parallesim a bit of improvement

csvreader.cpp

@@ -31,10 +31,10 @@ namespace usql {
 		size_t len = 0;
 
 		// TODO handle it by settings
-		const std::size_t hw_concurrency = std::max(0, (int)(std::thread::hardware_concurrency() - 2));
+		const std::size_t hw_concurrency = getConcurrency();
 		const bool use_threadpool = hw_concurrency > 1;
 
-		thread_pool tp{hw_concurrency};
+		ThreadPool tp{hw_concurrency};
 		std::mutex row_cnt_mutex;
 
 		try {
@@ -140,4 +140,22 @@ namespace usql {
 		return row_cnt;
 	}
 
+	size_t CsvReader::getConcurrency() {
+		auto value = Settings::get_setting("MAX_PARALLELISM");
+		if (value == "1" || value == "0")
+			return 0;
+		else if (value == "AUTO")
+			return std::thread::hardware_concurrency();
+		else {
+		    char *end;
+		    long i = strtol( value.c_str(), &end, 10 );
+    		if ( *end == '\0' ) {	// Success
+				if (i >= 0) return i;
+				else return std::max(std::thread::hardware_concurrency() + i, 0l);
+			}
+		}
+		return 0;
+	}
+
+
 }    // namespace

csvreader.h

@@ -28,6 +28,8 @@ public:
 
 	size_t parseCSVFile(const std::string &filename, std::vector<ColDefNode> &cols_def, Table& table);
 	size_t parseCSVString(const std::string &csvSource, std::vector<ColDefNode> &cols_def, Table& table);
+
+	static size_t getConcurrency();
 };
 
 }	// namespace

settings.cpp

@@ -12,7 +12,8 @@ std::vector<std::pair<std::string, std::string>> Settings::m_settings =
 	      std::make_pair("BOOL_TRUE_LITERAL", "Y"),
 	      std::make_pair("BOOL_FALSE_LITERAL", "N"),
           std::make_pair("DOUBLE_FORMAT", "%.2f"),
-	      std::make_pair("USE_INDEXSCAN", "N") };
+	      std::make_pair("USE_INDEXSCAN", "N"),
+	      std::make_pair("MAX_PARALLELISM", "1") };		// values "AUTO" or number of workers; when number negative means std::thread::hardware_concurrency() - number 
 
 
 

threadpool.h

@@ -5,15 +5,15 @@
 #include <condition_variable>
 #include <vector>
 
-class thread_pool {
+class ThreadPool {
 public:
-	thread_pool(std::size_t size) : stop(false) {
+	ThreadPool(std::size_t size) : stop(false) {
 		for (std::size_t i = 0; i < size; ++i) {
 			workers.emplace_back([this] { spawn(); });
 		}
 	}
 
-	virtual ~thread_pool() {
+	virtual ~ThreadPool() {
 		if (!stop) join();
 	}
 
@@ -74,6 +74,6 @@ public:
 	bool stop;
 };
 
-inline void dispatch(thread_pool &pool, std::function<void()> f) {
+inline void dispatch(ThreadPool &pool, std::function<void()> f) {
 	pool.post(f);
 }