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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
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