Make NUM_MERGE_THREADS configurable (#2535)

* Make `NUM_MERGE_THREADS` configurable

* Remove unused import

* Reword comment src/index/index.rs

Co-authored-by: PSeitz <PSeitz@users.noreply.github.com>

---------

Co-authored-by: PSeitz <PSeitz@users.noreply.github.com>

src/index/index.rs

@@ -24,6 +24,8 @@ use crate::schema::{Field, FieldType, Schema};
 use crate::tokenizer::{TextAnalyzer, TokenizerManager};
 use crate::SegmentReader;
 
+const DEFAULT_NUM_MERGE_THREADS: usize = 4;
+
 fn load_metas(
     directory: &dyn Directory,
     inventory: &SegmentMetaInventory,
@@ -538,10 +540,11 @@ impl Index {
     /// If the lockfile already exists, returns `Error::DirectoryLockBusy` or an `Error::IoError`.
     /// If the memory arena per thread is too small or too big, returns
     /// `TantivyError::InvalidArgument`
-    pub fn writer_with_num_threads<D: Document>(
+    pub fn writer_with_num_threads_and_num_merge_threads<D: Document>(
         &self,
         num_threads: usize,
         overall_memory_budget_in_bytes: usize,
+        num_merge_threads: usize,
     ) -> crate::Result<IndexWriter<D>> {
         let directory_lock = self
             .directory
@@ -563,6 +566,25 @@ impl Index {
             num_threads,
             memory_arena_in_bytes_per_thread,
             directory_lock,
+            num_merge_threads,
+        )
+    }
+
+    /// Creates a multithreaded writer with 4 merge threads.
+    ///
+    /// # Errors
+    /// If the lockfile already exists, returns `Error::FileAlreadyExists`.
+    /// If the memory arena per thread is too small or too big, returns
+    /// `TantivyError::InvalidArgument`
+    pub fn writer_with_num_threads<D: Document>(
+        &self,
+        num_threads: usize,
+        overall_memory_budget_in_bytes: usize,
+    ) -> crate::Result<IndexWriter<D>> {
+        self.writer_with_num_threads_and_num_merge_threads(
+            num_threads,
+            overall_memory_budget_in_bytes,
+            DEFAULT_NUM_MERGE_THREADS,
         )
     }
 

src/indexer/index_writer.rs

@@ -71,6 +71,7 @@ pub struct IndexWriter<D: Document = TantivyDocument> {
     worker_id: usize,
 
     num_threads: usize,
+    num_merge_threads: usize,
 
     delete_queue: DeleteQueue,
 
@@ -268,6 +269,7 @@ impl<D: Document> IndexWriter<D> {
         num_threads: usize,
         memory_budget_in_bytes_per_thread: usize,
         directory_lock: DirectoryLock,
+        num_merge_threads: usize,
     ) -> crate::Result<Self> {
         if memory_budget_in_bytes_per_thread < MEMORY_BUDGET_NUM_BYTES_MIN {
             let err_msg = format!(
@@ -291,8 +293,12 @@ impl<D: Document> IndexWriter<D> {
 
         let stamper = Stamper::new(current_opstamp);
 
-        let segment_updater =
-            SegmentUpdater::create(index.clone(), stamper.clone(), &delete_queue.cursor())?;
+        let segment_updater = SegmentUpdater::create(
+            index.clone(),
+            stamper.clone(),
+            &delete_queue.cursor(),
+            num_merge_threads,
+        )?;
 
         let mut index_writer = Self {
             _directory_lock: Some(directory_lock),
@@ -307,6 +313,8 @@ impl<D: Document> IndexWriter<D> {
             workers_join_handle: vec![],
             num_threads,
 
+            num_merge_threads,
+
             delete_queue,
 
             committed_opstamp: current_opstamp,
@@ -558,6 +566,7 @@ impl<D: Document> IndexWriter<D> {
             self.num_threads,
             self.memory_budget_in_bytes_per_thread,
             directory_lock,
+            self.num_merge_threads,
         )?;
 
         // the current `self` is dropped right away because of this call.

src/indexer/segment_updater.rs

@@ -25,8 +25,6 @@ use crate::indexer::{
 };
 use crate::{FutureResult, Opstamp};
 
-const NUM_MERGE_THREADS: usize = 4;
-
 /// Save the index meta file.
 /// This operation is atomic:
 /// Either
@@ -273,6 +271,7 @@ impl SegmentUpdater {
         index: Index,
         stamper: Stamper,
         delete_cursor: &DeleteCursor,
+        num_merge_threads: usize,
     ) -> crate::Result<SegmentUpdater> {
         let segments = index.searchable_segment_metas()?;
         let segment_manager = SegmentManager::from_segments(segments, delete_cursor);
@@ -287,7 +286,7 @@ impl SegmentUpdater {
             })?;
         let merge_thread_pool = ThreadPoolBuilder::new()
             .thread_name(|i| format!("merge_thread_{i}"))
-            .num_threads(NUM_MERGE_THREADS)
+            .num_threads(num_merge_threads)
             .build()
             .map_err(|_| {
                 crate::TantivyError::SystemError(