Memory leak causing lag

Minecraft leaks memory, even when idle.
(attached animated gif shows what's happening)

What I expected to happen was...:
Minecraft not using so much memory when no chunks are being updated, or the player moving at all.

What actually happened was...:
The memory-use builds up from 20~% to 56~% and then resets back to 20~% and repeats.
The rapid memory use causes lag that makes movement harder.

Steps to Reproduce:
Load a world in 14w17a and play as normal