why MultiThreadedExecutor act as SingleThreaded?

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Hi

I'm testing rclcpp::executors::MultiThreadedExecutor. I wrote two Subscription in a node, and at each callback function sleep for 10 seconds. I thought those two callback funcs would be executed at the same time, but in fact, one of them would not be executed until 10 seconds after the other started. here is the code. Does anyone have any ideas? Thank for your time

#include <rclcpp/rclcpp.hpp> 
#include <std_msgs/msg/int32.hpp>
using namespace std::chrono_literals;
std::string getTime(){
    time_t tick = (time_t)(rclcpp::Clock().now().seconds());
    struct tm tm;
    char s[100];
    tm = *localtime(&tick);
    strftime(s, sizeof(s), "%Y-%m-%d %H:%M:%S", &tm);
    return std::string(s);
}
class MyNode: public rclcpp::Node{
public:
    MyNode(const rclcpp::NodeOptions & options);
    ~MyNode(){}
    void int1Sub(std_msgs::msg::Int32::SharedPtr msg);
    void int2Sub(std_msgs::msg::Int32::SharedPtr msg);

private:
    rclcpp::Subscription<std_msgs::msg::Int32>::SharedPtr int_sub1_;
    rclcpp::Subscription<std_msgs::msg::Int32>::SharedPtr int_sub2_;
}
MyNode::MyNode(const rclcpp::NodeOptions & options):
    rclcpp::Node("my_node",options){
int_sub1_ = nh_->create_subscription<std_msgs::msg::Int32>("int1",10,
        std::bind(&MyNode::int1Sub,this,std::placeholders::_1));
int_sub2_ = nh_->create_subscription<std_msgs::msg::Int32>("int2",10,
        std::bind(&MyNode::int2Sub,this,std::placeholders::_1));
}
void MyNode::int1Sub(std_msgs::msg::Int32::SharedPtr msg){
    printf("int1 sub time:%s\n",getTime().c_str());
    rclcpp::sleep_for(10s);
    printf("int1 sub time:%s\n",getTime().c_str());
}
void MyNode::int2Sub(std_msgs::msg::Int32::SharedPtr msg){
    printf("int2 sub time:%s\n",getTime().c_str());
    rclcpp::sleep_for(10s);
    printf("int2 sub time:%s\n",getTime().c_str());
}
int main(int argc, char **argv) {
    rclcpp::init(argc, argv);
    rclcpp::executors::MultiThreadedExecutor executor(rclcpp::executor::ExecutorArgs(),5,true);

    auto node = std::make_shared<MyNode>(rclcpp::NodeOptions());
    executor.add_node(node);
    printf("threads   %d\n",executor.get_number_of_threads());
    executor.spin();
    rclcpp::shutdown();
    return EXIT_SUCCESS;
}