martin-g commented on code in PR #20047:
URL: https://github.com/apache/datafusion/pull/20047#discussion_r2799001039
##########
datafusion/execution/src/cache/cache_unit.rs:
##########
@@ -36,37 +36,160 @@ pub use crate::cache::DefaultFilesMetadataCache;
/// 2. If `Some(cached)`, validate with `cached.is_valid_for(¤t_meta)`
/// 3. If invalid or missing, compute new value and call `put(path, new_value)`
///
-/// Uses DashMap for lock-free concurrent access.
+/// # Internal details
+///
+/// The `memory_limit` controls the maximum size of the cache, which uses a
+/// Least Recently Used eviction algorithm. When adding a new entry, if the
total
+/// size of the cached entries exceeds `memory_limit`, the least recently used
entries
+/// are evicted until the total size is lower than `memory_limit`.
+///
///
/// [`FileStatisticsCache`]: crate::cache::cache_manager::FileStatisticsCache
#[derive(Default)]
pub struct DefaultFileStatisticsCache {
- cache: DashMap<Path, CachedFileMetadata>,
+ state: Mutex<DefaultFileStatisticsCacheState>,
+}
+
+impl DefaultFileStatisticsCache {
+ pub fn new(memory_limit: usize) -> Self {
+ Self {
+ state:
Mutex::new(DefaultFileStatisticsCacheState::new(memory_limit)),
+ }
+ }
+
+ /// Returns the size of the cached memory, in bytes.
+ pub fn memory_used(&self) -> usize {
+ let state = self.state.lock().unwrap();
+ state.memory_used
+ }
+}
+
+pub struct DefaultFileStatisticsCacheState {
+ lru_queue: LruQueue<Path, CachedFileMetadata>,
+ memory_limit: usize,
+ memory_used: usize,
+}
+
+pub const DEFAULT_FILE_STATISTICS_MEMORY_LIMIT: usize = 1024 * 1024; // 1MiB
+
+impl Default for DefaultFileStatisticsCacheState {
+ fn default() -> Self {
+ Self {
+ lru_queue: LruQueue::new(),
+ memory_limit: DEFAULT_FILE_STATISTICS_MEMORY_LIMIT,
+ memory_used: 0,
+ }
+ }
}
+impl DefaultFileStatisticsCacheState {
+ fn new(memory_limit: usize) -> Self {
+ Self {
+ lru_queue: LruQueue::new(),
+ memory_limit,
+ memory_used: 0,
+ }
+ }
+ fn get(&mut self, key: &Path) -> Option<CachedFileMetadata> {
+ self.lru_queue.get(key).cloned()
+ }
+
+ fn put(
+ &mut self,
+ key: &Path,
+ value: CachedFileMetadata,
+ ) -> Option<CachedFileMetadata> {
+ let key_size = key.heap_size();
+ let entry_size = value.heap_size();
+
+ if entry_size + key_size > self.memory_limit {
+ // Remove potential stale entry
+ self.remove(key);
+ return None;
+ }
+
+ let old_value = self.lru_queue.put(key.clone(), value);
+ self.memory_used += entry_size;
+
+ if let Some(old_entry) = &old_value {
+ self.memory_used -= old_entry.heap_size();
+ } else {
+ self.memory_used += key.heap_size();
+ }
+
+ self.evict_entries();
+
+ old_value
+ }
+
+ fn remove(&mut self, k: &Path) -> Option<CachedFileMetadata> {
+ if let Some(old_entry) = self.lru_queue.remove(k) {
+ self.memory_used -= k.heap_size();
+ self.memory_used -= old_entry.heap_size();
+ Some(old_entry)
+ } else {
+ None
+ }
+ }
+
+ fn contains_key(&self, k: &Path) -> bool {
+ self.lru_queue.contains_key(k)
+ }
+
+ fn len(&self) -> usize {
+ self.lru_queue.len()
+ }
+
+ fn clear(&mut self) {
+ self.lru_queue.clear();
+ self.memory_used = 0;
+ }
+
+ fn evict_entries(&mut self) {
+ while self.memory_used > self.memory_limit {
+ if let Some(removed) = self.lru_queue.pop() {
+ self.memory_used -= removed.0.heap_size();
+ self.memory_used -= removed.1.heap_size();
+ } else {
+ // cache is empty while memory_used > memory_limit, cannot
happen
+ debug_assert!(
+ false,
+ "This is a bug! Please report it to the Apache DataFusion
developers"
+ );
Review Comment:
```suggestion
log::error!(
"File statistics cache memory accounting bug:
memory_used={} but cache is empty. \
Please report this to the Apache DataFusion
developers.",
self.memory_used
);
debug_assert!(
false,
"memory_used={} but cache is empty",
self.memory_used
);
self.memory_used = 0;
```
1) log an error, so that it is also noticeable in production
2) reset `memory_used`, so that the cache might be used again
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