Add a version of the save graph without taking locks, when the caller guarantees no index update operations are sent to the index.

Author: Denis Deyneko

include/index.h

@@ -87,8 +87,16 @@ template <typename T, typename TagT = uint32_t, typename LabelT = uint32_t> clas
     DISKANN_DLLEXPORT ~Index();
 
     // Saves graph, data, metadata and associated tags.
+    // Takes exclusive locks on update, tags, consolidate_delete and delete. Some of these locks
+    // are used in search and lookup methods which means those will be blocked until save completes.
     DISKANN_DLLEXPORT void save(const char *filename, bool compact_before_save = false);
 
+    // Same as above, but adds ability to skip btaining locks.
+    // if no_lock is true, the function will not acquire any locks. This is useful for customers
+    // that can guarantee no updates to the index will happen while saving. In this case, the query 
+    // part continue to work.
+    DISKANN_DLLEXPORT void save(const char *filename, bool compact_before_save, bool no_lock);
+
     // Load functions
 #ifdef EXEC_ENV_OLS
     DISKANN_DLLEXPORT void load(AlignedFileReader &reader, uint32_t num_threads, uint32_t search_l);

nuget/DiskANN-preview.nuspec

@@ -2,7 +2,7 @@
 <package xmlns="http://schemas.microsoft.com/packaging/2013/05/nuspec.xsd">
   <metadata>
     <id>DiskANN-preview.win-x64</id>
-    <version>0.13.729.3-temp</version>
+    <version>0.13.729.3</version>
     <authors>DiskANN team</authors>
     <owners>DiskANN team</owners>
     <requireLicenseAcceptance>false</requireLicenseAcceptance>

src/index.cpp

@@ -411,14 +411,28 @@ _u64 Index<T, TagT, LabelT>::save_delete_list(const std::string& filename) const
 }
 
 template <typename T, typename TagT, typename LabelT>
-void Index<T, TagT, LabelT>::save(const char *filename, bool compact_before_save)
+void Index<T, TagT, LabelT>::save(const char *filename, bool compact_before_save /* = false */)
+{
+    save(filename, compact_before_save, false);
+}
+
+template <typename T, typename TagT, typename LabelT>
+void Index<T, TagT, LabelT>::save(const char *filename, bool compact_before_save, bool no_lock)
 {
     diskann::Timer timer;
 
-    std::unique_lock<std::shared_timed_mutex> ul(_update_lock);
-    std::unique_lock<std::shared_timed_mutex> cl(_consolidate_lock);
-    std::unique_lock<std::shared_timed_mutex> tl(_tag_lock);
-    std::unique_lock<std::shared_timed_mutex> dl(_delete_lock);
+    std::unique_lock<std::shared_timed_mutex> ul(_update_lock, std::defer_lock);
+    std::unique_lock<std::shared_timed_mutex> cl(_consolidate_lock, std::defer_lock);
+    std::unique_lock<std::shared_timed_mutex> tl(_tag_lock, std::defer_lock);
+    std::unique_lock<std::shared_timed_mutex> dl(_delete_lock, std::defer_lock);
+
+    if (!no_lock)
+    {
+        ul.lock();
+        cl.lock();
+        tl.lock();
+        dl.lock();
+    }
 
     bool frozen_points_compacted = false;
     if (compact_before_save)