implemented parallel flush

include/hepnos/Exception.hpp

@@ -17,7 +17,10 @@ class Exception : public std::exception
 
     public:
 
+    Exception() = default;
     Exception(const std::string& msg) : m_msg(msg){}
+    Exception(const Exception&) = default;
+    Exception& operator=(const Exception&) = default;
 
     virtual const char* what() const noexcept override
     {

src/private/WriteBatchImpl.hpp

@@ -17,6 +17,29 @@ namespace hepnos {
 
 class WriteBatch::Impl {
 
+    struct writer_thread_args {
+        unsigned long db_idx = 0;
+        std::pair<std::vector<std::string>, std::vector<std::string>>* keyvals = nullptr;
+        Impl* batch = nullptr;
+        Exception ex;
+        bool ok = true;
+    };
+
+    static void writer_thread(void* x) {
+        writer_thread_args* args = static_cast<writer_thread_args*>(x);
+        auto batch   = args->batch;
+        auto db_idx  = args->db_idx;
+        auto keyvals = args->keyvals;
+        const auto& keys = keyvals->first;
+        const auto& vals = keyvals->second;
+        try {
+            batch->m_datastore->m_impl->storeMultiple(db_idx, keys, vals);
+        } catch(Exception& ex) {
+            args->ok = false;
+            args->ex = ex;
+        }
+    }
+
     public:
 
         DataStore*   m_datastore;
@@ -46,12 +69,26 @@ class WriteBatch::Impl {
         }
 
         void flush() {
-            for(const auto& e : m_entries) {
+            ABT_xstream es = ABT_XSTREAM_NULL;
+            ABT_xstream_self(&es);
+            auto num_threads = m_entries.size();
+            std::vector<ABT_thread> threads(num_threads);
+            std::vector<writer_thread_args> args(num_threads);
+            unsigned i=0;
+            for(auto& e : m_entries) {
                 auto db_idx = e.first;
-                const auto& keys = e.second.first;
-                const auto& vals = e.second.second;
-                m_datastore->m_impl->storeMultiple(db_idx, keys, vals);
+                args[i].db_idx = e.first;
+                args[i].batch = this;
+                args[i].keyvals = &e.second;
+                ABT_thread_create_on_xstream(es, &writer_thread, &args[i], ABT_THREAD_ATTR_NULL, &threads[i]);
+                i += 1;
+            }
+            ABT_thread_join_many(num_threads, threads.data());
+            ABT_thread_free_many(num_threads, threads.data());
+            for(auto& a : args) {
+                if(not a.ok) throw a.ex;
             }
+            m_entries.clear();
         }
 
         ~Impl() {