Hey, I did fun experiments in UE5.5.4 recently. That seems nobody cares, but I’m really interested about it.
I was wondering that while AsyncSaveGameToSlot() on Slot1 is saving its data, if SaveGameToSlot() on the same slot(Slot1) is called, what will happen?
The answer that I discovered is actually.. SaveGameToSlot() is waiting for AsyncSaveGameToSlot() to be all done???
I actually expected some crash things.. but UE somehow smoothly does save. I looked up the engine code but I couldn’t figure it out. I really hope someone knows and explains about this please..
Below is how I experimented
SaveGameInstance->PlayerName = TEXT("Player One");
SaveGameInstance->Arr.Init(10, 250000000); // int32 array, about 1GB
UGameplayStatics::AsyncSaveGameToSlot(SaveGameInstance, TEXT("Slot1"), 0); // takes 1-2 seconds to write to slot(disk)
SaveGameInstance->PlayerName = TEXT("Player Two");
SaveGameInstance->Arr.Init(10, 1); // resize
UGameplayStatics::SaveGameToSlot(SaveGameInstance, TEXT("Slot1"), 0); // takes less than 0.1 seconds
Let’s call AsyncSaveGameToSlot() asyncSave and the other normalSave.
asyncSave actually Serialize() on the game thread, and write to slot on the worker thread. So when asyncSave finishes Serialize(), it passes remaining works to other thread, and goes back to the scope it is called. When asyncSave is still writing to slot, normalSave is called. But normalSave’s writing to slot is delayed until asyncSave is done its writing. (I checked all the times.) So Slot1.sav in SaveGames folder becomes only 2KB, not 1GB.
If slot name is different? normalSave don’t wait for asyncSave to finish its job. It is only done synchronously when the slot name is the same.