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