Merge #635

635: Use an unstable algorithm for `grenad::Sorter` when possible r=Kerollmops a=loiclec

# Pull Request
## What does this PR do?

Use an unstable algorithm to sort the internal vector used by `grenad::Sorter` whenever possible to speed up indexing.

In practice, every time the merge function creates a `RoaringBitmap`, we use an unstable sort. For every other merge function, such as `keep_first`, `keep_last`, etc., a stable sort is used.


Co-authored-by: Loïc Lecrenier <loic@meilisearch.com>

milli/src/update/index_documents/extract/extract_docid_word_positions.rs

@@ -32,6 +32,7 @@ pub fn extract_docid_word_positions<R: io::Read + io::Seek>(
 
     let mut documents_ids = RoaringBitmap::new();
     let mut docid_word_positions_sorter = create_sorter(
+        grenad::SortAlgorithm::Stable,
         concat_u32s_array,
         indexer.chunk_compression_type,
         indexer.chunk_compression_level,

milli/src/update/index_documents/extract/extract_facet_number_docids.rs

@@ -21,6 +21,7 @@ pub fn extract_facet_number_docids<R: io::Read + io::Seek>(
     let max_memory = indexer.max_memory_by_thread();
 
     let mut facet_number_docids_sorter = create_sorter(
+        grenad::SortAlgorithm::Unstable,
         merge_cbo_roaring_bitmaps,
         indexer.chunk_compression_type,
         indexer.chunk_compression_level,

milli/src/update/index_documents/extract/extract_facet_string_docids.rs

@@ -23,6 +23,7 @@ pub fn extract_facet_string_docids<R: io::Read + io::Seek>(
     let max_memory = indexer.max_memory_by_thread();
 
     let mut facet_string_docids_sorter = create_sorter(
+        grenad::SortAlgorithm::Stable,
         keep_first_prefix_value_merge_roaring_bitmaps,
         indexer.chunk_compression_type,
         indexer.chunk_compression_level,

milli/src/update/index_documents/extract/extract_fid_docid_facet_values.rs

@@ -28,6 +28,7 @@ pub fn extract_fid_docid_facet_values<R: io::Read + io::Seek>(
     let max_memory = indexer.max_memory_by_thread();
 
     let mut fid_docid_facet_numbers_sorter = create_sorter(
+        grenad::SortAlgorithm::Stable,
         keep_first,
         indexer.chunk_compression_type,
         indexer.chunk_compression_level,
@@ -36,6 +37,7 @@ pub fn extract_fid_docid_facet_values<R: io::Read + io::Seek>(
     );
 
     let mut fid_docid_facet_strings_sorter = create_sorter(
+        grenad::SortAlgorithm::Stable,
         keep_first,
         indexer.chunk_compression_type,
         indexer.chunk_compression_level,

milli/src/update/index_documents/extract/extract_fid_word_count_docids.rs

@@ -25,6 +25,7 @@ pub fn extract_fid_word_count_docids<R: io::Read + io::Seek>(
     let max_memory = indexer.max_memory_by_thread();
 
     let mut fid_word_count_docids_sorter = create_sorter(
+        grenad::SortAlgorithm::Unstable,
         merge_cbo_roaring_bitmaps,
         indexer.chunk_compression_type,
         indexer.chunk_compression_level,

milli/src/update/index_documents/extract/extract_word_docids.rs

@@ -30,6 +30,7 @@ pub fn extract_word_docids<R: io::Read + io::Seek>(
     let max_memory = indexer.max_memory_by_thread();
 
     let mut word_docids_sorter = create_sorter(
+        grenad::SortAlgorithm::Unstable,
         merge_roaring_bitmaps,
         indexer.chunk_compression_type,
         indexer.chunk_compression_level,
@@ -38,6 +39,7 @@ pub fn extract_word_docids<R: io::Read + io::Seek>(
     );
 
     let mut exact_word_docids_sorter = create_sorter(
+        grenad::SortAlgorithm::Unstable,
         merge_roaring_bitmaps,
         indexer.chunk_compression_type,
         indexer.chunk_compression_level,

milli/src/update/index_documents/extract/extract_word_pair_proximity_docids.rs

@@ -24,6 +24,7 @@ pub fn extract_word_pair_proximity_docids<R: io::Read + io::Seek>(
     let max_memory = indexer.max_memory_by_thread();
 
     let mut word_pair_proximity_docids_sorter = create_sorter(
+        grenad::SortAlgorithm::Unstable,
         merge_cbo_roaring_bitmaps,
         indexer.chunk_compression_type,
         indexer.chunk_compression_level,

milli/src/update/index_documents/extract/extract_word_position_docids.rs

@@ -21,6 +21,7 @@ pub fn extract_word_position_docids<R: io::Read + io::Seek>(
     let max_memory = indexer.max_memory_by_thread();
 
     let mut word_position_docids_sorter = create_sorter(
+        grenad::SortAlgorithm::Unstable,
         merge_cbo_roaring_bitmaps,
         indexer.chunk_compression_type,
         indexer.chunk_compression_level,

milli/src/update/index_documents/helpers/grenad_helpers.rs

@@ -27,6 +27,7 @@ pub fn create_writer<R: io::Write>(
 }
 
 pub fn create_sorter(
+    sort_algorithm: grenad::SortAlgorithm,
     merge: MergeFn,
     chunk_compression_type: grenad::CompressionType,
     chunk_compression_level: Option<u32>,
@@ -45,6 +46,7 @@ pub fn create_sorter(
         builder.dump_threshold(memory);
         builder.allow_realloc(false);
     }
+    builder.sort_algorithm(sort_algorithm);
     builder.build()
 }
 

milli/src/update/index_documents/mod.rs

@@ -1488,6 +1488,7 @@ mod tests {
         assert_eq!(count, 4);
     }
 
+    #[cfg(feature = "default")]
     #[test]
     fn test_meilisearch_1714() {
         let index = TempIndex::new();

milli/src/update/index_documents/transform.rs

@@ -99,6 +99,7 @@ impl<'a, 'i> Transform<'a, 'i> {
 
         // We initialize the sorter with the user indexing settings.
         let original_sorter = create_sorter(
+            grenad::SortAlgorithm::Stable,
             merge_function,
             indexer_settings.chunk_compression_type,
             indexer_settings.chunk_compression_level,
@@ -108,6 +109,7 @@ impl<'a, 'i> Transform<'a, 'i> {
 
         // We initialize the sorter with the user indexing settings.
         let flattened_sorter = create_sorter(
+            grenad::SortAlgorithm::Stable,
             merge_function,
             indexer_settings.chunk_compression_type,
             indexer_settings.chunk_compression_level,

milli/src/update/word_prefix_docids.rs

@@ -48,6 +48,7 @@ impl<'t, 'u, 'i> WordPrefixDocids<'t, 'u, 'i> {
         // It is forbidden to keep a mutable reference into the database
         // and write into it at the same time, therefore we write into another file.
         let mut prefix_docids_sorter = create_sorter(
+            grenad::SortAlgorithm::Unstable,
             merge_roaring_bitmaps,
             self.chunk_compression_type,
             self.chunk_compression_level,

milli/src/update/words_prefix_position_docids.rs

@@ -65,6 +65,7 @@ impl<'t, 'u, 'i> WordPrefixPositionDocids<'t, 'u, 'i> {
         debug!("Computing and writing the word levels positions docids into LMDB on disk...");
 
         let mut prefix_position_docids_sorter = create_sorter(
+            grenad::SortAlgorithm::Unstable,
             merge_cbo_roaring_bitmaps,
             self.chunk_compression_type,
             self.chunk_compression_level,