Making a separate thread for rendering made my code slower - c++

Question

I had a method called run in which I am updating and rendering the game objects.
void Run(olc::PixelGameEngine* pge) noexcept
{
Update(pge);
Render(pge);
}
Frame rate then was fluctuating 300~400 frames in release mode and 200~300 frames in debug mode. I have yet to add lodes of game logic, so I thought I would do rendering in a separate thread, so after a quick tutorial later I changed it to this.
void Run(olc::PixelGameEngine* pge) noexcept
{
Update(pge);
std::thread renderer(&GameManager::Render, this, pge);
renderer.join();
}
Now the frame rate is around 100~150 frames in release mode and 60~100 frames in debug mode.

Answer 1

std::thread creates a thread.
renderer.join() waits until the thread has finished.
Basically the same logic as your first example, but you create and destroy a thread in your 'loop'. Much more work than before, not surprising that the framerate goes down.
What you can do:
define two functions, one for the update and one for the render thread
introduce an global atomic flag
set it in the update function (indicates scene has been updated)
clear it in the render function (indicates that the changes has been presented)
create a thread (runnable, either update or render)
if the scene gets updated, set the flag accordingly
the renderer can decide based on the flag to render the scene or wait until the scene gets updated or render anyway (clear the flag)
the updater can decide based on the flag to wait until the scene has been rendered or update it anyway
Example (c++11):
#include <atomic>
#include <thread>
atomic_bool active = true;
atomic_bool scene_flag = false;
void render(olc::PixelGameEngine* pge) noexcept
{
while (active) {
//while (scene_flag == false) //wait for scene update
renderScene(pge);
scene_flag = false; //clear flag
}
}
void update(olc::PixelGameEngine* pge) noexcept
{
while (active) {
//while (scene_flag == true) //wait for scene render
updateScene(pge);
scene_flag = true; //set flag
}
}
int main()
{
thread u(update, nullptr);
thread r(render, nullptr);
/*while (some_condition) ...*/
active = false;
u.join();
r.join();
return 0;
}
Note: The above code could/will update the scene while it is being rendered (could/will lead to several problems). Proper synchronization is recommended.