Completely randomly with Minecraft open, Minecraft will go from using 1GB of RAM up to 12GB in under 30 seconds, lagging my computer extremely hard and making me have to stop it from task manager and/or restart my computer. I've had it happen up to 15 times a day, some days more some days less. It doesn't matter what JVM arguments I have, I have consulted many people about this, or how much RAM I allocate to it. my computer has 16GB of ram total, but Minecraft has never gotten past 12GB, it just tops out at there.