lib/thread_safe/atomic_reference_cache_backend.rb
module ThreadSafe
# A Ruby port of the Doug Lea's jsr166e.ConcurrentHashMapV8 class version 1.59
# available in public domain.
#
# Original source code available here:
# http://gee.cs.oswego.edu/cgi-bin/viewcvs.cgi/jsr166/src/jsr166e/ConcurrentHashMapV8.java?revision=1.59
#
# The Ruby port skips out the +TreeBin+ (red-black trees for use in bins whose
# size exceeds a threshold).
#
# A hash table supporting full concurrency of retrievals and high expected
# concurrency for updates. However, even though all operations are
# thread-safe, retrieval operations do _not_ entail locking, and there is
# _not_ any support for locking the entire table in a way that prevents all
# access.
#
# Retrieval operations generally do not block, so may overlap with update
# operations. Retrievals reflect the results of the most recently _completed_
# update operations holding upon their onset. (More formally, an update
# operation for a given key bears a _happens-before_ relation with any (non
# +nil+) retrieval for that key reporting the updated value.) For aggregate
# operations such as +clear()+, concurrent retrievals may reflect insertion or
# removal of only some entries. Similarly, the +each_pair+ iterator yields
# elements reflecting the state of the hash table at some point at or since
# the start of the +each_pair+. Bear in mind that the results of aggregate
# status methods including +size()+ and +empty?+} are typically useful only
# when a map is not undergoing concurrent updates in other threads. Otherwise
# the results of these methods reflect transient states that may be adequate
# for monitoring or estimation purposes, but not for program control.
#
# The table is dynamically expanded when there are too many collisions (i.e.,
# keys that have distinct hash codes but fall into the same slot modulo the
# table size), with the expected average effect of maintaining roughly two
# bins per mapping (corresponding to a 0.75 load factor threshold for
# resizing). There may be much variance around this average as mappings are
# added and removed, but overall, this maintains a commonly accepted
# time/space tradeoff for hash tables. However, resizing this or any other
# kind of hash table may be a relatively slow operation. When possible, it is
# a good idea to provide a size estimate as an optional :initial_capacity
# initializer argument. An additional optional :load_factor constructor
# argument provides a further means of customizing initial table capacity by
# specifying the table density to be used in calculating the amount of space
# to allocate for the given number of elements. Note that using many keys with
# exactly the same +hash+ is a sure way to slow down performance of any hash
# table.
#
# ## Design overview
#
# The primary design goal of this hash table is to maintain concurrent
# readability (typically method +[]+, but also iteration and related methods)
# while minimizing update contention. Secondary goals are to keep space
# consumption about the same or better than plain +Hash+, and to support high
# initial insertion rates on an empty table by many threads.
#
# Each key-value mapping is held in a +Node+. The validation-based approach
# explained below leads to a lot of code sprawl because retry-control
# precludes factoring into smaller methods.
#
# The table is lazily initialized to a power-of-two size upon the first
# insertion. Each bin in the table normally contains a list of +Node+s (most
# often, the list has only zero or one +Node+). Table accesses require
# volatile/atomic reads, writes, and CASes. The lists of nodes within bins are
# always accurately traversable under volatile reads, so long as lookups check
# hash code and non-nullness of value before checking key equality.
#
# We use the top two bits of +Node+ hash fields for control purposes -- they
# are available anyway because of addressing constraints. As explained further
# below, these top bits are used as follows:
#
# - 00 - Normal
# - 01 - Locked
# - 11 - Locked and may have a thread waiting for lock
# - 10 - +Node+ is a forwarding node
#
# The lower 28 bits of each +Node+'s hash field contain a the key's hash code,
# except for forwarding nodes, for which the lower bits are zero (and so
# always have hash field == +MOVED+).
#
# Insertion (via +[]=+ or its variants) of the first node in an empty bin is
# performed by just CASing it to the bin. This is by far the most common case
# for put operations under most key/hash distributions. Other update
# operations (insert, delete, and replace) require locks. We do not want to
# waste the space required to associate a distinct lock object with each bin,
# so instead use the first node of a bin list itself as a lock. Blocking
# support for these locks relies +Util::CheapLockable. However, we also need a
# +try_lock+ construction, so we overlay these by using bits of the +Node+
# hash field for lock control (see above), and so normally use builtin
# monitors only for blocking and signalling using
# +cheap_wait+/+cheap_broadcast+ constructions. See +Node#try_await_lock+.
#
# Using the first node of a list as a lock does not by itself suffice though:
# When a node is locked, any update must first validate that it is still the
# first node after locking it, and retry if not. Because new nodes are always
# appended to lists, once a node is first in a bin, it remains first until
# deleted or the bin becomes invalidated (upon resizing). However, operations
# that only conditionally update may inspect nodes until the point of update.
# This is a converse of sorts to the lazy locking technique described by
# Herlihy & Shavit.
#
# The main disadvantage of per-bin locks is that other update operations on
# other nodes in a bin list protected by the same lock can stall, for example
# when user +eql?+ or mapping functions take a long time. However,
# statistically, under random hash codes, this is not a common problem.
# Ideally, the frequency of nodes in bins follows a Poisson distribution
# (http://en.wikipedia.org/wiki/Poisson_distribution) with a parameter of
# about 0.5 on average, given the resizing threshold of 0.75, although with a
# large variance because of resizing granularity. Ignoring variance, the
# expected occurrences of list size k are (exp(-0.5) * pow(0.5, k) /
# factorial(k)). The first values are:
#
# - 0: 0.60653066
# - 1: 0.30326533
# - 2: 0.07581633
# - 3: 0.01263606
# - 4: 0.00157952
# - 5: 0.00015795
# - 6: 0.00001316
# - 7: 0.00000094
# - 8: 0.00000006
# - more: less than 1 in ten million
#
# Lock contention probability for two threads accessing distinct elements is
# roughly 1 / (8 * #elements) under random hashes.
#
# The table is resized when occupancy exceeds a percentage threshold
# (nominally, 0.75, but see below). Only a single thread performs the resize
# (using field +size_control+, to arrange exclusion), but the table otherwise
# remains usable for reads and updates. Resizing proceeds by transferring
# bins, one by one, from the table to the next table. Because we are using
# power-of-two expansion, the elements from each bin must either stay at same
# index, or move with a power of two offset. We eliminate unnecessary node
# creation by catching cases where old nodes can be reused because their next
# fields won't change. On average, only about one-sixth of them need cloning
# when a table doubles. The nodes they replace will be garbage collectable as
# soon as they are no longer referenced by any reader thread that may be in
# the midst of concurrently traversing table. Upon transfer, the old table bin
# contains only a special forwarding node (with hash field +MOVED+) that
# contains the next table as its key. On encountering a forwarding node,
# access and update operations restart, using the new table.
#
# Each bin transfer requires its bin lock. However, unlike other cases, a
# transfer can skip a bin if it fails to acquire its lock, and revisit it
# later. Method +rebuild+ maintains a buffer of TRANSFER_BUFFER_SIZE bins that
# have been skipped because of failure to acquire a lock, and blocks only if
# none are available (i.e., only very rarely). The transfer operation must
# also ensure that all accessible bins in both the old and new table are
# usable by any traversal. When there are no lock acquisition failures, this
# is arranged simply by proceeding from the last bin (+table.size - 1+) up
# towards the first. Upon seeing a forwarding node, traversals arrange to move
# to the new table without revisiting nodes. However, when any node is skipped
# during a transfer, all earlier table bins may have become visible, so are
# initialized with a reverse-forwarding node back to the old table until the
# new ones are established. (This sometimes requires transiently locking a
# forwarding node, which is possible under the above encoding.) These more
# expensive mechanics trigger only when necessary.
#
# The traversal scheme also applies to partial traversals of
# ranges of bins (via an alternate Traverser constructor)
# to support partitioned aggregate operations. Also, read-only
# operations give up if ever forwarded to a null table, which
# provides support for shutdown-style clearing, which is also not
# currently implemented.
#
# Lazy table initialization minimizes footprint until first use.
#
# The element count is maintained using a +ThreadSafe::Util::Adder+,
# which avoids contention on updates but can encounter cache thrashing
# if read too frequently during concurrent access. To avoid reading so
# often, resizing is attempted either when a bin lock is
# contended, or upon adding to a bin already holding two or more
# nodes (checked before adding in the +x_if_absent+ methods, after
# adding in others). Under uniform hash distributions, the
# probability of this occurring at threshold is around 13%,
# meaning that only about 1 in 8 puts check threshold (and after
# resizing, many fewer do so). But this approximation has high
# variance for small table sizes, so we check on any collision
# for sizes <= 64. The bulk putAll operation further reduces
# contention by only committing count updates upon these size
# checks.
class AtomicReferenceCacheBackend
class Table < Util::PowerOfTwoTuple
def cas_new_node(i, hash, key, value)
cas(i, nil, Node.new(hash, key, value))
end
def try_to_cas_in_computed(i, hash, key)
succeeded = false
new_value = nil
new_node = Node.new(locked_hash = hash | LOCKED, key, NULL)
if cas(i, nil, new_node)
begin
if NULL == (new_value = yield(NULL))
was_null = true
else
new_node.value = new_value
end
succeeded = true
ensure
volatile_set(i, nil) if !succeeded || was_null
new_node.unlock_via_hash(locked_hash, hash)
end
end
return succeeded, new_value
end
def try_lock_via_hash(i, node, node_hash)
node.try_lock_via_hash(node_hash) do
yield if volatile_get(i) == node
end
end
def delete_node_at(i, node, predecessor_node)
if predecessor_node
predecessor_node.next = node.next
else
volatile_set(i, node.next)
end
end
end
# Key-value entry. Nodes with a hash field of +MOVED+ are special, and do
# not contain user keys or values. Otherwise, keys are never +nil+, and
# +NULL+ +value+ fields indicate that a node is in the process of being
# deleted or created. For purposes of read-only access, a key may be read
# before a value, but can only be used after checking value to be +!= NULL+.
class Node
extend Util::Volatile
attr_volatile :hash, :value, :next
include Util::CheapLockable
bit_shift = Util::FIXNUM_BIT_SIZE - 2 # need 2 bits for ourselves
# Encodings for special uses of Node hash fields. See above for explanation.
MOVED = ('10' << ('0' * bit_shift)).to_i(2) # hash field for forwarding nodes
LOCKED = ('01' << ('0' * bit_shift)).to_i(2) # set/tested only as a bit
WAITING = ('11' << ('0' * bit_shift)).to_i(2) # both bits set/tested together
HASH_BITS = ('00' << ('1' * bit_shift)).to_i(2) # usable bits of normal node hash
SPIN_LOCK_ATTEMPTS = Util::CPU_COUNT > 1 ? Util::CPU_COUNT * 2 : 0
attr_reader :key
def initialize(hash, key, value, next_node = nil)
super()
@key = key
self.lazy_set_hash(hash)
self.lazy_set_value(value)
self.next = next_node
end
# Spins a while if +LOCKED+ bit set and this node is the first of its bin,
# and then sets +WAITING+ bits on hash field and blocks (once) if they are
# still set. It is OK for this method to return even if lock is not
# available upon exit, which enables these simple single-wait mechanics.
#
# The corresponding signalling operation is performed within callers: Upon
# detecting that +WAITING+ has been set when unlocking lock (via a failed
# CAS from non-waiting +LOCKED+ state), unlockers acquire the
# +cheap_synchronize+ lock and perform a +cheap_broadcast+.
def try_await_lock(table, i)
if table && i >= 0 && i < table.size # bounds check, TODO: why are we bounds checking?
spins = SPIN_LOCK_ATTEMPTS
randomizer = base_randomizer = Util::XorShiftRandom.get
while equal?(table.volatile_get(i)) && self.class.locked_hash?(my_hash = hash)
if spins >= 0
if (randomizer = (randomizer >> 1)).even? # spin at random
if (spins -= 1) == 0
Thread.pass # yield before blocking
else
randomizer = base_randomizer = Util::XorShiftRandom.xorshift(base_randomizer) if randomizer.zero?
end
end
elsif cas_hash(my_hash, my_hash | WAITING)
force_aquire_lock(table, i)
break
end
end
end
end
def key?(key)
@key.eql?(key)
end
def matches?(key, hash)
pure_hash == hash && key?(key)
end
def pure_hash
hash & HASH_BITS
end
def try_lock_via_hash(node_hash = hash)
if cas_hash(node_hash, locked_hash = node_hash | LOCKED)
begin
yield
ensure
unlock_via_hash(locked_hash, node_hash)
end
end
end
def locked?
self.class.locked_hash?(hash)
end
def unlock_via_hash(locked_hash, node_hash)
unless cas_hash(locked_hash, node_hash)
self.hash = node_hash
cheap_synchronize { cheap_broadcast }
end
end
private
def force_aquire_lock(table, i)
cheap_synchronize do
if equal?(table.volatile_get(i)) && (hash & WAITING) == WAITING
cheap_wait
else
cheap_broadcast # possibly won race vs signaller
end
end
end
class << self
def locked_hash?(hash)
(hash & LOCKED) != 0
end
end
end
# shorthands
MOVED = Node::MOVED
LOCKED = Node::LOCKED
WAITING = Node::WAITING
HASH_BITS = Node::HASH_BITS
NOW_RESIZING = -1
DEFAULT_CAPACITY = 16
MAX_CAPACITY = Util::MAX_INT
# The buffer size for skipped bins during transfers. The
# value is arbitrary but should be large enough to avoid
# most locking stalls during resizes.
TRANSFER_BUFFER_SIZE = 32
extend Util::Volatile
attr_volatile :table, # The array of bins. Lazily initialized upon first insertion. Size is always a power of two.
# Table initialization and resizing control. When negative, the
# table is being initialized or resized. Otherwise, when table is
# null, holds the initial table size to use upon creation, or 0
# for default. After initialization, holds the next element count
# value upon which to resize the table.
:size_control
def initialize(options = nil)
super()
@counter = Util::Adder.new
initial_capacity = options && options[:initial_capacity] || DEFAULT_CAPACITY
self.size_control = (capacity = table_size_for(initial_capacity)) > MAX_CAPACITY ? MAX_CAPACITY : capacity
end
def get_or_default(key, else_value = nil)
hash = key_hash(key)
current_table = table
while current_table
node = current_table.volatile_get_by_hash(hash)
current_table =
while node
if (node_hash = node.hash) == MOVED
break node.key
elsif (node_hash & HASH_BITS) == hash && node.key?(key) && NULL != (value = node.value)
return value
end
node = node.next
end
end
else_value
end
def [](key)
get_or_default(key)
end
def key?(key)
get_or_default(key, NULL) != NULL
end
def []=(key, value)
get_and_set(key, value)
value
end
def compute_if_absent(key)
hash = key_hash(key)
current_table = table || initialize_table
while true
if !(node = current_table.volatile_get(i = current_table.hash_to_index(hash)))
succeeded, new_value = current_table.try_to_cas_in_computed(i, hash, key) { yield }
if succeeded
increment_size
return new_value
end
elsif (node_hash = node.hash) == MOVED
current_table = node.key
elsif NULL != (current_value = find_value_in_node_list(node, key, hash, node_hash & HASH_BITS))
return current_value
elsif Node.locked_hash?(node_hash)
try_await_lock(current_table, i, node)
else
succeeded, value = attempt_internal_compute_if_absent(key, hash, current_table, i, node, node_hash) { yield }
return value if succeeded
end
end
end
def compute_if_present(key)
new_value = nil
internal_replace(key) do |old_value|
if (new_value = yield(NULL == old_value ? nil : old_value)).nil?
NULL
else
new_value
end
end
new_value
end
def compute(key)
internal_compute(key) do |old_value|
if (new_value = yield(NULL == old_value ? nil : old_value)).nil?
NULL
else
new_value
end
end
end
def merge_pair(key, value)
internal_compute(key) do |old_value|
if NULL == old_value || !(value = yield(old_value)).nil?
value
else
NULL
end
end
end
def replace_pair(key, old_value, new_value)
NULL != internal_replace(key, old_value) { new_value }
end
def replace_if_exists(key, new_value)
if (result = internal_replace(key) { new_value }) && NULL != result
result
end
end
def get_and_set(key, value) # internalPut in the original CHMV8
hash = key_hash(key)
current_table = table || initialize_table
while true
if !(node = current_table.volatile_get(i = current_table.hash_to_index(hash)))
if current_table.cas_new_node(i, hash, key, value)
increment_size
break
end
elsif (node_hash = node.hash) == MOVED
current_table = node.key
elsif Node.locked_hash?(node_hash)
try_await_lock(current_table, i, node)
else
succeeded, old_value = attempt_get_and_set(key, value, hash, current_table, i, node, node_hash)
break old_value if succeeded
end
end
end
def delete(key)
replace_if_exists(key, NULL)
end
def delete_pair(key, value)
result = internal_replace(key, value) { NULL }
if result && NULL != result
!!result
else
false
end
end
def each_pair
return self unless current_table = table
current_table_size = base_size = current_table.size
i = base_index = 0
while base_index < base_size
if node = current_table.volatile_get(i)
if node.hash == MOVED
current_table = node.key
current_table_size = current_table.size
else
begin
if NULL != (value = node.value) # skip deleted or special nodes
yield node.key, value
end
end while node = node.next
end
end
if (i_with_base = i + base_size) < current_table_size
i = i_with_base # visit upper slots if present
else
i = base_index += 1
end
end
self
end
def size
(sum = @counter.sum) < 0 ? 0 : sum # ignore transient negative values
end
def empty?
size == 0
end
# Implementation for clear. Steps through each bin, removing all nodes.
def clear
return self unless current_table = table
current_table_size = current_table.size
deleted_count = i = 0
while i < current_table_size
if !(node = current_table.volatile_get(i))
i += 1
elsif (node_hash = node.hash) == MOVED
current_table = node.key
current_table_size = current_table.size
elsif Node.locked_hash?(node_hash)
decrement_size(deleted_count) # opportunistically update count
deleted_count = 0
node.try_await_lock(current_table, i)
else
current_table.try_lock_via_hash(i, node, node_hash) do
begin
deleted_count += 1 if NULL != node.value # recheck under lock
node.value = nil
end while node = node.next
current_table.volatile_set(i, nil)
i += 1
end
end
end
decrement_size(deleted_count)
self
end
private
# Internal versions of the insertion methods, each a
# little more complicated than the last. All have
# the same basic structure:
# 1. If table uninitialized, create
# 2. If bin empty, try to CAS new node
# 3. If bin stale, use new table
# 4. Lock and validate; if valid, scan and add or update
#
# The others interweave other checks and/or alternative actions:
# * Plain +get_and_set+ checks for and performs resize after insertion.
# * compute_if_absent prescans for mapping without lock (and fails to add
# if present), which also makes pre-emptive resize checks worthwhile.
#
# Someday when details settle down a bit more, it might be worth
# some factoring to reduce sprawl.
def internal_replace(key, expected_old_value = NULL, &block)
hash = key_hash(key)
current_table = table
while current_table
if !(node = current_table.volatile_get(i = current_table.hash_to_index(hash)))
break
elsif (node_hash = node.hash) == MOVED
current_table = node.key
elsif (node_hash & HASH_BITS) != hash && !node.next # precheck
break # rules out possible existence
elsif Node.locked_hash?(node_hash)
try_await_lock(current_table, i, node)
else
succeeded, old_value = attempt_internal_replace(key, expected_old_value, hash, current_table, i, node, node_hash, &block)
return old_value if succeeded
end
end
NULL
end
def attempt_internal_replace(key, expected_old_value, hash, current_table, i, node, node_hash)
current_table.try_lock_via_hash(i, node, node_hash) do
predecessor_node = nil
old_value = NULL
begin
if node.matches?(key, hash) && NULL != (current_value = node.value)
if NULL == expected_old_value || expected_old_value == current_value # NULL == expected_old_value means whatever value
old_value = current_value
if NULL == (node.value = yield(old_value))
current_table.delete_node_at(i, node, predecessor_node)
decrement_size
end
end
break
end
predecessor_node = node
end while node = node.next
return true, old_value
end
end
def find_value_in_node_list(node, key, hash, pure_hash)
do_check_for_resize = false
while true
if pure_hash == hash && node.key?(key) && NULL != (value = node.value)
return value
elsif node = node.next
do_check_for_resize = true # at least 2 nodes -> check for resize
pure_hash = node.pure_hash
else
return NULL
end
end
ensure
check_for_resize if do_check_for_resize
end
def internal_compute(key, &block)
hash = key_hash(key)
current_table = table || initialize_table
while true
if !(node = current_table.volatile_get(i = current_table.hash_to_index(hash)))
succeeded, new_value = current_table.try_to_cas_in_computed(i, hash, key, &block)
if succeeded
if NULL == new_value
break nil
else
increment_size
break new_value
end
end
elsif (node_hash = node.hash) == MOVED
current_table = node.key
elsif Node.locked_hash?(node_hash)
try_await_lock(current_table, i, node)
else
succeeded, new_value = attempt_compute(key, hash, current_table, i, node, node_hash, &block)
break new_value if succeeded
end
end
end
def attempt_internal_compute_if_absent(key, hash, current_table, i, node, node_hash)
added = false
current_table.try_lock_via_hash(i, node, node_hash) do
while true
if node.matches?(key, hash) && NULL != (value = node.value)
return true, value
end
last = node
unless node = node.next
last.next = Node.new(hash, key, value = yield)
added = true
increment_size
return true, value
end
end
end
ensure
check_for_resize if added
end
def attempt_compute(key, hash, current_table, i, node, node_hash)
added = false
current_table.try_lock_via_hash(i, node, node_hash) do
predecessor_node = nil
while true
if node.matches?(key, hash) && NULL != (value = node.value)
if NULL == (node.value = value = yield(value))
current_table.delete_node_at(i, node, predecessor_node)
decrement_size
value = nil
end
return true, value
end
predecessor_node = node
unless node = node.next
if NULL == (value = yield(NULL))
value = nil
else
predecessor_node.next = Node.new(hash, key, value)
added = true
increment_size
end
return true, value
end
end
end
ensure
check_for_resize if added
end
def attempt_get_and_set(key, value, hash, current_table, i, node, node_hash)
node_nesting = nil
current_table.try_lock_via_hash(i, node, node_hash) do
node_nesting = 1
old_value = nil
found_old_value = false
while node
if node.matches?(key, hash) && NULL != (old_value = node.value)
found_old_value = true
node.value = value
break
end
last = node
unless node = node.next
last.next = Node.new(hash, key, value)
break
end
node_nesting += 1
end
return true, old_value if found_old_value
increment_size
true
end
ensure
check_for_resize if node_nesting && (node_nesting > 1 || current_table.size <= 64)
end
def initialize_copy(other)
super
@counter = Util::Adder.new
self.table = nil
self.size_control = (other_table = other.table) ? other_table.size : DEFAULT_CAPACITY
self
end
def try_await_lock(current_table, i, node)
check_for_resize # try resizing if can't get lock
node.try_await_lock(current_table, i)
end
def key_hash(key)
key.hash & HASH_BITS
end
# Returns a power of two table size for the given desired capacity.
def table_size_for(entry_count)
size = 2
size <<= 1 while size < entry_count
size
end
# Initializes table, using the size recorded in +size_control+.
def initialize_table
until current_table ||= table
if (size_ctrl = size_control) == NOW_RESIZING
Thread.pass # lost initialization race; just spin
else
try_in_resize_lock(current_table, size_ctrl) do
initial_size = size_ctrl > 0 ? size_ctrl : DEFAULT_CAPACITY
current_table = self.table = Table.new(initial_size)
initial_size - (initial_size >> 2) # 75% load factor
end
end
end
current_table
end
# If table is too small and not already resizing, creates next table and
# transfers bins. Rechecks occupancy after a transfer to see if another
# resize is already needed because resizings are lagging additions.
def check_for_resize
while (current_table = table) && MAX_CAPACITY > (table_size = current_table.size) && NOW_RESIZING != (size_ctrl = size_control) && size_ctrl < @counter.sum
try_in_resize_lock(current_table, size_ctrl) do
self.table = rebuild(current_table)
(table_size << 1) - (table_size >> 1) # 75% load factor
end
end
end
def try_in_resize_lock(current_table, size_ctrl)
if cas_size_control(size_ctrl, NOW_RESIZING)
begin
if current_table == table # recheck under lock
size_ctrl = yield # get new size_control
end
ensure
self.size_control = size_ctrl
end
end
end
# Moves and/or copies the nodes in each bin to new table. See above for explanation.
def rebuild(table)
old_table_size = table.size
new_table = table.next_in_size_table
# puts "#{old_table_size} -> #{new_table.size}"
forwarder = Node.new(MOVED, new_table, NULL)
rev_forwarder = nil
locked_indexes = nil # holds bins to revisit; nil until needed
locked_arr_idx = 0
bin = old_table_size - 1
i = bin
while true
if !(node = table.volatile_get(i))
# no lock needed (or available) if bin >= 0, because we're not popping values from locked_indexes until we've run through the whole table
redo unless (bin >= 0 ? table.cas(i, nil, forwarder) : lock_and_clean_up_reverse_forwarders(table, old_table_size, new_table, i, forwarder))
elsif Node.locked_hash?(node_hash = node.hash)
locked_indexes ||= Array.new
if bin < 0 && locked_arr_idx > 0
locked_arr_idx -= 1
i, locked_indexes[locked_arr_idx] = locked_indexes[locked_arr_idx], i # swap with another bin
redo
end
if bin < 0 || locked_indexes.size >= TRANSFER_BUFFER_SIZE
node.try_await_lock(table, i) # no other options -- block
redo
end
rev_forwarder ||= Node.new(MOVED, table, NULL)
redo unless table.volatile_get(i) == node && node.locked? # recheck before adding to list
locked_indexes << i
new_table.volatile_set(i, rev_forwarder)
new_table.volatile_set(i + old_table_size, rev_forwarder)
else
redo unless split_old_bin(table, new_table, i, node, node_hash, forwarder)
end
if bin > 0
i = (bin -= 1)
elsif locked_indexes && !locked_indexes.empty?
bin = -1
i = locked_indexes.pop
locked_arr_idx = locked_indexes.size - 1
else
return new_table
end
end
end
def lock_and_clean_up_reverse_forwarders(old_table, old_table_size, new_table, i, forwarder)
# transiently use a locked forwarding node
locked_forwarder = Node.new(moved_locked_hash = MOVED | LOCKED, new_table, NULL)
if old_table.cas(i, nil, locked_forwarder)
new_table.volatile_set(i, nil) # kill the potential reverse forwarders
new_table.volatile_set(i + old_table_size, nil) # kill the potential reverse forwarders
old_table.volatile_set(i, forwarder)
locked_forwarder.unlock_via_hash(moved_locked_hash, MOVED)
true
end
end
# Splits a normal bin with list headed by e into lo and hi parts; installs in given table.
def split_old_bin(table, new_table, i, node, node_hash, forwarder)
table.try_lock_via_hash(i, node, node_hash) do
split_bin(new_table, i, node, node_hash)
table.volatile_set(i, forwarder)
end
end
def split_bin(new_table, i, node, node_hash)
bit = new_table.size >> 1 # bit to split on
run_bit = node_hash & bit
last_run = nil
low = nil
high = nil
current_node = node
# this optimises for the lowest amount of volatile writes and objects created
while current_node = current_node.next
unless (b = current_node.hash & bit) == run_bit
run_bit = b
last_run = current_node
end
end
if run_bit == 0
low = last_run
else
high = last_run
end
current_node = node
until current_node == last_run
pure_hash = current_node.pure_hash
if (pure_hash & bit) == 0
low = Node.new(pure_hash, current_node.key, current_node.value, low)
else
high = Node.new(pure_hash, current_node.key, current_node.value, high)
end
current_node = current_node.next
end
new_table.volatile_set(i, low)
new_table.volatile_set(i + bit, high)
end
def increment_size
@counter.increment
end
def decrement_size(by = 1)
@counter.add(-by)
end
end
end