class Sidekiq::WorkSet

which can easily get out of sync with crashy processes.
processes but the alternative is a global counter
Not very efficient if you have lots of Sidekiq

which happens every 5 seconds. It is NOT real-time.
Note that #size is only as accurate as Sidekiq's heartbeat,

def size
  Sidekiq.redis do |conn|
    procs = conn.sscan("processes").to_a
    if procs.empty?
      0
    else
      conn.pipelined { |pipeline|
        procs.each do |key|
          pipeline.hget(key, "busy")
        end
      }.sum(&:to_i)
    end
  end
end