twitter-status-bot/.gems/gems/thread_safe-0.3.4/lib/thread_safe/util/striped64.rb

module ThreadSafe
  module Util
    # A Ruby port of the Doug Lea's jsr166e.Striped64 class version 1.6 available in public domain.
    # Original source code available here: http://gee.cs.oswego.edu/cgi-bin/viewcvs.cgi/jsr166/src/jsr166e/Striped64.java?revision=1.6
    #
    # Class holding common representation and mechanics for classes supporting dynamic striping on 64bit values.
    #
    # This class maintains a lazily-initialized table of atomically
    # updated variables, plus an extra +base+ field. The table size
    # is a power of two. Indexing uses masked per-thread hash codes.
    # Nearly all methods on this class are private, accessed directly
    # by subclasses.
    #
    # Table entries are of class +Cell+; a variant of AtomicLong padded
    # to reduce cache contention on most processors. Padding is
    # overkill for most Atomics because they are usually irregularly
    # scattered in memory and thus don't interfere much with each
    # other. But Atomic objects residing in arrays will tend to be
    # placed adjacent to each other, and so will most often share
    # cache lines (with a huge negative performance impact) without
    # this precaution.
    #
    # In part because +Cell+s are relatively large, we avoid creating
    # them until they are needed.  When there is no contention, all
    # updates are made to the +base+ field.  Upon first contention (a
    # failed CAS on +base+ update), the table is initialized to size 2.
    # The table size is doubled upon further contention until
    # reaching the nearest power of two greater than or equal to the
    # number of CPUS. Table slots remain empty (+nil+) until they are
    # needed.
    #
    # A single spinlock (+busy+) is used for initializing and
    # resizing the table, as well as populating slots with new +Cell+s.
    # There is no need for a blocking lock: When the lock is not
    # available, threads try other slots (or the base).  During these
    # retries, there is increased contention and reduced locality,
    # which is still better than alternatives.
    #
    # Per-thread hash codes are initialized to random values.
    # Contention and/or table collisions are indicated by failed
    # CASes when performing an update operation (see method
    # +retry_update+). Upon a collision, if the table size is less than
    # the capacity, it is doubled in size unless some other thread
    # holds the lock. If a hashed slot is empty, and lock is
    # available, a new +Cell+ is created. Otherwise, if the slot
    # exists, a CAS is tried.  Retries proceed by "double hashing",
    # using a secondary hash (XorShift) to try to find a
    # free slot.
    #
    # The table size is capped because, when there are more threads
    # than CPUs, supposing that each thread were bound to a CPU,
    # there would exist a perfect hash function mapping threads to
    # slots that eliminates collisions. When we reach capacity, we
    # search for this mapping by randomly varying the hash codes of
    # colliding threads.  Because search is random, and collisions
    # only become known via CAS failures, convergence can be slow,
    # and because threads are typically not bound to CPUS forever,
    # may not occur at all. However, despite these limitations,
    # observed contention rates are typically low in these cases.
    #
    # It is possible for a +Cell+ to become unused when threads that
    # once hashed to it terminate, as well as in the case where
    # doubling the table causes no thread to hash to it under
    # expanded mask.  We do not try to detect or remove such cells,
    # under the assumption that for long-running instances, observed
    # contention levels will recur, so the cells will eventually be
    # needed again; and for short-lived ones, it does not matter.
    class Striped64
      # Padded variant of AtomicLong supporting only raw accesses plus CAS.
      # The +value+ field is placed between pads, hoping that the JVM doesn't
      # reorder them.
      #
      # Optimisation note: It would be possible to use a release-only
      # form of CAS here, if it were provided.
      class Cell < AtomicReference
        # TODO: this only adds padding after the :value slot, need to find a way to add padding before the slot
        attr_reader *(Array.new(12).map {|i| :"padding_#{i}"})

        alias_method :cas, :compare_and_set

        def cas_computed
          cas(current_value = value, yield(current_value))
        end
      end

      extend Volatile
      attr_volatile :cells, # Table of cells. When non-null, size is a power of 2.
                    :base,  # Base value, used mainly when there is no contention, but also as a fallback during table initialization races. Updated via CAS.
                    :busy   # Spinlock (locked via CAS) used when resizing and/or creating Cells.

      alias_method :busy?, :busy

      def initialize
        super()
        self.busy = false
        self.base = 0
      end

      # Handles cases of updates involving initialization, resizing,
      # creating new Cells, and/or contention. See above for
      # explanation. This method suffers the usual non-modularity
      # problems of optimistic retry code, relying on rechecked sets of
      # reads.
      #
      # Arguments:
      # [+x+]
      #   the value
      # [+hash_code+]
      #   hash code used
      # [+x+]
      #   false if CAS failed before call
      def retry_update(x, hash_code, was_uncontended) # :yields: current_value
        hash     = hash_code
        collided = false # True if last slot nonempty
        while true
          if current_cells = cells
            if !(cell = current_cells.volatile_get_by_hash(hash))
              if busy?
                collided = false
              else # Try to attach new Cell
                if try_to_install_new_cell(Cell.new(x), hash) # Optimistically create and try to insert new cell
                  break
                else
                  redo # Slot is now non-empty
                end
              end
            elsif !was_uncontended # CAS already known to fail
              was_uncontended = true # Continue after rehash
            elsif cell.cas_computed {|current_value| yield current_value}
              break
            elsif current_cells.size >= CPU_COUNT || cells != current_cells # At max size or stale
              collided = false
            elsif collided && expand_table_unless_stale(current_cells)
              collided = false
              redo # Retry with expanded table
            else
              collided = true
            end
            hash = XorShiftRandom.xorshift(hash)

          elsif try_initialize_cells(x, hash) || cas_base_computed {|current_base| yield current_base}
            break
          end
        end
        self.hash_code = hash
      end

      private
      # Static per-thread hash code key. Shared across all instances to
      # reduce Thread locals pollution and because adjustments due to
      # collisions in one table are likely to be appropriate for
      # others.
      THREAD_LOCAL_KEY = "#{name}.hash_code".to_sym

      # A thread-local hash code accessor. The code is initially
      # random, but may be set to a different value upon collisions.
      def hash_code
        Thread.current[THREAD_LOCAL_KEY] ||= XorShiftRandom.get
      end

      def hash_code=(hash)
        Thread.current[THREAD_LOCAL_KEY] = hash
      end

      # Sets base and all +cells+ to the given value.
      def internal_reset(initial_value)
        current_cells = cells
        self.base     = initial_value
        if current_cells
          current_cells.each do |cell|
            cell.value = initial_value if cell
          end
        end
      end

      def cas_base_computed
        cas_base(current_base = base, yield(current_base))
      end

      def free?
        !busy?
      end

      def try_initialize_cells(x, hash)
        if free? && !cells
          try_in_busy do
            unless cells # Recheck under lock
              new_cells = PowerOfTwoTuple.new(2)
              new_cells.volatile_set_by_hash(hash, Cell.new(x))
              self.cells = new_cells
            end
          end
        end
      end

      def expand_table_unless_stale(current_cells)
        try_in_busy do
          if current_cells == cells # Recheck under lock
            new_cells = current_cells.next_in_size_table
            current_cells.each_with_index {|x, i| new_cells.volatile_set(i, x)}
            self.cells = new_cells
          end
        end
      end

      def try_to_install_new_cell(new_cell, hash)
        try_in_busy do
          # Recheck under lock
          if (current_cells = cells) && !current_cells.volatile_get(i = current_cells.hash_to_index(hash))
            current_cells.volatile_set(i, new_cell)
          end
        end
      end

      def try_in_busy
        if cas_busy(false, true)
          begin
            yield
          ensure
            self.busy = false
          end
        end
      end
    end
  end
end