mysql meta connection pool issue (#2413)

Signed-off-by: Ngroot <yihua.mo@zilliz.com>

core/src/db/meta/MySQLConnectionPool.cpp

@@ -11,6 +11,7 @@
 
 #include "db/meta/MySQLConnectionPool.h"
 #include <fiu-local.h>
+#include <thread>
 
 namespace milvus {
 namespace engine {
@@ -24,7 +25,7 @@ namespace meta {
 mysqlpp::Connection*
 MySQLConnectionPool::grab() {
     while (conns_in_use_ > max_pool_size_) {
-        sleep(1);
+        std::this_thread::sleep_for(std::chrono::milliseconds(10));
     }
 
     ++conns_in_use_;

core/src/db/meta/MySQLMetaImpl.cpp

@@ -314,9 +314,9 @@ MySQLMetaImpl::Initialize() {
     }
 
     // step 3: connect mysql
-    int thread_hint = std::thread::hardware_concurrency();
-    int max_pool_size = (thread_hint == 0) ? 8 : thread_hint;
-    unsigned int port = 0;
+    unsigned int thread_hint = std::thread::hardware_concurrency();
+    int max_pool_size = (thread_hint > 8) ? thread_hint : 8;
+    int port = 0;
     if (!uri_info.port_.empty()) {
         port = std::stoi(uri_info.port_);
     }