ruby-thread-safe 0.3.6-1 / usr / lib / ruby / vendor_ruby / 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