etseidl commented on code in PR #8898:
URL: https://github.com/apache/arrow-rs/pull/8898#discussion_r2547767386
##########
parquet/src/file/metadata/memory.rs:
##########
@@ -95,21 +95,16 @@ impl<K: HeapSize, V: HeapSize> HeapSize for HashMap<K, V> {
impl<T: HeapSize> HeapSize for Arc<T> {
fn heap_size(&self) -> usize {
- // Arc stores weak and strong counts on the heap alongside an instance
of T
- 2 * std::mem::size_of::<usize>() + std::mem::size_of::<T>() +
self.as_ref().heap_size()
- }
-}
-
-impl HeapSize for Arc<dyn HeapSize> {
- fn heap_size(&self) -> usize {
- 2 * std::mem::size_of::<usize>()
- + std::mem::size_of_val(self.as_ref())
- + self.as_ref().heap_size()
+ // Do not count the size of the Arc as that is accounted for by the
+ // caller (the object that contains the Arc).
+ self.as_ref().heap_size()
Review Comment:
With this change:
```rust
let v = vec![0i32; 20];
println!("filled vec heap size {}", v.heap_size());
println!("size of vec {}", std::mem::size_of::<Vec<i32>>());
let av = Arc::new(v);
println!("arc<vec> heap size {}", av.heap_size());
```
```
filled vec heap size 80
size of vec 24
arc<vec> heap size 80
```
The `Vec` is now on the heap, but that is not accounted for.
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