Minecraft barely uses RAM?

I have recently the problem that the chunks in Minecraft not load well, but I've read and read that it is on the Internet or server, so I can't really do anything. Nevertheless, I have assigned Minecraft half of my available RAM (3 out of 6 gigabytes) and now I have seen in F3 that after activating a shader, I have only 30 to 46 fps. However, only about 46% of my RAM is used, so why does not Minecraft use more automatically and can I force Minecraft to use a specific RAM?

Yes it is possible to set the ram to a certain minimum usage, but that you have only 30-50fps can also be on many other things. It does not have to be the ram that makes problems, but it may also be that your CPU or graphics card is too weak, probably the graphics card if you have one.

Why should the application full pop up the memory if they have no data with which they can fill the memory? Just because you provide the api or the application exclusively the memory available, this must not make it fully seamlessly up to the top. With what even if the data is not there? With one until it crashes to make it look like it's full?

Minecraft uses as much as it needs. More important for laggfreies games are CPU and with Shadern the GPU, not the Ram.