alamb opened a new issue, #10684: URL: https://github.com/apache/arrow-rs/issues/10684
**Is your feature request related to a problem or challenge?** - Found while testing upgrade of arrow in DataFusion: https://github.com/apache/datafusion/pull/24366 - Related to https://github.com/apache/arrow-rs/pull/10075 While updating DataFusion to arrow 60 I found the new `Metadata` struct introduced in https://github.com/apache/arrow-rs/pull/10075 does not expose its underlying reference-counted map. This hurts in two places: 1. **Memory accounting**: DataFusion tracks heap usage of plans/schemas and dedupes allocations shared via `Arc` by pointer. Since `Metadata` hides its allocation, DataFusion can only approximate its size and double-counts clones that share the same map: ```rust impl DFHeapSize for Metadata { fn heap_size(&self, ctx: &mut DFHeapSizeCtx) -> usize { // `Metadata` does not expose its underlying reference-counted map, so // this approximates the `BTreeMap` entries' sizes and cannot dedupe // instances that share the same allocation. self.iter() .map(|(k, v)| { size_of::<(String, String)>() + k.heap_size(ctx) + v.heap_size(ctx) }) .sum() } } ``` 2. **Zero-copy conversion to wrapper types**: DataFusion's `FieldMetadata` also stores an `Arc<BTreeMap<String, String>>`. The inbound direction is cheap (`Metadata: From<Arc<BTreeMap<String, String>>>`), but the outbound direction has to go through `BTreeMap`, which clones the map whenever the `Arc` is shared: ```rust impl From<Metadata> for FieldMetadata { fn from(value: Metadata) -> Self { // From<Metadata> for BTreeMap clones the map when the Arc is shared Self::new(value.into()) } } ``` **Describe the solution you'd like** Accessors that expose the shared map, for example `Metadata::as_arc(&self)` and `Metadata::into_arc(self)`. Then the conversion is always cheap: ```rust impl From<Metadata> for FieldMetadata { fn from(value: Metadata) -> Self { Self::new_from_arc(value.into_arc()) } } ``` and memory accounting can dedupe by allocation: ```rust impl DFHeapSize for Metadata { fn heap_size(&self, ctx: &mut DFHeapSizeCtx) -> usize { let map = self.as_arc(); if !ctx.count_allocation_once(Arc::as_ptr(map)) { return 0; // already counted a clone sharing this allocation } map.iter().map(|(k, v)| ...).sum() } } ``` **Describe alternatives you've considered** Approximating the size and accepting the extra clone, as shown above. **Additional context** -- This is an automated message from the Apache Git Service. To respond to the message, please log on to GitHub and use the URL above to go to the specific comment. To unsubscribe, e-mail: [email protected] For queries about this service, please contact Infrastructure at: [email protected]
