Execution not switching between other thread c++11

I am a beginner in C++11 multithreading. I am working with small codes and came into this problem. Here is the code:

#include<iostream>
#include<thread>
#include<vector>
#include<mutex>


std::mutex print_mutex;

void function1()
{
    std::cout << "Thread1 started" << std::endl;

    while(true)
    {

            std::unique_lock<std::mutex> lock(print_mutex);
            for(size_t i=0;i<=1000000000;i++)
                continue;
            std::cout << "This is function1" << std::endl;
            lock.unlock();


    }
}

void function2()
{
    std::cout << "Thread2 started" << std::endl;
    while(true)
    {

            std::unique_lock<std::mutex> lock(print_mutex);
            for(size_t i=0;i<=1000000000;i++)
                continue;
            std::cout << "This is function2" << std::endl;
            lock.unlock();
    }
}
int main()
{
    std::thread t1(function1);
    std::thread t2(function2);

    t1.join();
    t2.join();

    return 0;
}

I have written code with the intuition of expecting the following output:

Thread1 started
Thread2 started

This is function1
This is function2
This is function1
. .
.
.

But the output shown is as follows:

Thread1 started
Thread2 started

This is function1

This is function1
This is function1
.
.
.

Where am I going wrong?