BlakeOrth commented on code in PR #18855: URL: https://github.com/apache/datafusion/pull/18855#discussion_r2561961610
########## datafusion/execution/src/cache/list_files_cache.rs: ########## @@ -0,0 +1,731 @@ +// 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. + +use std::{ + sync::{Arc, Mutex}, + time::Duration, +}; + +use datafusion_common::instant::Instant; +use object_store::{path::Path, ObjectMeta}; + +use crate::cache::{cache_manager::ListFilesCache, lru_queue::LruQueue, CacheAccessor}; + +/// Default implementation of [`ListFilesCache`] +/// +/// Caches file metadata for file listing operations. +/// +/// # Internal details +/// +/// The `memory_limit` parameter controls the maximum size of the cache, which uses a Least +/// Recently Used eviction algorithm. When adding a new entry, if the total number of entries in +/// the cache exceeds `memory_limit`, the least recently used entries are evicted until the total +/// size is lower than the `memory_limit`. +/// +/// # `Extra` Handling +/// +/// Users should use the [`Self::get`] and [`Self::put`] methods. The +/// [`Self::get_with_extra`] and [`Self::put_with_extra`] methods simply call +/// `get` and `put`, respectively. +#[derive(Default)] +pub struct DefaultListFilesCache { + state: Mutex<DefaultListFilesCacheState>, +} + +impl DefaultListFilesCache { + /// Creates a new instance of [`DefaultListFilesCache`]. + /// + /// # Arguments + /// * `memory_limit` - The maximum size of the cache, in bytes. + /// * `ttl` - The TTL (time-to-live) of entries in the cache. + pub fn new(memory_limit: usize, ttl: Option<Duration>) -> Self { + Self { + state: Mutex::new(DefaultListFilesCacheState::new(memory_limit, ttl)), + } + } + + /// Returns the cache's memory limit in bytes. + pub fn cache_limit(&self) -> usize { + self.state.lock().unwrap().memory_limit + } + + /// Updates the cache with a new memory limit in bytes. + pub fn update_cache_limit(&self, limit: usize) { + let mut state = self.state.lock().unwrap(); + state.memory_limit = limit; + state.evict_entries(); + } + + /// Returns the TTL (time-to-live) applied to cache entries. + pub fn cache_ttl(&self) -> Option<Duration> { + self.state.lock().unwrap().ttl + } +} + +struct ListFilesEntry { + metas: Arc<Vec<ObjectMeta>>, + size_bytes: usize, + expires: Option<Instant>, +} + +impl ListFilesEntry { + fn try_new(metas: Arc<Vec<ObjectMeta>>, ttl: Option<Duration>) -> Option<Self> { + let size_bytes = (metas.capacity() * size_of::<ObjectMeta>()) + + metas.iter().map(meta_heap_bytes).reduce(|acc, b| acc + b)?; + + Some(Self { + metas, + size_bytes, + expires: ttl.map(|t| Instant::now() + t), + }) + } +} + +/// Calculates the number of bytes an [`ObjectMeta`] occupies in the heap. +fn meta_heap_bytes(object_meta: &ObjectMeta) -> usize { + let mut size = object_meta.location.as_ref().len(); + + if let Some(e) = &object_meta.e_tag { + size += e.len(); + } + if let Some(v) = &object_meta.version { + size += v.len(); + } + + size +} + +/// The default memory limit for the [`DefaultListFilesCache`] +pub(super) const DEFAULT_LIST_FILES_CACHE_MEMORY_LIMIT: usize = 1024 * 1024; // 1MiB + +/// Handles the inner state of the [`DefaultListFilesCache`] struct. +pub struct DefaultListFilesCacheState { + lru_queue: LruQueue<Path, ListFilesEntry>, + memory_limit: usize, + memory_used: usize, + ttl: Option<Duration>, +} + +impl Default for DefaultListFilesCacheState { + fn default() -> Self { + Self { + lru_queue: LruQueue::new(), + memory_limit: DEFAULT_LIST_FILES_CACHE_MEMORY_LIMIT, + memory_used: 0, + ttl: None, + } + } +} + +impl DefaultListFilesCacheState { + fn new(memory_limit: usize, ttl: Option<Duration>) -> Self { + Self { + lru_queue: LruQueue::new(), + memory_limit, + memory_used: 0, + ttl, + } + } + + /// Returns the respective entry from the cache, if it exists and the entry has not expired. + /// If the entry exists it becomes the most recently used. If the entry has expired it is + /// removed from the cache + fn get(&mut self, key: &Path) -> Option<Arc<Vec<ObjectMeta>>> { + let entry = self.lru_queue.get(key)?; + + match entry.expires { + Some(exp) if Instant::now() > exp => { + self.remove(key); + None + } + _ => Some(Arc::clone(&entry.metas)), + } + } + + /// Checks if the respective entry is currently cached. If the entry has expired it is removed + /// from the cache. + /// The LRU queue is not updated. + fn contains_key(&mut self, k: &Path) -> bool { + let Some(entry) = self.lru_queue.peek(k) else { + return false; + }; + + match entry.expires { + Some(exp) if Instant::now() > exp => { + self.remove(k); + false + } + _ => true, + } + } + + /// Adds a new key-value pair to cache, meaning LRU entries might be evicted if required. + /// If the key is already in the cache, the previous entry is returned. + /// If the size of the entry is greater than the `memory_limit`, the value is not inserted. + fn put( + &mut self, + key: &Path, + value: Arc<Vec<ObjectMeta>>, + ) -> Option<Arc<Vec<ObjectMeta>>> { + let entry = ListFilesEntry::try_new(value, self.ttl)?; + let entry_size = entry.size_bytes; + + // no point in trying to add this value to the cache if it cannot fit entirely + if entry_size > self.memory_limit { + return None; + } + + // if the key is already in the cache, the old value is removed + let old_value = self.lru_queue.put(key.clone(), entry); + self.memory_used += entry_size; + + if let Some(entry) = &old_value { + self.memory_used -= entry.size_bytes; + } + + self.evict_entries(); + + old_value.map(|v| v.metas) + } + + /// Evicts entries from the LRU cache until `memory_used` is lower than `memory_limit`. + fn evict_entries(&mut self) { + while self.memory_used > self.memory_limit { + if let Some(removed) = self.lru_queue.pop() { + self.memory_used -= removed.1.size_bytes; + } else { + // cache is empty while memory_used > memory_limit, cannot happen + debug_assert!( + false, + "cache is empty while memory_used > memory_limit, cannot happen" + ); + return; + } + } + } + + /// Removes an entry from the cache and returns it, if it exists. + fn remove(&mut self, k: &Path) -> Option<Arc<Vec<ObjectMeta>>> { + if let Some(entry) = self.lru_queue.remove(k) { + self.memory_used -= entry.size_bytes; + Some(entry.metas) + } else { + None + } + } + + /// Returns the number of entries currently cached. + fn len(&self) -> usize { + self.lru_queue.len() + } + + /// Removes all entries from the cache. + fn clear(&mut self) { + self.lru_queue.clear(); + self.memory_used = 0; + } +} + +impl ListFilesCache for DefaultListFilesCache { + fn cache_limit(&self) -> usize { + let state = self.state.lock().unwrap(); + state.memory_limit + } + + fn cache_ttl(&self) -> Option<Duration> { + let state = self.state.lock().unwrap(); + state.ttl + } + + fn update_cache_limit(&self, limit: usize) { + let mut state = self.state.lock().unwrap(); + state.memory_limit = limit; + state.evict_entries(); + } +} + +impl CacheAccessor<Path, Arc<Vec<ObjectMeta>>> for DefaultListFilesCache { + type Extra = ObjectMeta; + + fn get(&self, k: &Path) -> Option<Arc<Vec<ObjectMeta>>> { + let mut state = self.state.lock().unwrap(); + state.get(k) + } + + fn get_with_extra(&self, k: &Path, _e: &Self::Extra) -> Option<Arc<Vec<ObjectMeta>>> { + self.get(k) + } Review Comment: Looking forward, I'm wondering if these methods can end up supporting this cache's ability to be "prefix aware". Maybe it makes sense to have `type Extra = Path;` where `k: &Path` is the table path and `e: &Self::Extra` is the prefix. That way the cache can internally differentiate between entries that are just table list entries vs entries that have prefixes. -- 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] --------------------------------------------------------------------- To unsubscribe, e-mail: [email protected] For additional commands, e-mail: [email protected]
