Work allocation

work_allocation.rb

# frozen_string_literal: true

class WorkAllocation
  # Runtime process allocations. Last reviewed: 24/9/2019

  # Half a gig system allocation for agents and monitors and, unfortunately,
  # a couple of deploy-time asset compiles due to dependencies.  It's
  # okay to push some processes into swap on smaller instances during
  # deploy.   Ideally we wouldn't compile on-instance at all.
  OPS_MEMORY_ALLOWANCE = 512

  # Our app processes currently hover around 300MiB RSS in production.
  # Allow a little headroom because they sometimes crunch big imports/exports.
  PROCESS_SIZE_ESTIMATE = 350

  def initialize memory
    @system_memory = to_megabytes memory
  end

  # Allocate one background job processor per available GiB
  def job_worker_count
    at_least_one available_memory >> 10
  end

  # Divvy up the rest for application servers
  def app_server_count
    at_least_one (memory_for_app_servers.fdiv PROCESS_SIZE_ESTIMATE).round
  end

  # Nginx is cheap and websockets elevate the demand.
  # Run eight per app server.
  def web_server_count
    app_server_count << 3
  end

  private
    attr_reader :system_memory

    # Ohai, why can't you just give us integers?
    def to_megabytes memory
      case memory.to_s
      when /\A([0-9]+)TB\z/; $1.to_i << 20
      when /\A([0-9]+)GB\z/; $1.to_i << 10
      when /\A([0-9]+)MB\z/; $1.to_i
      when /\A([0-9]+)kB\z/; $1.to_i >> 10
      when /\A([0-9]+)\z/;   $1.to_i >> 20
      else
        raise ArgumentError, "invalid memory value #{memory.inspect}"
      end
    end

    def at_least_one count
      [count.to_i, 1].max
    end

    def available_memory
      system_memory - OPS_MEMORY_ALLOWANCE
    end

    def memory_for_app_servers
      available_memory - job_worker_count * PROCESS_SIZE_ESTIMATE
    end
end