junction/details/Linear.h

   1 /*------------------------------------------------------------------------
   2   Junction: Concurrent data structures in C++
   3   Copyright (c) 2016 Jeff Preshing
   4
   5   Distributed under the Simplified BSD License.
   6   Original location: https://github.com/preshing/junction
   7
   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 ------------------------------------------------------------------------*/
  12
  13 #ifndef JUNCTION_DETAILS_LINEAR_H
  14 #define JUNCTION_DETAILS_LINEAR_H
  15
  16 #include <junction/Core.h>
  17 #include <turf/Atomic.h>
  18 #include <turf/Mutex.h>
  19 #include <turf/ManualResetEvent.h>
  20 #include <turf/Util.h>
  21 #include <junction/MapTraits.h>
  22 #include <turf/Trace.h>
  23 #include <turf/Heap.h>
  24 #include <junction/SimpleJobCoordinator.h>
  25 #include <junction/QSBR.h>
  26
  27 namespace junction {
  28 namespace details {
  29
  30 TURF_TRACE_DECLARE(Linear, 22)
  31
  32 template <class Map>
  33 struct Linear {
  34     typedef typename Map::Hash Hash;
  35     typedef typename Map::Value Value;
  36     typedef typename Map::KeyTraits KeyTraits;
  37     typedef typename Map::ValueTraits ValueTraits;
  38
  39     static const ureg InitialSize = 8;
  40     static const ureg TableMigrationUnitSize = 32;
  41     static const ureg LinearSearchLimit = 128;
  42     static const ureg CellsInUseSample = LinearSearchLimit;
  43     TURF_STATIC_ASSERT(LinearSearchLimit > 0 && LinearSearchLimit < 256);              // Must fit in CellGroup::links
  44     TURF_STATIC_ASSERT(CellsInUseSample > 0 && CellsInUseSample <= LinearSearchLimit); // Limit sample to failed search chain
  45
  46     struct Cell {
  47         turf::Atomic<Hash> hash;
  48         turf::Atomic<Value> value;
  49     };
  50
  51     struct Table {
  52         const ureg sizeMask; // a power of two minus one
  53         const ureg limitNumValues;
  54         turf::Atomic<sreg> cellsRemaining;
  55         turf::Atomic<sreg> valuesRemaining;
  56         turf::Mutex mutex;                   // to DCLI the TableMigration (stored in the jobCoordinator)
  57         SimpleJobCoordinator jobCoordinator; // makes all blocked threads participate in the migration
  58
  59         Table(ureg sizeMask, ureg limitNumValues)
  60             : sizeMask(sizeMask), limitNumValues(limitNumValues), cellsRemaining(limitNumValues),
  61               valuesRemaining(limitNumValues) {
  62         }
  63
  64         static Table* create(ureg tableSize, ureg limitNumValues) {
  65             TURF_ASSERT(turf::util::isPowerOf2(tableSize));
  66             Table* table = (Table*) TURF_HEAP.alloc(sizeof(Table) + sizeof(Cell) * tableSize);
  67             new (table) Table(tableSize - 1, limitNumValues);
  68             for (ureg j = 0; j < tableSize; j++) {
  69                 table->getCells()[j].hash.storeNonatomic(KeyTraits::NullHash);
  70                 table->getCells()[j].value.storeNonatomic(Value(ValueTraits::NullValue));
  71             }
  72             return table;
  73         }
  74
  75         void destroy() {
  76             this->Table::~Table();
  77             TURF_HEAP.free(this);
  78         }
  79
  80         Cell* getCells() const {
  81             return (Cell*) (this + 1);
  82         }
  83
  84         ureg getNumMigrationUnits() const {
  85             return sizeMask / TableMigrationUnitSize + 1;
  86         }
  87     };
  88
  89     class TableMigration : public SimpleJobCoordinator::Job {
  90     public:
  91         Map& m_map;
  92         Table* m_source;
  93         turf::Atomic<ureg> m_sourceIndex;
  94         Table* m_destination;
  95         turf::Atomic<ureg> m_workerStatus; // number of workers + end flag
  96         turf::Atomic<sreg> m_unitsRemaining;
  97
  98         TableMigration(Map& map) : m_map(map), m_sourceIndex(0), m_workerStatus(0), m_unitsRemaining(0) {
  99             // Caller is responsible for filling in source & destination
 100         }
 101
 102         virtual ~TableMigration() TURF_OVERRIDE {
 103             // Destroy source table.
 104             m_source->destroy();
 105         }
 106
 107         void destroy() {
 108             delete this;
 109         }
 110
 111         bool migrateRange(ureg startIdx);
 112         virtual void run() TURF_OVERRIDE;
 113     };
 114
 115     static Cell* find(Hash hash, Table* table) {
 116         TURF_TRACE(Linear, 0, "[find] called", uptr(table), hash);
 117         TURF_ASSERT(table);
 118         TURF_ASSERT(hash != KeyTraits::NullHash);
 119         ureg sizeMask = table->sizeMask;
 120         for (ureg idx = hash;; idx++) {
 121             idx &= sizeMask;
 122             Cell* cell = table->getCells() + idx;
 123             // Load the hash that was there.
 124             uptr probeHash = cell->hash.load(turf::Relaxed);
 125             if (probeHash == hash) {
 126                 TURF_TRACE(Linear, 1, "[find] found existing cell", uptr(table), idx);
 127                 return cell;
 128             } else if (probeHash == KeyTraits::NullHash) {
 129                 return NULL;
 130             }
 131         }
 132     }
 133
 134     // FIXME: Possible optimization: Dedicated insert for migration? It wouldn't check for InsertResult_AlreadyFound.
 135     enum InsertResult { InsertResult_AlreadyFound, InsertResult_InsertedNew, InsertResult_Overflow };
 136     static InsertResult insert(Hash hash, Table* table, Cell*& cell) {
 137         TURF_TRACE(Linear, 2, "[insert] called", uptr(table), hash);
 138         TURF_ASSERT(table);
 139         TURF_ASSERT(hash != KeyTraits::NullHash);
 140         ureg sizeMask = table->sizeMask;
 141
 142         for (ureg idx = hash;; idx++) {
 143             idx &= sizeMask;
 144             cell = table->getCells() + idx;
 145             // Load the existing hash.
 146             uptr probeHash = cell->hash.load(turf::Relaxed);
 147             if (probeHash == hash) {
 148                 TURF_TRACE(Linear, 3, "[insert] found existing cell", uptr(table), idx);
 149                 return InsertResult_AlreadyFound; // Key found in table. Return the existing cell.
 150             }
 151             if (probeHash == KeyTraits::NullHash) {
 152                 // It's an empty cell. Try to reserve it.
 153                 // But first, decrement cellsRemaining to ensure we have permission to create new getCells().
 154                 s32 prevCellsRemaining = table->cellsRemaining.fetchSub(1, turf::Relaxed);
 155                 if (prevCellsRemaining <= 0) {
 156                     // Table is overpopulated.
 157                     TURF_TRACE(Linear, 4, "[insert] ran out of cellsRemaining", prevCellsRemaining, 0);
 158                     table->cellsRemaining.fetchAdd(1, turf::Relaxed); // Undo cellsRemaining decrement
 159                     return InsertResult_Overflow;
 160                 }
 161                 // Try to reserve this cell.
 162                 uptr prevHash = cell->hash.compareExchange(KeyTraits::NullHash, hash, turf::Relaxed);
 163                 if (prevHash == KeyTraits::NullHash) {
 164                     // Success. We reserved a new cell.
 165                     TURF_TRACE(Linear, 5, "[insert] reserved cell", prevCellsRemaining, idx);
 166                     return InsertResult_InsertedNew;
 167                 }
 168                 // There was a race and another thread reserved that cell from under us.
 169                 TURF_TRACE(Linear, 6, "[insert] detected race to reserve cell", ureg(hash), idx);
 170                 table->cellsRemaining.fetchAdd(1, turf::Relaxed); // Undo cellsRemaining decrement
 171                 if (prevHash == hash) {
 172                     TURF_TRACE(Linear, 7, "[insert] race reserved same hash", ureg(hash), idx);
 173                     return InsertResult_AlreadyFound; // They inserted the same key. Return the existing cell.
 174                 }
 175             }
 176             // Try again in the next cell.
 177         }
 178     }
 179
 180     static void beginTableMigration(Map& map, Table* table) {
 181         // Create new migration by DCLI.
 182         TURF_TRACE(Linear, 8, "[beginTableMigration] called", 0, 0);
 183         SimpleJobCoordinator::Job* job = table->jobCoordinator.loadConsume();
 184         if (job) {
 185             TURF_TRACE(Linear, 9, "[beginTableMigration] new migration already exists", 0, 0);
 186         } else {
 187             turf::LockGuard<turf::Mutex> guard(table->mutex);
 188             job = table->jobCoordinator.loadConsume(); // Non-atomic would be sufficient, but that's OK.
 189             if (job) {
 190                 TURF_TRACE(Linear, 10, "[beginTableMigration] new migration already exists (double-checked)", 0, 0);
 191             } else {
 192                 // Determine new migration size and cap the number of values that can be added concurrent to the migration.
 193                 sreg oldValuesLimit = table->limitNumValues;
 194                 sreg oldValuesRemaining = table->valuesRemaining.load(turf::Relaxed);
 195                 sreg oldValuesInUse = oldValuesLimit - oldValuesRemaining;
 196             calculateNextTableSize:
 197                 sreg nextTableSize = turf::util::roundUpPowerOf2(oldValuesInUse * 2);
 198                 sreg nextLimitNumValues = nextTableSize * 3 / 4;
 199                 if (nextLimitNumValues < oldValuesLimit) {
 200                     // Set the new limitNumValues on the *current* table.
 201                     // This prevents other threads, while the migration is in progress, from concurrently
 202                     // re-inserting more values than the new table can hold.
 203                     // To set the new limitNumValues on the current table in an atomic fashion,
 204                     // we update its valuesRemaining via CAS loop:
 205                     for (;;) {
 206                         // We must recalculate desiredValuesRemaining on each iteration of the CAS loop
 207                         oldValuesInUse = oldValuesLimit - oldValuesRemaining;
 208                         sreg desiredValuesRemaining = nextLimitNumValues - oldValuesInUse;
 209                         if (desiredValuesRemaining < 0) {
 210                             TURF_TRACE(Linear, 11, "[table] restarting valuesRemaining CAS loop", nextLimitNumValues,
 211                                        desiredValuesRemaining);
 212                             // Must recalculate nextTableSize. Goto, baby!
 213                             goto calculateNextTableSize;
 214                         }
 215                         if (table->valuesRemaining.compareExchangeWeak(oldValuesRemaining, desiredValuesRemaining, turf::Relaxed,
 216                                                                        turf::Relaxed))
 217                             break; // Success!
 218                         // CAS failed because table->valuesRemaining was modified by another thread.
 219                         // An updated value has been reloaded into oldValuesRemaining (modified by reference).
 220                         // Recalculate desiredValuesRemaining to account for the updated value, and try again.
 221                         TURF_TRACE(Linear, 12, "[table] valuesRemaining CAS failed", oldValuesRemaining, desiredValuesRemaining);
 222                     }
 223                 }
 224                 // Now we are assured that the new table will not become overpopulated during the migration process.
 225                 // Create new migration.
 226                 TableMigration* migration = new TableMigration(map);
 227                 migration->m_source = table;
 228                 migration->m_destination = Table::create(nextTableSize, nextLimitNumValues);
 229                 migration->m_unitsRemaining.storeNonatomic(table->getNumMigrationUnits());
 230                 // Publish the new migration.
 231                 table->jobCoordinator.storeRelease(migration);
 232             }
 233         }
 234     }
 235 }; // Linear
 236
 237 template <class Map>
 238 bool Linear<Map>::TableMigration::migrateRange(ureg startIdx) {
 239     ureg srcSizeMask = m_source->sizeMask;
 240     ureg endIdx = turf::util::min(startIdx + TableMigrationUnitSize, srcSizeMask + 1);
 241     sreg valuesMigrated = 0;
 242     // Iterate over source range.
 243     for (ureg srcIdx = startIdx; srcIdx < endIdx; srcIdx++) {
 244         Cell* srcCell = m_source->getCells() + (srcIdx & srcSizeMask);
 245         Hash srcHash;
 246         Value srcValue;
 247         // Fetch the srcHash and srcValue.
 248         for (;;) {
 249             srcHash = srcCell->hash.load(turf::Relaxed);
 250             if (srcHash == KeyTraits::NullHash) {
 251                 // An unused cell. Try to put a Redirect marker in its value.
 252                 srcValue =
 253                     srcCell->value.compareExchange(Value(ValueTraits::NullValue), Value(ValueTraits::Redirect), turf::Relaxed);
 254                 if (srcValue == Value(ValueTraits::Redirect)) {
 255                     // srcValue is already marked Redirect due to previous incomplete migration.
 256                     TURF_TRACE(Linear, 13, "[migrateRange] empty cell already redirected", uptr(m_source), srcIdx);
 257                     break;
 258                 }
 259                 if (srcValue == Value(ValueTraits::NullValue))
 260                     break; // Redirect has been placed. Break inner loop, continue outer loop.
 261                 TURF_TRACE(Linear, 14, "[migrateRange] race to insert key", uptr(m_source), srcIdx);
 262                 // Otherwise, somebody just claimed the cell. Read srcHash again...
 263             } else {
 264                 // Check for deleted/uninitialized value.
 265                 srcValue = srcCell->value.load(turf::Relaxed);
 266                 if (srcValue == Value(ValueTraits::NullValue)) {
 267                     // Try to put a Redirect marker.
 268                     if (srcCell->value.compareExchangeStrong(srcValue, Value(ValueTraits::Redirect), turf::Relaxed))
 269                         break; // Redirect has been placed. Break inner loop, continue outer loop.
 270                     TURF_TRACE(Linear, 15, "[migrateRange] race to insert value", uptr(m_source), srcIdx);
 271                 }
 272
 273                 // We've got a key/value pair to migrate.
 274                 // Reserve a destination cell in the destination.
 275                 TURF_ASSERT(srcHash != KeyTraits::NullHash);
 276                 TURF_ASSERT(srcValue != Value(ValueTraits::NullValue));
 277                 TURF_ASSERT(srcValue != Value(ValueTraits::Redirect)); // Incomplete/concurrent migrations are impossible.
 278                 Cell* dstCell;
 279                 InsertResult result = insert(srcHash, m_destination, dstCell);
 280                 // During migration, a hash can only exist in one place among all the source tables,
 281                 // and it is only migrated by one thread. Therefore, the hash will never already exist
 282                 // in the destination table:
 283                 TURF_ASSERT(result != InsertResult_AlreadyFound);
 284                 TURF_ASSERT(result != InsertResult_Overflow);
 285                 // Migrate the old value to the new cell.
 286                 for (;;) {
 287                     // Copy srcValue to the destination.
 288                     dstCell->value.store(srcValue, turf::Relaxed);
 289                     // Try to place a Redirect marker in srcValue.
 290                     Value doubleCheckedSrcValue =
 291                         srcCell->value.compareExchange(srcValue, Value(ValueTraits::Redirect), turf::Relaxed);
 292                     TURF_ASSERT(doubleCheckedSrcValue !=
 293                                 Value(ValueTraits::Redirect)); // Only one thread can redirect a cell at a time.
 294                     if (doubleCheckedSrcValue == srcValue) {
 295                         // No racing writes to the src. We've successfully placed the Redirect marker.
 296                         // srcValue was non-NULL when we decided to migrate it, but it may have changed to NULL
 297                         // by a late-arriving erase.
 298                         if (srcValue == Value(ValueTraits::NullValue))
 299                             TURF_TRACE(Linear, 16, "[migrateRange] racing update was erase", uptr(m_source), srcIdx);
 300                         else
 301                             valuesMigrated++;
 302                         break;
 303                     }
 304                     // There was a late-arriving write (or erase) to the src. Migrate the new value and try again.
 305                     TURF_TRACE(Linear, 17, "[migrateRange] race to update migrated value", uptr(m_source), srcIdx);
 306                     srcValue = doubleCheckedSrcValue;
 307                 }
 308                 // Cell successfully migrated. Proceed to next source cell.
 309                 break;
 310             }
 311         }
 312     }
 313     sreg prevValuesRemaining = m_destination->valuesRemaining.fetchSub(valuesMigrated, turf::Relaxed);
 314     TURF_ASSERT(valuesMigrated <= prevValuesRemaining);
 315     TURF_UNUSED(prevValuesRemaining);
 316     // Range has been migrated successfully.
 317     return true;
 318 }
 319
 320 template <class Map>
 321 void Linear<Map>::TableMigration::run() {
 322     // Conditionally increment the shared # of workers.
 323     ureg probeStatus = m_workerStatus.load(turf::Relaxed);
 324     do {
 325         if (probeStatus & 1) {
 326             // End flag is already set, so do nothing.
 327             TURF_TRACE(Linear, 18, "[TableMigration::run] already ended", uptr(this), 0);
 328             return;
 329         }
 330     } while (!m_workerStatus.compareExchangeWeak(probeStatus, probeStatus + 2, turf::Relaxed, turf::Relaxed));
 331     // # of workers has been incremented, and the end flag is clear.
 332     TURF_ASSERT((probeStatus & 1) == 0);
 333
 334     // Loop over all migration units in the source table.
 335     for (;;) {
 336         if (m_workerStatus.load(turf::Relaxed) & 1) {
 337             TURF_TRACE(Linear, 19, "[TableMigration::run] detected end flag set", uptr(this), 0);
 338             goto endMigration;
 339         }
 340         ureg startIdx = m_sourceIndex.fetchAdd(TableMigrationUnitSize, turf::Relaxed);
 341         if (startIdx >= m_source->sizeMask + 1)
 342             break; // No more migration units.
 343         migrateRange(startIdx);
 344         sreg prevRemaining = m_unitsRemaining.fetchSub(1, turf::Relaxed);
 345         TURF_ASSERT(prevRemaining > 0);
 346         if (prevRemaining == 1) {
 347             // That was the last chunk to migrate.
 348             m_workerStatus.fetchOr(1, turf::Relaxed);
 349             goto endMigration;
 350         }
 351     }
 352     TURF_TRACE(Linear, 20, "[TableMigration::run] out of migration units", uptr(this), 0);
 353
 354 endMigration:
 355     // Decrement the shared # of workers.
 356     probeStatus = m_workerStatus.fetchSub(
 357         2, turf::AcquireRelease); // AcquireRelease makes all previous writes visible to the last worker thread.
 358     if (probeStatus >= 4) {
 359         // There are other workers remaining. Return here so that only the very last worker will proceed.
 360         TURF_TRACE(Linear, 21, "[TableMigration::run] not the last worker", uptr(this), uptr(probeStatus));
 361         return;
 362     }
 363
 364     // We're the very last worker thread.
 365     // Publish the new subtree.
 366     TURF_ASSERT(probeStatus == 3);
 367     m_map.publishTableMigration(this);
 368     // End the jobCoodinator.
 369     m_source->jobCoordinator.end();
 370
 371     // We're done with this TableMigration. Queue it for GC.
 372     DefaultQSBR.enqueue(&TableMigration::destroy, this);
 373 }
 374
 375 } // namespace details
 376 } // namespace junction
 377
 378 #endif // JUNCTION_DETAILS_LINEAR_H