Re: [PR] perf: optimize levenshtein function [datafusion-comet]

["sunchao (via GitHub)" <gi...@apache.org>]

sunchao commented on code in PR #5042:
URL: https://github.com/apache/datafusion-comet/pull/5042#discussion_r3837586414


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native/spark-expr/src/string_funcs/levenshtein.rs:
##########
@@ -26,67 +26,188 @@ use datafusion::common::{cast::as_generic_string_array, DataFusionError, Result}
 use datafusion::physical_plan::ColumnarValue;
 use std::sync::Arc;
 
+// Thread-local scratch buffers to avoid heap allocations in the row processing loop
+thread_local! {
+    static LEVENSHTEIN_SCRATCH: std::cell::RefCell<(Vec<i32>, Vec<i32>)> =
+        std::cell::RefCell::new((Vec::with_capacity(64), Vec::with_capacity(64)));

Review Comment:
   [P2] Bound the retained capacity of the scratch vectors
   
   Could you cap the cached capacity or keep these buffers at batch/task scope? `Vec::resize` reduces the length but never releases capacity, so moving the vectors into TLS retains the largest allocation until the worker thread exits, including after the query finishes. I ran the exact-head kernels with two equal 16,000,000-character ASCII strings and threshold 0, then dropped the input and evaluated a one-character pair. The two vectors still retained 128,000,008 bytes after that small call. These are valid inputs and the thresholded case completes in linear time. Comet's long-lived worker threads can therefore retain this allocation independently, outside the query memory pool. The previous call-local vectors released it on return.



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