Problem with Highres Compositing

Hi, I have to do a highres rendering with masks and need to use cryptomatte. The color pass, AO and so on without cryptomatte and compositing renders with no problems. As soon as I use compositing and cryptomatte my RAM Memory explodes. (64 GB) I need about 7 masks and I realy know it needs a lot of computing power and memory because of cryptomattes and/or compositing stuff. My system is not that bad but the obvious problem to me is that blender (latest 2.8) does fill up ram memory and does not work anymore when this 64Gb are reached. No Ram outsourcing to harddisks happens like it should and does with other software. Is there something I need to setup or missed in settings or is 2.8 not ready for ultra Highres Compositing? I realy mean compositing Ram Memory usage. It is not working when Ram memory is full. 2.8 does not outsource memory when compositing becomes more complex or highres. btw it is a 18k image... rendering is great when I dont use cryptomatte or compositing stuff.... hmmmppff ;) thanks in advance for any tips or tricks or workarrounds