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| // Licensed to the Apache Software Foundation (ASF) under one | ||
| // or more contributor license agreements. See the NOTICE file | ||
| // distributed with this work for additional information | ||
| // regarding copyright ownership. The ASF licenses this file | ||
| // to you under the Apache License, Version 2.0 (the | ||
| // "License"); you may not use this file except in compliance | ||
| // with the License. You may obtain a copy of the License at | ||
| // | ||
| // http://www.apache.org/licenses/LICENSE-2.0 | ||
| // | ||
| // Unless required by applicable law or agreed to in writing, | ||
| // software distributed under the License is distributed on an | ||
| // "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY | ||
| // KIND, either express or implied. See the License for the | ||
| // specific language governing permissions and limitations | ||
| // under the License. | ||
|
|
||
| //! This module provides a function to estimate the memory size of a HashTable prior to alloaction | ||
| use crate::{DataFusionError, Result}; | ||
|
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| /// 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 | ||
| /// | ||
| /// ```rust | ||
| /// # use datafusion_common::utils::memory::estimate_memory_size; | ||
| /// # use datafusion_common::Result; | ||
| /// | ||
| /// 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 | ||
| /// | ||
| /// ```rust | ||
| /// # use datafusion_common::utils::memory::estimate_memory_size; | ||
| /// # use std::collections::HashMap; | ||
| /// | ||
| /// 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"); | ||
| /// ``` | ||
| pub fn estimate_memory_size<T>(num_elements: usize, fixed_size: usize) -> Result<usize> { | ||
| // For the majority of cases hashbrown overestimates the bucket quantity | ||
| // to keep ~1/8 of them empty. We take this factor into account by | ||
| // multiplying the number of elements with a fixed ratio of 8/7 (~1.14). | ||
| // This formula leads to overallocation for small tables (< 8 elements) | ||
| // but should be fine overall. | ||
| num_elements | ||
| .checked_mul(8) | ||
| .and_then(|overestimate| { | ||
| let estimated_buckets = (overestimate / 7).next_power_of_two(); | ||
| // + size of entry * number of buckets | ||
| // + 1 byte for each bucket | ||
| // + fixed size of collection (HashSet/HashTable) | ||
| std::mem::size_of::<T>() | ||
| .checked_mul(estimated_buckets)? | ||
| .checked_add(estimated_buckets)? | ||
| .checked_add(fixed_size) | ||
| }) | ||
| .ok_or_else(|| { | ||
| DataFusionError::Execution( | ||
| "usize overflow while estimating the number of buckets".to_string(), | ||
| ) | ||
| }) | ||
| } | ||
|
|
||
| #[cfg(test)] | ||
| mod tests { | ||
| use std::collections::HashSet; | ||
|
|
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| use super::estimate_memory_size; | ||
|
|
||
| #[test] | ||
| fn test_estimate_memory() { | ||
| // size (bytes): 48 | ||
| let fixed_size = std::mem::size_of::<HashSet<u32>>(); | ||
|
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| // estimated buckets: 16 = (8 * 8 / 7).next_power_of_two() | ||
| let num_elements = 8; | ||
| // size (bytes): 128 = 16 * 4 + 16 + 48 | ||
| let estimated = estimate_memory_size::<u32>(num_elements, fixed_size).unwrap(); | ||
| assert_eq!(estimated, 128); | ||
|
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| // estimated buckets: 64 = (40 * 8 / 7).next_power_of_two() | ||
| let num_elements = 40; | ||
| // size (bytes): 368 = 64 * 4 + 64 + 48 | ||
| let estimated = estimate_memory_size::<u32>(num_elements, fixed_size).unwrap(); | ||
| assert_eq!(estimated, 368); | ||
| } | ||
|
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| #[test] | ||
| fn test_estimate_memory_overflow() { | ||
| let num_elements = usize::MAX; | ||
| let fixed_size = std::mem::size_of::<HashSet<u32>>(); | ||
| let estimated = estimate_memory_size::<u32>(num_elements, fixed_size); | ||
|
|
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| assert!(estimated.is_err()); | ||
| } | ||
| } |
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