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- lib/thread_safe/util/striped64.rb

### Parent

Object

### Namespace

- CLASS ThreadSafe::Util::Striped64::Cell

### Methods

- ::new
- #retry_update

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# ThreadSafe::Util::Striped64

A Ruby port of the Doug Lea's jsr166e.Striped64 class version 1.6 available in public domain.

Original source code available here: 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.

### Constants

THREAD_LOCAL_KEY

  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.

### Public Class Methods

new() click to toggle source

    # File lib/thread_safe/util/striped64.rb, line 89
    def initialize
      super()
      self.busy = false
      self.base = 0
    end

### Public Instance Methods

retry_update(x, hash_code, was_uncontended) click to toggle source

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

    # File lib/thread_safe/util/striped64.rb, line 108
    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

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