innobase/include/ut0pool.h

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/** @file include/ut0pool.h
 Object pool.

 Created 2012-Feb-26 Sunny Bains
 ***********************************************************************/

#ifndef ut0pool_h
#define ut0pool_h

#include <functional>
#include <queue>
#include <vector>

#include "ut0new.h"

/** Allocate the memory for the object in blocks. We keep the objects sorted
on pointer so that they are closer together in case they have to be iterated
over in a list. */
template <typename Type, typename Factory, typename LockStrategy>
struct Pool {
  typedef Type value_type;

  // FIXME: Add an assertion to check alignment and offset is
  // as we expect it. Also, sizeof(void*) can be 8, can we impove on this.
  struct Element {
    Pool *m_pool;
    value_type m_type;
  };

  /** Constructor
  @param size size of the memory block */
  Pool(size_t size) : m_end(), m_start(), m_size(size), m_last() {
    ut_a(size >= sizeof(Element));

    m_lock_strategy.create();

    ut_a(m_start == nullptr);

    m_start = reinterpret_cast<Element *>(ut_zalloc_nokey(m_size));

    m_last = m_start;

    m_end = &m_start[m_size / sizeof(*m_start)];

    /* Note: Initialise only a small subset, even though we have
    allocated all the memory. This is required only because PFS
    (MTR) results change if we instantiate too many mutexes up
    front. */

    init(ut_min(size_t(16), size_t(m_end - m_start)));

    ut_ad(m_pqueue.size() <= size_t(m_last - m_start));
  }

  /** Destructor */
  ~Pool() {
    m_lock_strategy.destroy();

    for (Element *elem = m_start; elem != m_last; ++elem) {
      ut_ad(elem->m_pool == this);
      Factory::destroy(&elem->m_type);
    }

    ut_free(m_start);
    m_end = m_last = m_start = nullptr;
    m_size = 0;
  }

  /** Get an object from the pool.
  @return a free instance or NULL if exhausted. */
  Type *get() {
    Element *elem;

    m_lock_strategy.enter();

    if (!m_pqueue.empty()) {
      elem = m_pqueue.top();
      m_pqueue.pop();

    } else if (m_last < m_end) {
      /* Initialise the remaining elements. */
      init(m_end - m_last);

      ut_ad(!m_pqueue.empty());

      elem = m_pqueue.top();
      m_pqueue.pop();
    } else {
      elem = nullptr;
    }

    m_lock_strategy.exit();

    return (elem != nullptr ? &elem->m_type : nullptr);
  }

  /** Add the object to the pool.
  @param ptr object to free */
  static void mem_free(value_type *ptr) {
    Element *elem;
    byte *p = reinterpret_cast<byte *>(ptr + 1);

    elem = reinterpret_cast<Element *>(p - sizeof(*elem));

    elem->m_pool->put(elem);
  }

 protected:
  // Disable copying
  Pool(const Pool &);
  Pool &operator=(const Pool &);

 private:
  /* We only need to compare on pointer address. */
  typedef std::priority_queue<Element *,
                              std::vector<Element *, ut_allocator<Element *>>,
                              std::greater<Element *>>
      pqueue_t;

  /** Release the object to the free pool
  @param elem element to free */
  void put(Element *elem) {
    m_lock_strategy.enter();

    ut_ad(elem >= m_start && elem < m_last);

    ut_ad(Factory::debug(&elem->m_type));

    m_pqueue.push(elem);

    m_lock_strategy.exit();
  }

  /** Initialise the elements.
  @param n_elems Number of elements to initialise */
  void init(size_t n_elems) {
    ut_ad(size_t(m_end - m_last) >= n_elems);

    for (size_t i = 0; i < n_elems; ++i, ++m_last) {
      m_last->m_pool = this;
      Factory::init(&m_last->m_type);
      m_pqueue.push(m_last);
    }

    ut_ad(m_last <= m_end);
  }

 private:
  /** Pointer to the last element */
  Element *m_end;

  /** Pointer to the first element */
  Element *m_start;

  /** Size of the block in bytes */
  size_t m_size;

  /** Upper limit of used space */
  Element *m_last;

  /** Priority queue ordered on the pointer addresse. */
  pqueue_t m_pqueue;

  /** Lock strategy to use */
  LockStrategy m_lock_strategy;
};

template <typename Pool, typename LockStrategy>
struct PoolManager {
  typedef Pool PoolType;
  typedef typename PoolType::value_type value_type;

  PoolManager(size_t size) : m_size(size) { create(); }

  ~PoolManager() {
    destroy();

    ut_a(m_pools.empty());
  }

  /** Get an element from one of the pools.
  @return instance or NULL if pool is empty. */
  value_type *get() {
    size_t index = 0;
    size_t delay = 1;
    value_type *ptr = nullptr;

    do {
      m_lock_strategy.enter();

      ut_ad(!m_pools.empty());

      size_t n_pools = m_pools.size();

      PoolType *pool = m_pools[index % n_pools];

      m_lock_strategy.exit();

      ptr = pool->get();

      if (ptr == nullptr && (index / n_pools) > 2) {
        if (!add_pool(n_pools)) {
          ib::error(ER_IB_MSG_FAILED_TO_ALLOCATE_WAIT, m_size, delay);

          /* There is nothing much we can do
          except crash and burn, however lets
          be a little optimistic and wait for
          a resource to be freed. */
          os_thread_sleep(delay * 1000000);

          if (delay < 32) {
            delay <<= 1;
          }

        } else {
          delay = 1;
        }
      }

      ++index;

    } while (ptr == nullptr);

    return (ptr);
  }

  static void mem_free(value_type *ptr) { PoolType::mem_free(ptr); }

 private:
  /** Add a new pool
  @param n_pools Number of pools that existed when the add pool was
                  called.
  @return true on success */
  bool add_pool(size_t n_pools) {
    bool added = false;

    m_lock_strategy.enter();

    if (n_pools < m_pools.size()) {
      /* Some other thread already added a pool. */
      added = true;
    } else {
      PoolType *pool;

      ut_ad(n_pools == m_pools.size());

      pool = UT_NEW_NOKEY(PoolType(m_size));

      if (pool != nullptr) {
        ut_ad(n_pools <= m_pools.size());

        m_pools.push_back(pool);

        ib::info(ER_IB_MSG_NUM_POOLS, m_pools.size());

        added = true;
      }
    }

    ut_ad(n_pools < m_pools.size() || !added);

    m_lock_strategy.exit();

    return (added);
  }

  /** Create the pool manager. */
  void create() {
    ut_a(m_size > sizeof(value_type));
    m_lock_strategy.create();

    add_pool(0);
  }

  /** Release the resources. */
  void destroy() {
    typename Pools::iterator it;
    typename Pools::iterator end = m_pools.end();

    for (it = m_pools.begin(); it != end; ++it) {
      PoolType *pool = *it;

      UT_DELETE(pool);
    }

    m_pools.clear();

    m_lock_strategy.destroy();
  }

 private:
  // Disable copying
  PoolManager(const PoolManager &);
  PoolManager &operator=(const PoolManager &);

  typedef std::vector<PoolType *, ut_allocator<PoolType *>> Pools;

  /** Size of each block */
  size_t m_size;

  /** Pools managed this manager */
  Pools m_pools;

  /** Lock strategy to use */
  LockStrategy m_lock_strategy;
};

#endif /* ut0pool_h */