten", inline(never))]
	#[cfg_attr(not(target_os = "emscripten"), inline)]
	fn capacity_to_buckets(cap: usize) -> Option<usize> {
	debug_assert_ne!(cap, 0);
	// For small tables we require at least 1 empty bucket so that lookups are
	// guaranteed to terminate if an element doesn't exist in the table.
	if cap < 8 {
	// We don't bother with a table size of 2 buckets since that can only
	// hold a single element. Instead we skip directly to a 4 bucket table
	// which can hold 3 elements.
	return Some(if cap < 4 { 4 } else { 8 });
	}
	// Otherwise require 1/8 buckets to be empty (87.5% load)
	//
	// Be careful when modifying this, calculate_layout relies on the
	// overflow check here.
	let adjusted_cap = cap.checked_mul(8)? / 7;
	// Any overflows will have been caught by the checked_mul. Also, any
	// rounding errors from the division above will be cleaned up by
	// next_power_of_two (which can't overflow because of the previous division).
	Some(adjusted_cap.next_power_of_two())
	}

All of that is subject to change, and of course if your K or V types have additional indirect memory then you'll need to account for that too.

[cuviper]

I missed RawTable::allocation_info when I answered before, and you can get access to that through HashMap::raw_table. However, the "raw" feature has been removed in #546 for the upcoming 0.15.