ConcurrentHashmapImpl.icc

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/*
 * Copyright (C) 2002-2025 CERN for the benefit of the ATLAS collaboration.
 */
/**
 * @file CxxUtils/ConcurrentHashmapImpl.h
 * @author scott snyder <snyder@bnl.gov>
 * @date Dec, 2020
 * @brief Hash table allowing concurrent, lockless reads.
 */
 
 
#include <cassert>
#include <bit>
 
 
namespace CxxUtils {
namespace detail {
 
 
/**
 * @brief Constructor.
 * @param hash The hash of the entry we're looking for.
 * @param mask Table mask; i.e., the table capacity - 1.
 * @param maskBits Number of 1 bits in mask.
 * @param probeLimit Maximum number of probes to try before failing.
 */
template <unsigned ENTRIES_PER_CACHELINE>
inline
CHMTableIterator<ENTRIES_PER_CACHELINE>::CHMTableIterator
  (size_t hash, size_t mask, size_t maskBits, size_t probeLimit)
    // Mask limited to the table.  Also mask off the part within a cache line.
  : m_mask ((mask & ~ENTRIES_PER_CACHELINE_MASK)),
    // Offset of hash within a cache line.
    m_boffs (hash & ENTRIES_PER_CACHELINE_MASK),
    // Starting increment between hash probes.
    // Must be large enough to go to a new cache line, which is why
    // we or in ENTRIES_PER_CACHELINE.
    m_stride (((hash >> maskBits) | hash | ENTRIES_PER_CACHELINE) & ENTRIES_PER_CACHELINE_MASK),
    m_probeLimit (probeLimit),
    // Index at the start of the cache line containing hash.
    m_bucket (hash & m_mask),
    m_nprobes (0)
{
}
 
 
/**
 * @brief Offset of the element currently being probed.
 */
template <unsigned ENTRIES_PER_CACHELINE>
inline
size_t CHMTableIterator<ENTRIES_PER_CACHELINE>::offset() const
{
  // m_bucket the starting index of the current cache line.
  // Count within the cache line in a circular manner starting
  // at m_boffs.
  return m_bucket + ((m_nprobes + m_boffs)&ENTRIES_PER_CACHELINE_MASK);
}
 
 
/**
 * @brief Return the number of probes performed so far.
 */
template <unsigned ENTRIES_PER_CACHELINE>
inline
size_t CHMTableIterator<ENTRIES_PER_CACHELINE>::nprobes() const
{
  return m_nprobes;
}
 
 
/**
 * @brief Move to the next probe.
 * Returns true if we should continue, or false if we've hit the maximum
 * number of probes.
 */
template <unsigned ENTRIES_PER_CACHELINE>
inline
bool CHMTableIterator<ENTRIES_PER_CACHELINE>::next()
{
  // Increment number of probes and stop if we've hit the maximum.
  if (++m_nprobes >= m_probeLimit) {
    return false;
  }
  // We've finished a cache line if the low bits are back to 0.
  if ((m_nprobes & ENTRIES_PER_CACHELINE_MASK) == 0) {
    // Move to a different cacheline.  
    // cf. knuth AOCP exercise 6.4.20.
    // By construction, the low bits (within the cacheline) of
    // m_bucket, m_nprobes, and m_stride should all be 0.
    m_bucket = (m_bucket + m_nprobes + m_stride) & m_mask;
  }
  return true;
}
 
 
//*****************************************************************************
 
 
#define T_CHMIMPL \
  template <template <class> class UPDATER_,     \
            typename HASHER_,                    \
            typename MATCHER_,                   \
            uintptr_t NULLVAL_,                  \
            uintptr_t TOMBSTONE_>
 
#define CHMIMPL ConcurrentHashmapImpl<UPDATER_, HASHER_, MATCHER_, NULLVAL_, TOMBSTONE_>
 
 
/**
 * @brief Constructor.
 * @param capacity Number of entries in the table.  Must be a power of 2.
 * @param hasher Hash object to use.
 * @param matcher Key match object to use.
 */
T_CHMIMPL
CHMIMPL::Table::Table (size_t capacity,
                       const Hasher_t& hasher /*= Hasher_t()*/,
                       const Matcher_t& matcher /*= Matcher_t()*/)
  : m_capacity (capacity),
    m_maxProbe (capacity / 4),
    m_mask (capacity-1),
    m_maskBits (std::countr_zero (capacity)),
    m_hasher (hasher),
    m_matcher (matcher),
    m_longestProbe (0)
{
  // Clear all the keys.
  for (size_t i = 0; i < capacity; i++) {
    m_entries[i].m_key = nullval;
  }
}
 
 
/**
 * @brief Allocator for table objects.
 * @param capacity Size of the table (must be a power of 2).
 *
 * Allocate with enough space for the table of entries.
 * Also align on a cache line.
 */
T_CHMIMPL
void* CHMIMPL::Table::operator new (size_t, size_t capacity)
{
  void* memptr = nullptr;
  // Allocate aligned memory block.
  // The Table structure includes one entry at the end,
  // so subtract 1 from capacity.
  posix_memalign (&memptr, CACHELINE, sizeof(Table) + (capacity-1)*sizeof(entry_t));
  if (!memptr) std::abort();
  return memptr;
}
 
 
/**
 * @brief Deallocator for table objects.
 */
T_CHMIMPL
void CHMIMPL::Table::operator delete (void* p)
{
  free (p);
}
 
 
/**
 * @brief Find a table entry for reading.
 * @param key The key for which to search.
 * @param hash The hash of the key.
 *
 * Returns the matching entry, or nullptr.
 */
T_CHMIMPL
size_t CHMIMPL::Table::probeRead (val_t key, size_t hash) const
{
  // Iterate over table probes.
  // We don't need to check more than the longest probe sequence
  // used so far.
  TableIterator it (hash, m_mask, m_maskBits, m_longestProbe);
  do {
    size_t offset = it.offset();
    const entry_t* ent = m_entries + offset;
    if (ent->m_key == nullval) {
      // If we hit an empty key, report failure.
      return INVALID;
    }
    if (m_matcher (ent->m_key, key)) {
      // Found a matching key.
      return offset;
    }
  } while (it.next());
  // Too many probes --- return failure.
  return INVALID;
}
 
 
/**
 * @brief Find a table entry for writing.
 * @param key The key for which to search.
 * @param hash The hash of the key.
 * @param entry[out] The entry found.
 *
 * If we find the entry, return true with @c entry pointing at it.
 * If we don't find it, and there's still room in the table, return false
 * with @c entry pointing at the next empty entry.
 * Otherwise, return false with @c entry set to nullptr.
 */
T_CHMIMPL
size_t CHMIMPL::Table::probeWrite (val_t key, size_t hash, bool& insert)
{
  // Iterate over table probes.
  TableIterator it (hash, m_mask, m_maskBits, m_maxProbe);
  do {
    size_t offset = it.offset();
    entry_t* ent = m_entries + offset;
    if (ent->m_key == nullval) {
      // We hit an empty key; a new entry could be added here.
      // Update the longest probe count.
      CxxUtils::atomic_fetch_max (&m_longestProbe, it.nprobes()+1);
      insert = true;
      return offset;
    }
    if (m_matcher (ent->m_key, key)) {
      // Found a matching key.
      insert = false;
      return offset;
    }
  } while (it.next());
  // Too many probes --- return failure.
  return INVALID;
}
 
 
/**
 * @brief The number of entries in the table.
 */
T_CHMIMPL
inline
size_t CHMIMPL::Table::capacity() const
{
  return m_capacity;
}
 
 
/**
 * @brief Return the entry for an offset.
 * @param offset The index of the desired entry.
 */
T_CHMIMPL
inline
const typename CHMIMPL::entry_t& CHMIMPL::Table::entry (size_t offset) const
{
  return m_entries[offset];
}
 
 
/**
 * @brief Return the entry for an offset (non-const).
 * @param offset The index of the desired entry.
 */
T_CHMIMPL
inline
typename CHMIMPL::entry_t& CHMIMPL::Table::entry (size_t offset)
{
  return m_entries[offset];
}
 
 
//*****************************************************************************
 
 
/**
 * @brief Constructor.
 * @param updater Object used to manage memory
 *                (see comments at the start of the class).
 * @param capacity Minimum initial table size.
 * @param hasher Hash object to use.
 * @param matcher Key match object to use.
 * @param ctx Execution context.
 */
T_CHMIMPL
CHMIMPL::ConcurrentHashmapImpl (Updater_t&& updater,
                                size_t capacity_in,
                                const Hasher_t& hasher,
                                const Matcher_t& matcher,
                                const typename Updater_t::Context_t& ctx)
  : m_updater (std::move (updater)),
    m_hasher (hasher),
    m_matcher (matcher),
    m_size (0),
    m_erased (0)
{
  // Round up capacity to a power of 2.
  size_t capacity = round_up (capacity_in);
 
  m_table = new (capacity) Table (capacity, hasher, matcher);
  m_updater.update (std::unique_ptr<Table> (m_table), ctx);
}
 
 
/**
 * @brief Take a lock on the container.
 *
 * Take a lock on the container.
 * The lock can then be passed to put(), allowing to factor out the locking
 * when put() gets used in a loop.  The lock will be released when the
 * lock object is destroyed.
 */
T_CHMIMPL
inline
typename CHMIMPL::Lock_t CHMIMPL::lock()
{
  return Lock_t (m_mutex);
}
 
 
/**
 * @brief Add an entry to the table, with external locking.
 * @param lock The lock object returned from lock().
 * @param key The key to insert.
 * @param hash The hash of the key.
 * @param val The value to insert.
 * @param overwrite If true, then overwrite an existing entry.
 *                  If false, an existing entry will not be changed.
 * @param ctx Execution context.
 *
 * If the key already exists, then its value will be updated.
 * Returns an iterator pointing at the entry and a flag which is
 * true if a new element was added.
 */
T_CHMIMPL
std::pair<typename CHMIMPL::const_iterator, bool>
CHMIMPL::put (const Lock_t& lock,
              val_t key, size_t hash, val_t val, bool overwrite,
              const typename Updater_t::Context_t& ctx)
{
  assert (key != nullval && key != tombstone);
 
  do {
    bool insert;
    size_t offset = m_table->probeWrite (key, hash, insert);
    if (offset != INVALID) {
      entry_t& ent = m_table->entry (offset);
      if (insert) {
        // Found a place to put it.
        // Be sure not to set the key until the value is in place.
        ent.m_val = val;
        ent.m_key = key;
        ++m_size;
      }
      else {
        // Found --- update the entry if wanted.
        if (overwrite) {
          if (val != ent.m_val) {
            ent.m_val = val;
          }
        }
      }
      return std::make_pair (const_iterator (*m_table, offset), insert);
    }
 
    // Need to grow the table.
  } while (grow (lock, ctx));
 
  // grow() failed.
  return std::make_pair (end(), false);
}
 
 
/**
 * @brief Add an entry to the table.
 * @param key The key to insert.
 * @param hash The hash of the key.
 * @param val The value to insert.
 * @param overwrite If true, then overwrite an existing entry.
 *                  If false, an existing entry will not be changed.
 * @param ctx Execution context.
 *
 * If the key already exists, then its value will be updated.
 * Returns an iterator pointing at the entry and a flag which is
 * true if a new element was added.
 */
T_CHMIMPL
inline
std::pair<typename CHMIMPL::const_iterator, bool>
CHMIMPL::put (val_t key, size_t hash, val_t val, bool overwrite,
              const typename Updater_t::Context_t& ctx)
{
  Lock_t l (m_mutex);
  return put (l, key, hash, val, overwrite, ctx);
}
 
 
/**
 * @brief Look up an entry in the table.
 * @param key The key to find.
 * @param hash The hash of the key.
 *
 * Returns an iterator pointing at the found entry, or end().
 */
T_CHMIMPL
typename CHMIMPL::const_iterator CHMIMPL::get (val_t key, size_t hash) const
{
  const Table& table = m_updater.get();
  size_t offset = table.probeRead (key, hash);
  // Offset will be -1 if not found --- invalid iterator.
  return const_iterator (table, offset);
}
 
 
/**
 * @brief Erase an entry from the table, with external locking.
 * @param lock The lock object returned from lock().
 * @param key The key to find.
 * @param hash The hash of the key.
 *
 * Mark the corresponding entry as deleted.
 * Return true on success, false on failure (key not found).
 *
 * The tombstone value must be different from the null value.
 *
 * Take care if the key or value types require memory allocation.
 *
 * This may cause the key type returned by an iterator to change
 * asynchronously to the tombstone value.
 **/
T_CHMIMPL
bool
CHMIMPL::erase (const Lock_t& /*lock*/, val_t key, size_t hash)
{
  static_assert (nullval != tombstone);
  const Table& table = m_updater.get();
  size_t offset = table.probeRead (key, hash);
  if (offset != INVALID) {
    entry_t& ent = m_table->entry (offset);
    ++m_erased;
    --m_size;
    ent.m_key = tombstone;
    return true;
  }
  return false;
}
 
 
/**
 * @brief Erase an entry from the table.
 * @param key The key to find.
 * @param hash The hash of the key.
 *
 * Mark the corresponding entry as deleted.
 * Return true on success, false on failure (key not found).
 *
 * The tombstone value must be different from the null value.
 *
 * Take care if the key or value types require memory allocation.
 *
 * This may cause the key type returned by an iterator to change
 * asynchronously to the tombstone value.
 **/
T_CHMIMPL
inline
bool
CHMIMPL::erase (val_t key, size_t hash)
{
  return erase (lock(), key, hash);
}
 
 
/**
 * @brief Return the number of items currently stored.
 */
T_CHMIMPL
size_t CHMIMPL::size() const
{
  return m_size;
}
 
 
/**
 * @brief Return the current table size.
 */
T_CHMIMPL
size_t CHMIMPL::capacity() const
{
  const Table& table = m_updater.get();
  return table.capacity();
}
 
 
/**
 * @brief The number of erased elements in the current table.
 */
T_CHMIMPL
size_t CHMIMPL::erased() const
{
  return m_erased;
}
 
 
/** 
 * @brief Return the hasher object.
 */
T_CHMIMPL
inline
const typename CHMIMPL::Hasher_t& CHMIMPL::hasher() const
{
  return m_hasher;
}
 
 
/** 
 * @brief Return the matcher object.
 */
T_CHMIMPL
inline
const typename CHMIMPL::Matcher_t& CHMIMPL::matcher() const
{
  return m_matcher;
}
 
 
/**
 * @brief Constructor.
 * @param table The table instance we're referencing.
 * @param end If true, initialize this to an end iterator.
 *            Otherwise, initialize it to a a begin iterator.
 */
T_CHMIMPL
inline
CHMIMPL::const_iterator::const_iterator (const Table& table, bool end)
  : m_table (table),
    m_offset (INVALID)
{
  // For an end iterator, we want offset to be -1.
  // For a begin iterator, we need to advance to the first non-null entry.
  if (!end) {
    next();
  }
}
 
 
/**
 * @brief Constructor.
 * @param table The table instance we're referencing.
 * @param offset Offset of the iterator within the table.
 *               (Must point at an occupied entry.)
 */
T_CHMIMPL
CHMIMPL::const_iterator::const_iterator (const Table& table, size_t offset)
  : m_table (table),
    m_offset (offset)
{
  assert (offset == INVALID || m_table.entry (offset).m_key != nullval);
}
 
 
/**
 * @brief Advance the iterator to the next occupied entry.
 */
T_CHMIMPL
inline
void CHMIMPL::const_iterator::next()
{
  val_t key;
  do {
    ++m_offset;
    if (m_offset >= m_table.capacity()) {
      m_offset = INVALID;
      break;
    }
    key = m_table.entry (m_offset).m_key;
  } while (key == nullval || key == tombstone);
}
 
 
/**
 * @brief Move the iterator back to the previous occupied entry.
 */
T_CHMIMPL
inline
void CHMIMPL::const_iterator::prev()
{
  if (m_offset == INVALID) {
    m_offset = m_table.capacity();
  }
  val_t key;
  do {
    --m_offset;
    if (m_offset >= m_table.capacity()) {
      m_offset = INVALID;
      break;
    }
    key = m_table.entry (m_offset).m_key;
  } while (key == nullval || key == tombstone);
}
 
 
/**
 * @brief Return the key for this iterator.
 *        If deletions are allowed, then the key may change asynchronously
 *        to the tombstone value.
 */
T_CHMIMPL
inline
typename CHMIMPL::val_t CHMIMPL::const_iterator::key() const
{
  return m_table.entry (m_offset).m_key;
}
    
 
/**
 * @brief Return the value for this iterator.
 */
T_CHMIMPL
inline
typename CHMIMPL::val_t CHMIMPL::const_iterator::value() const
{
  return m_table.entry (m_offset).m_val;
}
 
 
/**
 * @brief Compare two iterators.
 */
T_CHMIMPL
inline
bool CHMIMPL::const_iterator::operator!= (const const_iterator& other) const
{
  if (m_offset != other.m_offset) return true;
  if (m_offset == INVALID) return false;
  return &m_table != &other.m_table;
}
 
 
/**
 * @brief Check that the iterator is valid (not pointing at the end).
 */
T_CHMIMPL
inline
bool CHMIMPL::const_iterator::valid() const
{
  return m_offset != INVALID;
}
 
 
/**
 * @brief Return a range that can be used to iterate over the container.
 */
T_CHMIMPL
inline
typename CHMIMPL::const_iterator_range CHMIMPL::range() const
{
  const Table& table = m_updater.get();
  return const_iterator_range (const_iterator (table, false),
                               const_iterator (table, true));
}
 
 
/** 
 * @brief A begin iterator for the container.
 */
T_CHMIMPL
inline
typename CHMIMPL::const_iterator CHMIMPL::begin() const
{
  const Table& table = m_updater.get();
  return const_iterator (table, false);
}
 
 
/** 
 * @brief An end iterator for the container.
 */
T_CHMIMPL
inline
typename CHMIMPL::const_iterator CHMIMPL::end() const
{
  const Table& table = m_updater.get();
  return const_iterator (table, true);
}
 
 
/**
 * @brief Erase the table and change the capacity.
 * @param capacity The new table capacity.
 * @param ctx Execution context.
 *
 * Returns an iterator pointing at the start of the old table.
 */
T_CHMIMPL
typename CHMIMPL::const_iterator
CHMIMPL::clear (size_t capacity,
                const typename Updater_t::Context_t& ctx)
{
  capacity = round_up (capacity);
  Lock_t lock (m_mutex);
  std::unique_ptr<Table> new_table (new (capacity) Table (capacity,
                                                          m_hasher,
                                                          m_matcher));
 
  // Save an iterator to the old table.
  const_iterator it = begin();
 
  // Publish the new table.
  m_size = 0;
  m_erased = 0;
  m_table = new_table.get();
  m_updater.update (std::move (new_table), ctx);
  return it;
}
 
 
/**
 * @brief Erase the table (don't change the capacity).
 * @param ctx Execution context.
 *
 * Returns an iterator pointing at the start of the old table.
 */
T_CHMIMPL
typename CHMIMPL::const_iterator
CHMIMPL::clear (const typename Updater_t::Context_t& ctx)
{
  const Table& table = m_updater.get();
  // Possible race here in that the container capacity could increase
  // before we take out the lock in clear().  In practice, this should
  // not actually be a problem.
  return clear (table.capacity(), ctx);
}
 
 
/**
 * @brief Erase the table by filling it with nulls.
 *
 * This method is not safe to use concurrently --- no other threads
 * may be accessing the container at the same time, either for read
 * or write.
 */
T_CHMIMPL
void CHMIMPL::forceClear()
{
  Lock_t lock (m_mutex);
  size_t cap = m_table->capacity();
  for (size_t i = 0; i < cap; i++) {
    m_table->entry(i).m_key.store (nullval, std::memory_order_relaxed);
  }
  m_size = 0;
  m_erased = 0;
  std::atomic_thread_fence (std::memory_order_seq_cst);
}
 
 
/**
 * @brief Increase the table capacity.
 * @param capacity The new table capacity.
 * @param ctx Execution context.
 *
 * No action will be taken if @c capacity is smaller
 * than the current capacity.
 */
T_CHMIMPL
void CHMIMPL::reserve (size_t capacity,
                       const typename Updater_t::Context_t& ctx)
{
  Lock_t lock (m_mutex);
  if (capacity < m_table->capacity()) return;
  grow (lock, round_up (capacity), ctx);
}
 
 
/**
 * @brief Called when this thread is no longer referencing anything
 *        from this container.
 * @param ctx Execution context.
 */
T_CHMIMPL
void CHMIMPL::quiescent (const typename Updater_t::Context_t& ctx)
{
  m_updater.quiescent (ctx);
}
 
 
/**
 * @brief Swap this container with another.
 * @param other The container with which to swap.
 *
 * This will also call swap on the Updater object; hence, the Updater
 * object must also support swap.  The Hasher and Matcher instances
 * are NOT swapped.
 *
 * This operation is NOT thread-safe.  No other threads may be accessing
 * either container during this operation.
 */
T_CHMIMPL
void CHMIMPL::swap (ConcurrentHashmapImpl& other)
{
  // Shouldn't be needed since we specified that no other threads can be
  // accessing this...
  Lock_t lock (m_mutex);
  Lock_t lock_other (other.m_mutex);
 
  m_updater.swap (other.m_updater);
  std::swap (m_table, other.m_table);
 
  auto swap_atomic = [] (std::atomic<size_t>& a, std::atomic<size_t>& b)
  {
    size_t tmp = a.load (std::memory_order_relaxed);
    a.store (b.load (std::memory_order_relaxed),
             std::memory_order_relaxed);
    b.store (tmp, std::memory_order_relaxed);
  };
 
  swap_atomic (m_size, other.m_size);
  swap_atomic (m_erased, other.m_erased);
}
 
 
/**
 * @brief Access the Updater instance.
 */
T_CHMIMPL
inline
typename CHMIMPL::Updater_t& CHMIMPL::updater()
{
  return m_updater;
}
 
 
/**
 * @brief Make the table larger.
 * @param ctx Execution context.
 *
 * Must be holding a lock on the mutex to call this.
 */
T_CHMIMPL
bool CHMIMPL::grow (const Lock_t& lock, const typename Updater_t::Context_t& ctx)
{
  // Allocate a new table with twice the capacity, unless there
  // have been many erasures.
  size_t new_capacity = m_erased >= m_table->capacity()/2 ?
    m_table->capacity() : 2*m_table->capacity();
  return grow (lock, new_capacity, ctx);
}
 
 
/**
 * @brief Make the table larger.
 * @param new_capacity The new table capacity (must be a power of 2).
 * @param ctx Execution context.
 *
 * Must be holding a lock on the mutex to call this.
 */
T_CHMIMPL
bool CHMIMPL::grow (const Lock_t& /*lock*/,
                    size_t new_capacity,
                    const typename Updater_t::Context_t& ctx)
{
  // The current table.
  const Table& table = *m_table;
 
  std::unique_ptr<Table> new_table (new (new_capacity) Table (new_capacity,
                                                              m_hasher,
                                                              m_matcher));
 
  // Copy data from the existing table to the new one.
  size_t capacity = table.capacity();
  for (size_t i = 0; i < capacity; i++) {
    const entry_t& ent = table.entry(i);
    if (ent.m_key != nullval) {
      size_t hash = m_hasher (ent.m_key);
      bool insert;
      size_t offset = new_table->probeWrite (ent.m_key, hash, insert);
      if (offset == INVALID) {
        std::abort();
      }
      entry_t& new_entry = new_table->entry (offset);
      new_entry.m_key = static_cast<val_t> (ent.m_key);
      new_entry.m_val = static_cast<val_t> (ent.m_val);
    }
  }
 
  m_erased = 0;
 
  // Publish the new table.
  m_table = new_table.get();
  m_updater.update (std::move (new_table), ctx);
    
  return true;
}
 
 
/**
 * @brief Round up to a power of 2.
 */
T_CHMIMPL
uint64_t CHMIMPL::round_up (uint64_t x)
{
  if (x <= 64) return 64;
  return std::bit_ceil (x);
}
 
 
#undef CHMIMPL
#undef T_CHMIMPL
 
 
 
} // namespace detail
} // namespace CxxUtils