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Enable developer menu by tapping 7 times on the build number under phone information.
Then access the Menu that you will find under settings. You will find the task manager with active task and memory usage.
Post an hardcopy of memory usage please: I am curious. Thanks.
Hi, Bhonder
it is 2.1 of 4 Gb as screen shoot below.
Thanks!
Besides, now I know how to write "three hours" in chinese too! 
I can't find the Memory display in XZ1, apart from using Developer options.
Does anyone know were it went?
Bought my phone a few days ago. In my case, the screenshot was from Settings > System > About phone > Diagnostic.
It's a little trickier now. You have to follow this path:
Config->system->developer option->memory.
Gosh.. Hav less than 20% memory left. But no noticeable lag
@Zulz you shouldn't worry about the memory usage until you notice any lagging or slowing down. the way Android manages memory is different from how Windows does it (i.e. what most people are used to). it keeps as many stuff in memory as possible so you can switch quickly between apps without them reloading every time.