Redmine

"high" is a rather subjective designation. Without specific numbers, it is impossible to verify this nor is it possible to tell you whether your numbers might be normal.

With that being said, it is quite normal to see a single application process of an application server such as unicorn or passenger to use between 256 and 512 MB. Depending on your workload, this might temporarily spike to more than 1 GB per process (e.g. if you have large issues or issue trees, make "expensive" queries, ...

Depending on how you have configured application caches, those might also affect memory usage (by default, Redmine uses an in-memory cache).

To restrict the amount of memory used by your application server, you might use options such as PassengerMemoryLimit for passenger or the Unicorn::WorkerKiller when using unicorn (or an equivalent option of your chosen application server).