Leave some room for other processes in unicorn calculation

Currently our calculation for determining how many unicorn workers is this:

default['gitlab']['unicorn']['worker_processes'] = [
  2, # Two is the minimum or HTTP(S) Git pushes will no longer work.                                                                                                                                                                                           
  [
    # Cores + 1 gives good CPU utilization.                                                                                                                                                                                                                    
    node['cpu']['total'].to_i + 1,
    # See how many 300MB worker processes fit in (total RAM - 1GB). We add                                                                                                                                                                                     
    # 128000 KB in the numerator to get rounding instead of integer truncation.                                                                                                                                                                                
    (node['memory']['total'].to_i - 1048576 + 128000) / 358400
  ].min # min because we want to exceed neither CPU nor RAM                                                                                                                                                                                                    
].max # max because we need at least 2 workers

On a 2-core system with 2GB ram, I see that there are 4 processes in total. In this configuration, that generally leaves no headroom for processes to run (e.g. gitlab-shell, du, etc.). I see lots of out-of-memory errors in Sidekiq as a result.

I have a few suggestions:

  1. Get rid of cores + 1 calculation--it should be driven by memory alone
  2. Leave 200-300 MB of headroom for some other processes to run

Thoughts?

/cc: @dblessing, @jacobvosmaer-gitlab, @marin