Use LRU cache

index-scheduler/src/index_mapper.rs

@@ -1,5 +1,3 @@
-use std::collections::hash_map::Entry;
-use std::collections::HashMap;
 use std::path::{Path, PathBuf};
 use std::sync::{Arc, RwLock};
 use std::{fs, thread};
@@ -14,6 +12,7 @@ use time::OffsetDateTime;
 use uuid::Uuid;
 
 use self::IndexStatus::{Available, BeingDeleted, BeingResized};
+use crate::lru::{InsertionOutcome, LruMap};
 use crate::uuid_codec::UuidCodec;
 use crate::{clamp_to_page_size, Error, Result};
 
@@ -29,7 +28,7 @@ const INDEX_MAPPING: &str = "index-mapping";
 #[derive(Clone)]
 pub struct IndexMapper {
     /// Keep track of the opened indexes. Used mainly by the index resolver.
-    index_map: Arc<RwLock<HashMap<Uuid, IndexStatus>>>,
+    index_map: Arc<RwLock<IndexMap>>,
 
     /// Map an index name with an index uuid currently available on disk.
     pub(crate) index_mapping: Database<Str, UuidCodec>,
@@ -40,6 +39,122 @@ pub struct IndexMapper {
     pub indexer_config: Arc<IndexerConfig>,
 }
 
+struct IndexMap {
+    unavailable: Vec<(Uuid, Option<Arc<SignalEvent>>)>,
+    available: LruMap<Uuid, Index>,
+}
+
+impl IndexMap {
+    pub fn new(cap: usize) -> IndexMap {
+        Self { unavailable: Vec::new(), available: LruMap::new(cap) }
+    }
+
+    pub fn get(&self, uuid: &Uuid) -> Option<IndexStatus> {
+        self.get_if_unavailable(uuid)
+            .map(|signal| {
+                if let Some(signal) = signal {
+                    IndexStatus::BeingResized(signal)
+                } else {
+                    IndexStatus::BeingDeleted
+                }
+            })
+            .or_else(|| self.available.get(uuid).map(|index| IndexStatus::Available(index.clone())))
+    }
+
+    /// Inserts a new index as available
+    ///
+    /// # Panics
+    ///
+    /// - If the index is already present, but currently unavailable.
+    pub fn insert(&mut self, uuid: &Uuid, index: Index) -> InsertionOutcome<Uuid, Index> {
+        assert!(
+            matches!(self.get_if_unavailable(uuid), None),
+            "Attempted to insert an index that was not available"
+        );
+
+        self.available.insert(*uuid, index)
+    }
+
+    /// Begins a resize operation.
+    ///
+    /// Returns `None` if the index is already unavailable, or not present at all.
+    pub fn start_resize(&mut self, uuid: &Uuid, signal: Arc<SignalEvent>) -> Option<Index> {
+        if self.get_if_unavailable(uuid).is_some() {
+            return None;
+        }
+
+        let index = self.available.remove(uuid)?;
+        self.unavailable.push((*uuid, Some(signal)));
+        Some(index)
+    }
+
+    /// Ends a resize operation that completed successfully.
+    ///
+    /// As the index becomes available again, it might evict another index from the cache. In that case, it is returned.
+    ///
+    /// # Panics
+    ///
+    /// - if the target index was not being resized.
+    /// - the index was also in the list of available indexes.
+    pub fn end_resize(
+        &mut self,
+        uuid: &Uuid,
+        index: Index,
+    ) -> (Arc<SignalEvent>, Option<(Uuid, Index)>) {
+        let signal =
+            self.pop_if_unavailable(uuid).flatten().expect("The index was not being resized");
+        let evicted = match self.available.insert(*uuid, index) {
+            InsertionOutcome::InsertedNew => None,
+            InsertionOutcome::Evicted(uuid, index) => Some((uuid, index)),
+            InsertionOutcome::Replaced(_) => panic!("Inconsistent map state"),
+        };
+        (signal, evicted)
+    }
+
+    /// Ends a resize operation that failed for some reason.
+    ///
+    /// # Panics
+    ///
+    /// - if the target index was not being resized.
+    pub fn end_resize_failed(&mut self, uuid: &Uuid) -> Arc<SignalEvent> {
+        self.pop_if_unavailable(uuid).flatten().expect("The index was not being resized")
+    }
+
+    /// Beings deleting an index.
+    ///
+    /// # Panics
+    ///
+    /// - if the index was already unavailable
+    pub fn start_deletion(&mut self, uuid: &Uuid) -> Option<Index> {
+        assert!(
+            matches!(self.get_if_unavailable(uuid), None),
+            "Attempt to start deleting an index that was already unavailable"
+        );
+
+        let index = self.available.remove(uuid)?;
+        self.unavailable.push((*uuid, None));
+        Some(index)
+    }
+
+    pub fn end_deletion(&mut self, uuid: &Uuid) {
+        self.pop_if_unavailable(uuid)
+            .expect("Attempted to delete an index that was not being deleted");
+    }
+
+    fn get_if_unavailable(&self, uuid: &Uuid) -> Option<Option<Arc<SignalEvent>>> {
+        self.unavailable
+            .iter()
+            .find_map(|(candidate_uuid, signal)| (uuid == candidate_uuid).then_some(signal.clone()))
+    }
+
+    fn pop_if_unavailable(&mut self, uuid: &Uuid) -> Option<Option<Arc<SignalEvent>>> {
+        self.unavailable
+            .iter()
+            .position(|(candidate_uuid, _)| candidate_uuid == uuid)
+            .map(|index| self.unavailable.swap_remove(index).1)
+    }
+}
+
 /// Whether the index is available for use or is forbidden to be inserted back in the index map
 #[allow(clippy::large_enum_variant)]
 #[derive(Clone)]
@@ -60,7 +175,7 @@ impl IndexMapper {
         indexer_config: IndexerConfig,
     ) -> Result<Self> {
         Ok(Self {
-            index_map: Arc::default(),
+            index_map: Arc::new(RwLock::new(IndexMap::new(20))),
             index_mapping: env.create_database(Some(INDEX_MAPPING))?,
             base_path,
             index_size,
@@ -112,9 +227,15 @@ impl IndexMapper {
                 // Error if the UUIDv4 somehow already exists in the map, since it should be fresh.
                 // This is very unlikely to happen in practice.
                 // TODO: it would be better to lazily create the index. But we need an Index::open function for milli.
-                if self.index_map.write().unwrap().insert(uuid, Available(index.clone())).is_some()
-                {
-                    panic!("Uuid v4 conflict: index with UUID {uuid} already exists.");
+                match self.index_map.write().unwrap().insert(&uuid, index.clone()) {
+                    InsertionOutcome::Evicted(uuid, evicted_index) => {
+                        log::info!("Closing index with UUID {uuid}");
+                        evicted_index.prepare_for_closing();
+                    }
+                    InsertionOutcome::Replaced(_) => {
+                        panic!("Uuid v4 conflict: index with UUID {uuid} already exists.")
+                    }
+                    _ => (),
                 }
 
                 Ok(index)
@@ -135,21 +256,18 @@ impl IndexMapper {
         assert!(self.index_mapping.delete(&mut wtxn, name)?);
 
         wtxn.commit()?;
+
         // We remove the index from the in-memory index map.
         let closing_event = loop {
             let mut lock = self.index_map.write().unwrap();
-            let resize_operation = match lock.insert(uuid, BeingDeleted) {
-                Some(Available(index)) => break Some(index.prepare_for_closing()),
-                // The target index is in the middle of a resize operation.
-                // Wait for this operation to complete, then try again.
+            let resize_operation = match lock.get(&uuid) {
+                Some(Available(index)) => {
+                    lock.start_deletion(&uuid);
+                    break index.prepare_for_closing();
+                }
                 Some(BeingResized(resize_operation)) => resize_operation.clone(),
-                // The index is already being deleted or doesn't exist.
-                // It's OK to remove it from the map again.
-                _ => break None,
+                Some(BeingDeleted) | None => return Ok(()),
             };
-
-            // Avoiding deadlocks: we need to drop the lock before waiting for the end of the resize, which
-            // will involve operations on the very map we're locking.
             drop(lock);
             resize_operation.wait();
         };
@@ -162,9 +280,7 @@ impl IndexMapper {
             .spawn(move || {
                 // We first wait to be sure that the previously opened index is effectively closed.
                 // This can take a lot of time, this is why we do that in a seperate thread.
-                if let Some(closing_event) = closing_event {
                 closing_event.wait();
-                }
 
                 // Then we remove the content from disk.
                 if let Err(e) = fs::remove_dir_all(&index_path) {
@@ -175,7 +291,7 @@ impl IndexMapper {
                 }
 
                 // Finally we remove the entry from the index map.
-                assert!(matches!(index_map.write().unwrap().remove(&uuid), Some(BeingDeleted)));
+                index_map.write().unwrap().end_deletion(&uuid);
             })
             .unwrap();
 
@@ -202,23 +318,9 @@ impl IndexMapper {
             .ok_or_else(|| Error::IndexNotFound(name.to_string()))?;
 
         // We remove the index from the in-memory index map.
-        let mut lock = self.index_map.write().unwrap();
         // signal that will be sent when the resize operation completes
         let resize_operation = Arc::new(SignalEvent::manual(false));
-        let index = match lock.insert(uuid, BeingResized(resize_operation)) {
-            Some(Available(index)) => index,
-            Some(previous_status) => {
-                lock.insert(uuid, previous_status);
-                panic!(
-                    "Attempting to resize index {name} that is already being resized or deleted."
-                )
-            }
-            None => {
-                panic!("Could not find the status of index {name} in the in-memory index mapper.")
-            }
-        };
-
-        drop(lock);
+        let Some(index) = self.index_map.write().unwrap().start_resize(&uuid, resize_operation) else { return Ok(()) };
 
         let resize_succeeded = (move || {
             let current_size = index.map_size()?;
@@ -242,21 +344,17 @@ impl IndexMapper {
         // Even if there was an error we don't want to leave the map in an inconsistent state as it would cause
         // deadlocks.
         let mut lock = self.index_map.write().unwrap();
-        let (resize_operation, resize_succeeded) = match resize_succeeded {
+        let (resize_operation, resize_succeeded, evicted) = match resize_succeeded {
             Ok(index) => {
                 // insert the resized index
-                let Some(BeingResized(resize_operation)) = lock.insert(uuid, Available(index)) else {
-                    panic!("Index state for index {name} was modified while it was being resized")
-                };
+                let (resize_operation, evicted) = lock.end_resize(&uuid, index);
 
-                (resize_operation, Ok(()))
+                (resize_operation, Ok(()), evicted)
             }
             Err(error) => {
                 // there was an error, not much we can do... delete the index from the in-memory map to prevent future errors
-                let Some(BeingResized(resize_operation)) = lock.remove(&uuid) else {
-                                    panic!("Index state for index {name} was modified while it was being resized")
-                                };
-                (resize_operation, Err(error))
+                let resize_operation = lock.end_resize_failed(&uuid);
+                (resize_operation, Err(error), None)
             }
         };
 
@@ -264,6 +362,11 @@ impl IndexMapper {
         drop(lock);
         resize_operation.signal();
 
+        if let Some((uuid, evicted_index)) = evicted {
+            log::info!("Closing index with UUID {uuid}");
+            evicted_index.prepare_for_closing();
+        }
+
         resize_succeeded
     }
 
@@ -275,11 +378,11 @@ impl IndexMapper {
             .ok_or_else(|| Error::IndexNotFound(name.to_string()))?;
 
         // we clone here to drop the lock before entering the match
-        let index = loop {
-            let index = self.index_map.read().unwrap().get(&uuid).cloned();
+        let (index, evicted_index) = loop {
+            let index = self.index_map.read().unwrap().get(&uuid);
 
             match index {
-                Some(Available(index)) => break index,
+                Some(Available(index)) => break (index, None),
                 Some(BeingResized(ref resize_operation)) => {
                     // Avoiding deadlocks: no lock taken while doing this operation.
                     resize_operation.wait();
@@ -290,36 +393,44 @@ impl IndexMapper {
                 None => {
                     let mut index_map = self.index_map.write().unwrap();
                     // between the read lock and the write lock it's not impossible
-                    // that someone already opened the index (eg if two search happens
+                    // that someone already opened the index (eg if two searches happen
                     // at the same time), thus before opening it we check a second time
                     // if it's not already there.
-                    // Since there is a good chance it's not already there we can use
-                    // the entry method.
-                    match index_map.entry(uuid) {
-                        Entry::Vacant(entry) => {
+                    match index_map.get(&uuid) {
+                        None => {
                             let index_path = self.base_path.join(uuid.to_string());
 
                             let index =
                                 self.create_or_open_index(&index_path, None, self.index_size)?;
-                            entry.insert(Available(index.clone()));
-                            break index;
+                            match index_map.insert(&uuid, index.clone()) {
+                                InsertionOutcome::InsertedNew => break (index, None),
+                                InsertionOutcome::Evicted(evicted_uuid, evicted_index) => {
+                                    break (index, Some((evicted_uuid, evicted_index)))
                                 }
-                        Entry::Occupied(entry) => match entry.get() {
-                            Available(index) => break index.clone(),
-                            BeingResized(resize_operation) => {
+                                InsertionOutcome::Replaced(_) => {
+                                    panic!("Inconsistent map state")
+                                }
+                            }
+                        }
+                        Some(Available(index)) => break (index, None),
+                        Some(BeingResized(resize_operation)) => {
                             // Avoiding the deadlock: we drop the lock before waiting
                             let resize_operation = resize_operation.clone();
                             drop(index_map);
                             resize_operation.wait();
                             continue;
                         }
-                            BeingDeleted => return Err(Error::IndexNotFound(name.to_string())),
-                        },
+                        Some(BeingDeleted) => return Err(Error::IndexNotFound(name.to_string())),
                     }
                 }
             }
         };
 
+        if let Some((evicted_uuid, evicted_index)) = evicted_index {
+            log::info!("Closing index with UUID {evicted_uuid}");
+            evicted_index.prepare_for_closing();
+        }
+
         Ok(index)
     }