Blender cycles rendering layer but keep to sum memory? something i really don't understand!
i made different layer for my render.
i have 5 layers, i set for everyone: layer to render, layer to exclude and eventualy also the mask layer.

if i render from composer editor every single node (clicking in render button in render layer node) i don't have problems!

when i start the render (f12) i have render out memory problems on last renderlayer!

every renderlayer work good but in rendering information i see the memory usage sum to previous renderlayer

example: when renderlayer1 finish mem = 100, when renderlayer2 start mem = 101, when finish mem = 200, when renderlayer3 start mem = 201!!!

in this way my GPU memory go full before finish the rendering!!!

N.B.: the last render layer is rendering just 1 layer, excluding all objects layers (keeping just layer for commons light) and have no mask layer active. if i render just this render layer i give to me 991M Mem usage. when i render this from f12 render i give me out memory over 3200M mem usage!!

i can set something for clear memory every render layer?
This look like bug and all what you can do is report as bug with most accurate info as you can. 

I noticed that Blender have problem with VRAM freeing sometimes. Try save every render layer separately. I recommend use multilayered .exr for keep render passes. Now you can composite everything in new .blend file. Optionally you can use other compositor like Natron or Fuse.

Last time I always use this workflow for more complicated projects. This method give me possibility easily rerender only layers I need later and without rendering everything again. 

I don't thing this is "stressfull way", ultimately this can save you a lot of time. Only you must link your images smartly and work will go fast :-)