class 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
# File lib/thread_safe/util/striped64.rb, line 89 def initialize super() self.busy = false self.base = 0 end
Public Instance Methods
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
Private Instance Methods
# File lib/thread_safe/util/striped64.rb, line 172 def cas_base_computed cas_base(current_base = base, yield(current_base)) end
# File lib/thread_safe/util/striped64.rb, line 192 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
# File lib/thread_safe/util/striped64.rb, line 176 def free? !busy? end
A thread-local hash code accessor. The code is initially random, but may be set to a different value upon collisions.
# File lib/thread_safe/util/striped64.rb, line 153 def hash_code Thread.current[THREAD_LOCAL_KEY] ||= XorShiftRandom.get end
# File lib/thread_safe/util/striped64.rb, line 157 def hash_code=(hash) Thread.current[THREAD_LOCAL_KEY] = hash end
Sets base and all cells
to the given value.
# File lib/thread_safe/util/striped64.rb, line 162 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
# File lib/thread_safe/util/striped64.rb, line 211 def try_in_busy if cas_busy(false, true) begin yield ensure self.busy = false end end end
# File lib/thread_safe/util/striped64.rb, line 180 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
# File lib/thread_safe/util/striped64.rb, line 202 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