Well, your Linux server will require some RAM for system processes so you can't use all of it for your needs
You can check it with this command:
$ free -m
total used free shared buffers cached
Mem: 496 237 259 0 45 102
-/+ buffers/cache: 89 407
Swap: 713 0 713
You can think that available RAM is total RAM minus (free+buffers+cached) because Linux will use free RAM to cache disk I/O if available.
Allocating 900 Mb to the PHP itself is not a good idea in general because you'll need memory for nginx, mysql, etc and most likely you will face the oom killer problem sooner or later.
Take a look at this article to understand how to measure your Linux server performance and resources usage, maybe it will help you.