gistfile1.rb

# Hook into unicorn, unicorn middleware, not rack middleware
#
# Since we need no knowledge about the request we can simply
#  hook unicorn
module Middleware::UnicornOobgc

  MIN_REQUESTS_PER_OOBGC = 5
  MAX_DELTAS = 20

  def self.init
    # hook up HttpServer intercept
    ObjectSpace.each_object(Unicorn::HttpServer) do |s|
      s.extend(self)
    end
  end

  def process_client(client)
    stat = GC.stat

    @previous_deltas ||= []
    @num_requests ||= 0
    @num_requests += 1

    # only track N deltas
    if @previous_deltas.length > MAX_DELTAS
      @previous_deltas.delete_at(0)
    end

    gc_count = stat[:count]
    live_num = stat[:heap_live_num]

    super(client) # Unicorn::HttpServer#process_client

    # at this point client is serviced
    stat = GC.stat
    new_gc_count = stat[:count]
    new_live_num = stat[:heap_live_num]

    # no GC happened during the request
    if new_gc_count == gc_count
      @previous_deltas << (new_live_num - live_num)

      if @gc_live_num && @num_requests > MIN_REQUESTS_PER_OOBGC
        largest = @previous_deltas.max
        if largest * (2 + Random.rand(2)) + new_live_num > @gc_live_num
          GC.start
          @num_requests = 0
        end
      end
    else
      puts "OobGC, GC live num adjusted, GC was not avoided: #{live_num}"
      @gc_live_num = live_num
    end

  end

end

test:

UNICORN_WORKERS=2 DISCOURSE_DISABLE_ANON_CACHE=1 RUBY_GC_MALLOC_LIMIT=20000000 unicorn -E production -c config/unicorn.conf.rb

ab -c 1 -n 300 http://127.0.0.1:3000/

BEFORE:

Percentage of the requests served within a certain time (ms)
50% 33
66% 34
75% 36
80% 37
90% 93
95% 178
98% 180
99% 184
100% 188 (longest request)

AFTER:

Percentage of the requests served within a certain time (ms)
50% 32
66% 34
75% 36
80% 38
90% 42
95% 46
98% 55
99% 81
100% 94 (longest request)

This will trigger a GC when the count of live objects is approaching the count it was at during last time a GC hit. It will track the delta number of objects a request added. If it is ever 2*largest delta of last 20 + random(2) close to the last time it detected a GC it will preempt

It is normal to see worker process count "OobGC, GC live num adjusted, GC was not avoided" on startup and a sporadic message when it needs to re-adjust.

If you are seeing a flood of these messages oobgc is not working probably cause you have your RUBY_GC_MALLOC_LIMIT is set too low.