1 /*------------------------------------------------------------------------
2 Junction: Concurrent data structures in C++
3 Copyright (c) 2016 Jeff Preshing
5 Distributed under the Simplified BSD License.
6 Original location: https://github.com/preshing/junction
8 This software is distributed WITHOUT ANY WARRANTY; without even the
9 implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
10 See the LICENSE file for more information.
11 ------------------------------------------------------------------------*/
13 #ifndef JUNCTION_CONCURRENTMAP_LEAPFROG_H
14 #define JUNCTION_CONCURRENTMAP_LEAPFROG_H
16 #include <junction/Core.h>
17 #include <junction/details/Leapfrog.h>
18 #include <junction/QSBR.h>
19 #include <turf/Heap.h>
20 #include <turf/Trace.h>
24 TURF_TRACE_DECLARE(ConcurrentMap_Leapfrog, 17)
26 template <typename K, typename V, class KT = DefaultKeyTraits<K>, class VT = DefaultValueTraits<V> >
27 class ConcurrentMap_Leapfrog {
32 typedef VT ValueTraits;
33 typedef typename turf::util::BestFit<Key>::Unsigned Hash;
34 typedef details::Leapfrog<ConcurrentMap_Leapfrog> Details;
37 turf::Atomic<typename Details::Table*> m_root;
40 ConcurrentMap_Leapfrog(ureg capacity = Details::InitialSize) : m_root(Details::Table::create(capacity)) {
43 ~ConcurrentMap_Leapfrog() {
44 typename Details::Table* table = m_root.loadNonatomic();
48 // publishTableMigration() is called by exactly one thread from Details::TableMigration::run()
49 // after all the threads participating in the migration have completed their work.
50 void publishTableMigration(typename Details::TableMigration* migration) {
51 // There are no racing calls to this function.
52 typename Details::Table* oldRoot = m_root.loadNonatomic();
53 m_root.store(migration->m_destination, turf::Release);
54 TURF_ASSERT(oldRoot == migration->getSources()[0].table);
55 // Caller will GC the TableMigration and the source table.
58 // A Mutator represents a known cell in the hash table.
59 // It's meant for manipulations within a temporary function scope.
60 // Obviously you must not call QSBR::Update while holding a Mutator.
61 // Any operation that modifies the table (exchangeValue, eraseValue)
62 // may be forced to follow a redirected cell, which changes the Mutator itself.
63 // Note that even if the Mutator was constructed from an existing cell,
64 // exchangeValue() can still trigger a resize if the existing cell was previously marked deleted,
65 // or if another thread deletes the key between the two steps.
68 friend class ConcurrentMap_Leapfrog;
70 ConcurrentMap_Leapfrog& m_map;
71 typename Details::Table* m_table;
72 typename Details::Cell* m_cell;
75 // Constructor: Find existing cell
76 Mutator(ConcurrentMap_Leapfrog& map, Key key, bool) : m_map(map), m_value(Value(ValueTraits::NullValue)) {
77 TURF_TRACE(ConcurrentMap_Leapfrog, 0, "[Mutator] find constructor called", uptr(0), uptr(key));
78 Hash hash = KeyTraits::hash(key);
80 m_table = m_map.m_root.load(turf::Consume);
81 m_cell = Details::find(hash, m_table);
84 Value value = m_cell->value.load(turf::Consume);
85 if (value != Value(ValueTraits::Redirect)) {
86 // Found an existing value
90 // We've encountered a Redirect value. Help finish the migration.
91 TURF_TRACE(ConcurrentMap_Leapfrog, 1, "[Mutator] find was redirected", uptr(m_table), 0);
92 m_table->jobCoordinator.participate();
93 // Try again using the latest root.
97 // Constructor: Insert or find cell
98 Mutator(ConcurrentMap_Leapfrog& map, Key key) : m_map(map), m_value(Value(ValueTraits::NullValue)) {
99 TURF_TRACE(ConcurrentMap_Leapfrog, 2, "[Mutator] insertOrFind constructor called", uptr(0), uptr(key));
100 Hash hash = KeyTraits::hash(key);
102 m_table = m_map.m_root.load(turf::Consume);
104 switch (Details::insertOrFind(hash, m_table, m_cell, overflowIdx)) { // Modifies m_cell
105 case Details::InsertResult_InsertedNew: {
106 // We've inserted a new cell. Don't load m_cell->value.
109 case Details::InsertResult_AlreadyFound: {
110 // The hash was already found in the table.
111 Value value = m_cell->value.load(turf::Consume);
112 if (value == Value(ValueTraits::Redirect)) {
113 // We've encountered a Redirect value.
114 TURF_TRACE(ConcurrentMap_Leapfrog, 3, "[Mutator] insertOrFind was redirected", uptr(m_table), uptr(m_value));
115 break; // Help finish the migration.
117 // Found an existing value
121 case Details::InsertResult_Overflow: {
122 // Unlike ConcurrentMap_Linear, we don't need to keep track of & pass a "mustDouble" flag.
123 // Passing overflowIdx is sufficient to prevent an infinite loop here.
124 // It defines the start of the range of cells to check while estimating total cells in use.
125 // After the first migration, deleted keys are purged, so if we hit this line during the
126 // second loop iteration, every cell in the range will be in use, thus the estimate will be 100%.
127 // (Concurrent deletes could result in further iterations, but it will eventually settle.)
128 Details::beginTableMigration(m_map, m_table, overflowIdx);
132 // A migration has been started (either by us, or another thread). Participate until it's complete.
133 m_table->jobCoordinator.participate();
134 // Try again using the latest root.
139 Value getValue() const {
140 // Return previously loaded value. Don't load it again.
141 return Value(m_value);
144 Value exchangeValue(Value desired) {
145 TURF_ASSERT(desired != Value(ValueTraits::NullValue));
146 TURF_ASSERT(desired != Value(ValueTraits::Redirect));
147 TURF_ASSERT(m_cell); // Cell must have been found or inserted
148 TURF_TRACE(ConcurrentMap_Leapfrog, 4, "[Mutator::exchangeValue] called", uptr(m_table), uptr(m_value));
150 Value oldValue = m_value;
151 if (m_cell->value.compareExchangeStrong(m_value, desired, turf::ConsumeRelease)) {
152 // Exchange was successful. Return previous value.
153 TURF_TRACE(ConcurrentMap_Leapfrog, 5, "[Mutator::exchangeValue] exchanged Value", uptr(m_value),
155 Value result = m_value;
156 m_value = desired; // Leave the mutator in a valid state
159 // The CAS failed and m_value has been updated with the latest value.
160 if (m_value != Value(ValueTraits::Redirect)) {
161 TURF_TRACE(ConcurrentMap_Leapfrog, 6, "[Mutator::exchangeValue] detected race to write value", uptr(m_table),
163 if (oldValue == Value(ValueTraits::NullValue) && m_value != Value(ValueTraits::NullValue)) {
164 TURF_TRACE(ConcurrentMap_Leapfrog, 7, "[Mutator::exchangeValue] racing write inserted new value",
165 uptr(m_table), uptr(m_value));
167 // There was a racing write (or erase) to this cell.
168 // Pretend we exchanged with ourselves, and just let the racing write win.
171 // We've encountered a Redirect value. Help finish the migration.
172 TURF_TRACE(ConcurrentMap_Leapfrog, 8, "[Mutator::exchangeValue] was redirected", uptr(m_table), uptr(m_value));
173 Hash hash = m_cell->hash.load(turf::Relaxed);
175 // Help complete the migration.
176 m_table->jobCoordinator.participate();
177 // Try again in the new table.
178 m_table = m_map.m_root.load(turf::Consume);
179 m_value = Value(ValueTraits::NullValue);
181 switch (Details::insertOrFind(hash, m_table, m_cell, overflowIdx)) { // Modifies m_cell
182 case Details::InsertResult_AlreadyFound:
183 m_value = m_cell->value.load(turf::Consume);
184 if (m_value == Value(ValueTraits::Redirect)) {
185 TURF_TRACE(ConcurrentMap_Leapfrog, 9, "[Mutator::exchangeValue] was re-redirected", uptr(m_table),
190 case Details::InsertResult_InsertedNew:
192 case Details::InsertResult_Overflow:
193 TURF_TRACE(ConcurrentMap_Leapfrog, 10, "[Mutator::exchangeValue] overflow after redirect", uptr(m_table),
195 Details::beginTableMigration(m_map, m_table, overflowIdx);
198 // We were redirected... again
201 // Try again in the new table.
205 void assignValue(Value desired) {
206 exchangeValue(desired);
210 TURF_ASSERT(m_cell); // Cell must have been found or inserted
211 TURF_TRACE(ConcurrentMap_Leapfrog, 11, "[Mutator::eraseValue] called", uptr(m_table), uptr(m_cell));
213 if (m_value == Value(ValueTraits::NullValue))
214 return Value(m_value);
215 TURF_ASSERT(m_cell); // m_value is non-NullValue, therefore cell must have been found or inserted.
216 if (m_cell->value.compareExchangeStrong(m_value, Value(ValueTraits::NullValue), turf::Consume)) {
217 // Exchange was successful and a non-NULL value was erased and returned by reference in m_value.
218 TURF_ASSERT(m_value != ValueTraits::NullValue); // Implied by the test at the start of the loop.
219 Value result = m_value;
220 m_value = Value(ValueTraits::NullValue); // Leave the mutator in a valid state
223 // The CAS failed and m_value has been updated with the latest value.
224 TURF_TRACE(ConcurrentMap_Leapfrog, 12, "[Mutator::eraseValue] detected race to write value", uptr(m_table),
226 if (m_value != Value(ValueTraits::Redirect)) {
227 // There was a racing write (or erase) to this cell.
228 // Pretend we erased nothing, and just let the racing write win.
229 return Value(ValueTraits::NullValue);
231 // We've been redirected to a new table.
232 TURF_TRACE(ConcurrentMap_Leapfrog, 13, "[Mutator::eraseValue] was redirected", uptr(m_table), uptr(m_cell));
233 Hash hash = m_cell->hash.load(turf::Relaxed); // Re-fetch hash
235 // Help complete the migration.
236 m_table->jobCoordinator.participate();
237 // Try again in the new table.
238 m_table = m_map.m_root.load(turf::Consume);
239 m_cell = Details::find(hash, m_table);
241 m_value = Value(ValueTraits::NullValue);
244 m_value = m_cell->value.load(turf::Relaxed);
245 if (m_value != Value(ValueTraits::Redirect))
247 TURF_TRACE(ConcurrentMap_Leapfrog, 14, "[Mutator::eraseValue] was re-redirected", uptr(m_table),
254 Mutator insertOrFind(Key key) {
255 return Mutator(*this, key);
258 Mutator find(Key key) {
259 return Mutator(*this, key, false);
262 // Lookup without creating a temporary Mutator.
264 Hash hash = KeyTraits::hash(key);
265 TURF_TRACE(ConcurrentMap_Leapfrog, 15, "[get] called", uptr(this), uptr(hash));
267 typename Details::Table* table = m_root.load(turf::Consume);
268 typename Details::Cell* cell = Details::find(hash, table);
270 return Value(ValueTraits::NullValue);
271 Value value = cell->value.load(turf::Consume);
272 if (value != Value(ValueTraits::Redirect))
273 return value; // Found an existing value
274 // We've been redirected to a new table. Help with the migration.
275 TURF_TRACE(ConcurrentMap_Leapfrog, 16, "[get] was redirected", uptr(table), uptr(hash));
276 table->jobCoordinator.participate();
277 // Try again in the new table.
281 Value assign(Key key, Value desired) {
282 Mutator iter(*this, key);
283 return iter.exchangeValue(desired);
286 Value exchange(Key key, Value desired) {
287 Mutator iter(*this, key);
288 return iter.exchangeValue(desired);
291 Value erase(Key key) {
292 Mutator iter(*this, key, false);
293 return iter.eraseValue();
296 // The easiest way to implement an Iterator is to prevent all Redirects.
297 // The currrent Iterator does that by forbidding concurrent inserts.
298 // To make it work with concurrent inserts, we'd need a way to block TableMigrations.
301 typename Details::Table* m_table;
307 Iterator(ConcurrentMap_Leapfrog& map) {
308 // Since we've forbidden concurrent inserts (for now), nonatomic would suffice here, but let's plan ahead:
309 m_table = map.m_root.load(turf::Consume);
315 TURF_ASSERT(m_table);
316 TURF_ASSERT(isValid() || m_idx == -1); // Either the Iterator is already valid, or we've just started iterating.
317 while (++m_idx <= m_table->sizeMask) {
318 // Index still inside range of table.
319 typename Details::CellGroup* group = m_table->getCellGroups() + (m_idx >> 2);
320 typename Details::Cell* cell = group->cells + (m_idx & 3);
321 m_hash = cell->hash.load(turf::Relaxed);
322 if (m_hash != KeyTraits::NullHash) {
323 // Cell has been reserved.
324 m_value = cell->value.load(turf::Relaxed);
325 TURF_ASSERT(m_value != Value(ValueTraits::Redirect));
326 if (m_value != Value(ValueTraits::NullValue))
327 return; // Yield this cell.
330 // That's the end of the map.
331 m_hash = KeyTraits::NullHash;
332 m_value = Value(ValueTraits::NullValue);
335 bool isValid() const {
336 return m_value != Value(ValueTraits::NullValue);
340 TURF_ASSERT(isValid());
341 // Since we've forbidden concurrent inserts (for now), nonatomic would suffice here, but let's plan ahead:
342 return KeyTraits::dehash(m_hash);
345 Value getValue() const {
346 TURF_ASSERT(isValid());
352 } // namespace junction
354 #endif // JUNCTION_CONCURRENTMAP_LEAPFROG_H