comphead commented on code in PR #7082: URL: https://github.com/apache/datafusion-comet/pull/7082#discussion_r4338821008
########## native/core/src/parquet/shared_metadata_cache.rs: ########## @@ -0,0 +1,324 @@ +// 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. + +//! A registry that lets the tasks of one Spark stage share a Parquet `FileMetadataCache`. +//! +//! Comet builds a `RuntimeEnv`, and with it a metadata cache, for every Spark task, so two tasks +//! that read splits of the same file each fetch and parse its footer and page index. This +//! registry hands every task of a stage on this executor the same cache instead. +//! +//! The scope is the stage, not the executor, on purpose. A cache that outlives the query would +//! answer a later query from metadata of a file that has since been deleted or rewritten, and +//! `(size, modification time)` validation cannot catch that: Spark's cached file index lists the +//! same size and time as the cached footer. Spark's `HiveMetadataCacheSuite` reads a table, +//! deletes one data file, reads again, and expects `FILE_NOT_EXIST`. Every new query is a new +//! stage, so it opens its files and sees the deletion, as it does without the registry. +//! +//! A stage attempt is part of the key because a retried stage may read a changed file. So is the +//! Spark application: stage ids restart at zero in every `SparkContext`, and this registry is +//! process-wide, so a context started after another one stopped in the same JVM must not match +//! the stages the old one left behind. +//! +//! The registry holds a weak reference to the cache of every stage with a running task, since the +//! tasks own it through their `RuntimeEnv`. It additionally holds strong references to the last +//! [`MAX_IDLE_CACHES`] caches it handed out. Those matter when an executor runs one task at a +//! time: the next task of the stage starts after the previous one released its plan. Whatever the +//! tasks do, at most [`MAX_IDLE_CACHES`] caches outlive the tasks that used them. +//! +//! The stage scopes the cache; it does not identify the files in it. DataFusion keys an entry by +//! the file's path within its object store, which drops the bucket, so [`scope_to_store`] gives +//! each scan a view of the cache that also keys by the scan's object store. + +use crate::execution::spark_config::{ + SparkConfig, COMET_APP_ID, COMET_PARQUET_SHARED_METADATA_CACHE_ENABLED, COMET_STAGE_ATTEMPT, + COMET_STAGE_ID, +}; +use datafusion::common::TableReference; +use datafusion::datasource::object_store::ObjectStoreUrl; +use datafusion::execution::cache::cache_manager::{ + CachedFileMetadataEntry, FileMetadataCache, DEFAULT_METADATA_CACHE_LIMIT, +}; +use datafusion::execution::cache::default_cache::DefaultCache; +use datafusion::execution::cache::{Cache, CacheEntryInfo}; +use object_store::path::Path; +use std::collections::{HashMap, VecDeque}; +use std::sync::{Arc, Mutex, OnceLock, Weak}; +use std::time::Duration; + +/// How many caches of stages with no running task the registry holds on to. +const MAX_IDLE_CACHES: usize = 2; + +#[derive(Debug, Clone, PartialEq, Eq, Hash)] +pub(crate) struct StageKey { + pub app_id: String, + pub stage_id: i32, + pub stage_attempt: i32, +} + +#[derive(Default)] +struct Registry { + /// Every cache that is still alive, running or retained. + alive: HashMap<StageKey, Weak<FileMetadataCache>>, + /// The caches kept alive for the next task of their stage, least recently requested first. + retained: VecDeque<(StageKey, Arc<FileMetadataCache>)>, +} + +#[derive(Default)] +pub(crate) struct SharedMetadataCaches { + registry: Mutex<Registry>, +} + +impl SharedMetadataCaches { + /// The cache shared by the tasks of `key`, created if no task of that stage has asked for one + /// yet. + pub(crate) fn cache_for(&self, key: StageKey) -> Arc<FileMetadataCache> { + let mut registry = self.registry.lock().unwrap(); + let cache = registry + .alive + .get(&key) + .and_then(Weak::upgrade) + .unwrap_or_else(|| { + Arc::new( + DefaultCache::new(DEFAULT_METADATA_CACHE_LIMIT) + .with_name("SharedFileMetadataCache"), + ) + }); + registry.alive.insert(key.clone(), Arc::downgrade(&cache)); + + registry.retained.retain(|(retained, _)| *retained != key); + registry.retained.push_back((key, Arc::clone(&cache))); + while registry.retained.len() > MAX_IDLE_CACHES { + registry.retained.pop_front(); + } + registry.alive.retain(|_, weak| weak.strong_count() > 0); + cache + } + + #[cfg(test)] + fn len(&self) -> usize { + self.registry.lock().unwrap().alive.len() + } +} + +/// A view of a [`FileMetadataCache`] that keys every entry by the object store as well as the +/// path within it. +/// +/// DataFusion keys an entry by `ObjectMeta.location`, and the location of a file in a scan has no +/// bucket or authority, so equal paths in two stores collide. Two files at the same path with the +/// same size then pass the cache's `(size, modification time)` check, and a task would plan its +/// scan with the other file's footer and statistics. Only the key changes; the entries, the memory +/// limit and the eviction are those of the cache underneath, which is why a view can be made per +/// scan from the cache of the whole stage. `len`, `clear` and `list_entries` act on the whole +/// cache, not on this scope. +#[derive(Debug)] +struct StoreScopedCache { + scope: String, + inner: Arc<FileMetadataCache>, +} + +impl StoreScopedCache { + fn scoped(&self, key: &Path) -> Path { + std::iter::once(self.scope.clone()) + .chain(key.parts().map(|part| part.as_ref().to_string())) + .collect() + } +} + +impl Cache<Path, CachedFileMetadataEntry> for StoreScopedCache { + fn get(&self, key: &Path) -> Option<CachedFileMetadataEntry> { + self.inner.get(&self.scoped(key)) + } + + fn put(&self, key: &Path, value: CachedFileMetadataEntry) -> Option<CachedFileMetadataEntry> { + self.inner.put(&self.scoped(key), value) + } + + fn remove(&self, key: &Path) -> Option<CachedFileMetadataEntry> { + self.inner.remove(&self.scoped(key)) + } + + fn contains_key(&self, key: &Path) -> bool { + self.inner.contains_key(&self.scoped(key)) + } + + fn len(&self) -> usize { + self.inner.len() + } + + fn clear(&self) { + self.inner.clear() + } + + fn name(&self) -> String { + self.inner.name() + } + + fn cache_limit(&self) -> usize { + self.inner.cache_limit() + } + + fn update_cache_limit(&self, limit: usize) { + self.inner.update_cache_limit(limit) + } + + fn cache_ttl(&self) -> Option<Duration> { + self.inner.cache_ttl() + } + + fn update_cache_ttl(&self, ttl: Option<Duration>) { + self.inner.update_cache_ttl(ttl) + } + + fn drop_table_entries(&self, table_ref: &TableReference) -> datafusion::error::Result<()> { + self.inner.drop_table_entries(table_ref) + } + + fn list_entries( + &self, + ) -> datafusion::common::HashMap<Path, CacheEntryInfo<CachedFileMetadataEntry>> { + self.inner.list_entries() + } +} + +/// The metadata cache a scan of `object_store_url` should use, given the one of its +/// `RuntimeEnv`. +/// +/// `object_store_url` is the URL the scan's store is registered under, which already carries the +/// backend and a hash of the store's configuration (see `object_store_registration_url`), so two +/// stores with equal paths never share an entry. The scan and any code that reads the footers +/// ahead of it, such as the Delta deletion vector planning, must pass the same URL to find each +/// other's entries. +pub(crate) fn scope_to_store( + cache: Arc<FileMetadataCache>, + object_store_url: &ObjectStoreUrl, +) -> Arc<FileMetadataCache> { + Arc::new(StoreScopedCache { + scope: object_store_url.as_str().to_string(), + inner: cache, + }) +} + +/// The cache the task with these Spark configs should use, or `None` when sharing is off or the +/// task's stage is unknown, in which case DataFusion's per-task cache applies. +pub(crate) fn shared_metadata_cache_for_task( + spark_config: &HashMap<String, String>, +) -> Option<Arc<FileMetadataCache>> { + static CACHES: OnceLock<SharedMetadataCaches> = OnceLock::new(); + + if !spark_config.get_bool(COMET_PARQUET_SHARED_METADATA_CACHE_ENABLED) { + return None; + } + let parse = |name: &str| spark_config.get(name)?.parse::<i32>().ok(); + let key = StageKey { + app_id: spark_config.get(COMET_APP_ID)?.clone(), + stage_id: parse(COMET_STAGE_ID)?, + stage_attempt: parse(COMET_STAGE_ATTEMPT)?, + }; + Some( + CACHES + .get_or_init(SharedMetadataCaches::default) + .cache_for(key), + ) +} + +#[cfg(test)] +mod tests { + use super::*; + + fn key(stage_id: i32, stage_attempt: i32) -> StageKey { + StageKey { + app_id: "app-1".to_string(), + stage_id, + stage_attempt, + } + } + + #[test] + fn tasks_of_one_stage_share_a_cache() { + let caches = SharedMetadataCaches::default(); + let a = caches.cache_for(key(1, 0)); + let b = caches.cache_for(key(1, 0)); + assert!(Arc::ptr_eq(&a, &b)); + } + + #[test] + fn stages_and_attempts_do_not_share() { + let caches = SharedMetadataCaches::default(); + let first = caches.cache_for(key(1, 0)); + let other_stage = caches.cache_for(key(2, 0)); + let retry = caches.cache_for(key(1, 1)); + assert!(!Arc::ptr_eq(&first, &other_stage)); + assert!(!Arc::ptr_eq(&first, &retry)); + } + + #[test] + fn an_idle_cache_is_kept_for_the_next_task_of_its_stage() { + let caches = SharedMetadataCaches::default(); + let first = caches.cache_for(key(1, 0)); + let address = Arc::as_ptr(&first) as *const () as usize; + drop(first); + let next = caches.cache_for(key(1, 0)); + assert_eq!(Arc::as_ptr(&next) as *const () as usize, address); + } + + #[test] + fn applications_do_not_share() { + // A context started after another one stopped in the same JVM reuses its stage ids. + let caches = SharedMetadataCaches::default(); + let old = caches.cache_for(key(1, 0)); + drop(old); + let mut other = key(1, 0); + other.app_id = "app-2".to_string(); + let new = caches.cache_for(other); + assert_eq!(new.len(), 0); + assert!(!Arc::ptr_eq(&new, &caches.cache_for(key(1, 0)))); + } + + #[test] + fn idle_retention_is_bounded_after_all_tasks_finish() { + let caches = SharedMetadataCaches::default(); + let handles: Vec<_> = (0..16) + .map(|stage| caches.cache_for(key(stage, 0))) + .collect(); + let weak: Vec<_> = handles.iter().map(Arc::downgrade).collect(); + drop(handles); + assert!(weak.iter().filter(|w| w.upgrade().is_some()).count() <= MAX_IDLE_CACHES); + } + + #[test] + fn a_dropped_cache_is_forgotten_by_the_next_request() { Review Comment: `idle_caches_are_bounded_but_running_ones_are_not_dropped` below already shows that dropped caches leave the registry, since its `len()` would be 11 otherwise. 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