I committed the grave mistake of purchasing a laptop with only 8GB ram and I constantly run out of memory as a result. When it happens, I just repeatedly mash alt+sysrq+f until it kills off some chromium tabs and unfreezes my machine. It essentially behaves like one of those extensions that lets you unload tabs. If needed, you can get the tab back by just reloading the page. The machine slows down to a crawl at 96% usage, and freezes at 97% usage (according to my i3 bar).