Kokkos_MemoryPool.hpp

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//                        Kokkos v. 2.0
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#ifndef KOKKOS_MEMORYPOOL_HPP
#define KOKKOS_MEMORYPOOL_HPP

#include <Kokkos_Core_fwd.hpp>
#include <Kokkos_Parallel.hpp>
#include <Kokkos_Atomic.hpp>
#include <impl/Kokkos_ConcurrentBitset.hpp>
#include <impl/Kokkos_Error.hpp>
#include <impl/Kokkos_SharedAlloc.hpp>

namespace Kokkos {
namespace Impl {
/* Report violation of size constraints:
 *   min_block_alloc_size <= max_block_alloc_size
 *   max_block_alloc_size <= min_superblock_size 
 *   min_superblock_size  <= max_superblock_size
 *   min_superblock_size  <= min_total_alloc_size
 *   min_superblock_size  <= min_block_alloc_size * 
 *                           max_block_per_superblock
 */
void memory_pool_bounds_verification
  ( size_t min_block_alloc_size
  , size_t max_block_alloc_size
  , size_t min_superblock_size
  , size_t max_superblock_size
  , size_t max_block_per_superblock
  , size_t min_total_alloc_size
  );
}
}

namespace Kokkos {

template< typename DeviceType >
class MemoryPool {
private:

  typedef typename Kokkos::Impl::concurrent_bitset CB ;

  enum : uint32_t { bits_per_int_lg2  = CB::bits_per_int_lg2 };
  enum : uint32_t { state_shift       = CB::state_shift };
  enum : uint32_t { state_used_mask   = CB::state_used_mask };
  enum : uint32_t { state_header_mask = CB::state_header_mask };
  enum : uint32_t { max_bit_count_lg2 = CB::max_bit_count_lg2 };
  enum : uint32_t { max_bit_count     = CB::max_bit_count };

  enum : uint32_t { HINT_PER_BLOCK_SIZE = 2 };

  /*  Each superblock has a concurrent bitset state
   *  which is an array of uint32_t integers.
   *    [ { block_count_lg2  : state_shift bits
   *      , used_block_count : ( 32 - state_shift ) bits
   *      }
   *    , { block allocation bit set }* ]
   *
   *  As superblocks are assigned (allocated) to a block size
   *  and released (deallocated) back to empty the superblock state
   *  is concurrently updated.
   */

  /*  Mapping between block_size <-> block_state
   *
   *  block_state = ( m_sb_size_lg2 - block_size_lg2 ) << state_shift
   *  block_size  = m_sb_size_lg2 - ( block_state >> state_shift )
   *
   *  Thus A_block_size < B_block_size  <=>  A_block_state > B_block_state
   */

  typedef typename DeviceType::memory_space base_memory_space ;

  enum { accessible =
           Kokkos::Impl::MemorySpaceAccess< Kokkos::HostSpace 
                                          , base_memory_space >::accessible };

  typedef Kokkos::Impl::SharedAllocationTracker Tracker ;
  typedef Kokkos::Impl::SharedAllocationRecord
    < base_memory_space >  Record ;

  Tracker    m_tracker ;
  uint32_t * m_sb_state_array ;
  uint32_t   m_sb_state_size ;
  uint32_t   m_sb_size_lg2 ;
  uint32_t   m_max_block_size_lg2 ;
  uint32_t   m_min_block_size_lg2 ;
  int32_t    m_sb_count ;
  int32_t    m_hint_offset ;   // Offset to K * #block_size array of hints
  int32_t    m_data_offset ;   // Offset to 0th superblock data
  int32_t    m_unused_padding ;

public:

  enum : uint32_t { max_superblock_size      = 1LU << 31 /* 2 gigabytes */ };
  enum : uint32_t { max_block_per_superblock = max_bit_count };

  //--------------------------------------------------------------------------

  KOKKOS_INLINE_FUNCTION
  size_t capacity() const noexcept
    { return size_t(m_sb_count) << m_sb_size_lg2 ; }

  KOKKOS_INLINE_FUNCTION
  size_t min_block_size() const noexcept
    { return ( 1LU << m_min_block_size_lg2 ); }

  KOKKOS_INLINE_FUNCTION
  size_t max_block_size() const noexcept
    { return ( 1LU << m_max_block_size_lg2 ); }

  struct usage_statistics {
    size_t capacity_bytes ;       
    size_t superblock_bytes ;     
    size_t max_block_bytes ;      
    size_t min_block_bytes ;      
    size_t capacity_superblocks ; 
    size_t consumed_superblocks ; 
    size_t consumed_blocks ;  
    size_t consumed_bytes ;   
    size_t reserved_blocks ;  
    size_t reserved_bytes ;   
  };

  void get_usage_statistics( usage_statistics & stats ) const
    {
      Kokkos::HostSpace host ;

      const size_t alloc_size = m_hint_offset * sizeof(uint32_t);

      uint32_t * const sb_state_array = 
        accessible ? m_sb_state_array : (uint32_t *) host.allocate(alloc_size);

      if ( ! accessible ) {
        Kokkos::Impl::DeepCopy< Kokkos::HostSpace , base_memory_space >
          ( sb_state_array , m_sb_state_array , alloc_size );
      }

      stats.superblock_bytes = ( 1LU << m_sb_size_lg2 );
      stats.max_block_bytes  = ( 1LU << m_max_block_size_lg2 );
      stats.min_block_bytes  = ( 1LU << m_min_block_size_lg2 );
      stats.capacity_bytes   = stats.superblock_bytes * m_sb_count ;
      stats.capacity_superblocks = m_sb_count ;
      stats.consumed_superblocks = 0 ;
      stats.consumed_blocks = 0 ;
      stats.consumed_bytes  = 0 ;
      stats.reserved_blocks = 0 ;
      stats.reserved_bytes  = 0 ;

      const uint32_t * sb_state_ptr = sb_state_array ;

      for ( int32_t i = 0 ; i < m_sb_count
          ; ++i , sb_state_ptr += m_sb_state_size ) {

        const uint32_t block_count_lg2 = (*sb_state_ptr) >> state_shift ;

        if ( block_count_lg2 ) {
          const uint32_t block_count    = 1u << block_count_lg2 ;
          const uint32_t block_size_lg2 = m_sb_size_lg2 - block_count_lg2 ;
          const uint32_t block_size     = 1u << block_size_lg2 ;
          const uint32_t block_used     = (*sb_state_ptr) & state_used_mask ;

          stats.consumed_superblocks++ ;
          stats.consumed_blocks += block_used ;
          stats.consumed_bytes  += block_used * block_size ;
          stats.reserved_blocks += block_count - block_used ;
          stats.reserved_bytes  += (block_count - block_used ) * block_size ;
        }
      }

      if ( ! accessible ) {
        host.deallocate( sb_state_array, alloc_size );
      }
    }

  void print_state( std::ostream & s ) const
    {
      Kokkos::HostSpace host ;

      const size_t alloc_size = m_hint_offset * sizeof(uint32_t);

      uint32_t * const sb_state_array = 
        accessible ? m_sb_state_array : (uint32_t *) host.allocate(alloc_size);

      if ( ! accessible ) {
        Kokkos::Impl::DeepCopy< Kokkos::HostSpace , base_memory_space >
          ( sb_state_array , m_sb_state_array , alloc_size );
      }

      const uint32_t * sb_state_ptr = sb_state_array ;

      s << "pool_size(" << ( size_t(m_sb_count) << m_sb_size_lg2 ) << ")"
        << " superblock_size(" << ( 1LU << m_sb_size_lg2 ) << ")" << std::endl ;

      for ( int32_t i = 0 ; i < m_sb_count
          ; ++i , sb_state_ptr += m_sb_state_size ) {

        if ( *sb_state_ptr ) {

          const uint32_t block_count_lg2 = (*sb_state_ptr) >> state_shift ;
          const uint32_t block_size_lg2  = m_sb_size_lg2 - block_count_lg2 ;
          const uint32_t block_count     = 1u << block_count_lg2 ;
          const uint32_t block_used      = (*sb_state_ptr) & state_used_mask ;

          s << "Superblock[ " << i << " / " << m_sb_count << " ] {"
            << " block_size(" << ( 1 << block_size_lg2 ) << ")"
            << " block_count( " << block_used
            << " / " << block_count  << " )"
            << std::endl ;
        }
      }

      if ( ! accessible ) {
        host.deallocate( sb_state_array, alloc_size );
      }
    }

  //--------------------------------------------------------------------------

#ifdef KOKKOS_CUDA_9_DEFAULTED_BUG_WORKAROUND
  KOKKOS_INLINE_FUNCTION MemoryPool( MemoryPool && rhs )
    : m_tracker(std::move(rhs.m_tracker))
    , m_sb_state_array(std::move(rhs.m_sb_state_array))
    , m_sb_state_size(std::move(rhs.m_sb_state_size))
    , m_sb_size_lg2(std::move(rhs.m_sb_size_lg2))
    , m_max_block_size_lg2(std::move(rhs.m_max_block_size_lg2))
    , m_min_block_size_lg2(std::move(rhs.m_min_block_size_lg2))
    , m_sb_count(std::move(rhs.m_sb_count))
    , m_hint_offset(std::move(rhs.m_hint_offset))
    , m_data_offset(std::move(rhs.m_data_offset))
  {
  }
  KOKKOS_INLINE_FUNCTION MemoryPool( const MemoryPool & rhs )
    : m_tracker(rhs.m_tracker)
    , m_sb_state_array(rhs.m_sb_state_array)
    , m_sb_state_size(rhs.m_sb_state_size)
    , m_sb_size_lg2(rhs.m_sb_size_lg2)
    , m_max_block_size_lg2(rhs.m_max_block_size_lg2)
    , m_min_block_size_lg2(rhs.m_min_block_size_lg2)
    , m_sb_count(rhs.m_sb_count)
    , m_hint_offset(rhs.m_hint_offset)
    , m_data_offset(rhs.m_data_offset)
  {
  }
  KOKKOS_INLINE_FUNCTION MemoryPool & operator = ( MemoryPool && rhs )
  {
    m_tracker = std::move(rhs.m_tracker);
    m_sb_state_array = std::move(rhs.m_sb_state_array);
    m_sb_state_size = std::move(rhs.m_sb_state_size);
    m_sb_size_lg2 = std::move(rhs.m_sb_size_lg2);
    m_max_block_size_lg2 = std::move(rhs.m_max_block_size_lg2);
    m_min_block_size_lg2 = std::move(rhs.m_min_block_size_lg2);
    m_sb_count = std::move(rhs.m_sb_count);
    m_hint_offset = std::move(rhs.m_hint_offset);
    m_data_offset = std::move(rhs.m_data_offset);
    return *this;
  }
  KOKKOS_INLINE_FUNCTION MemoryPool & operator = ( const MemoryPool & rhs )
  {
    m_tracker = rhs.m_tracker;
    m_sb_state_array = rhs.m_sb_state_array;
    m_sb_state_size = rhs.m_sb_state_size;
    m_sb_size_lg2 = rhs.m_sb_size_lg2;
    m_max_block_size_lg2 = rhs.m_max_block_size_lg2;
    m_min_block_size_lg2 = rhs.m_min_block_size_lg2;
    m_sb_count = rhs.m_sb_count;
    m_hint_offset = rhs.m_hint_offset;
    m_data_offset = rhs.m_data_offset;
    return *this;
  }
#else
  KOKKOS_INLINE_FUNCTION MemoryPool( MemoryPool && ) = default ;
  KOKKOS_INLINE_FUNCTION MemoryPool( const MemoryPool & ) = default ;
  KOKKOS_INLINE_FUNCTION MemoryPool & operator = ( MemoryPool && ) = default ;
  KOKKOS_INLINE_FUNCTION MemoryPool & operator = ( const MemoryPool & ) = default ;
#endif

  KOKKOS_INLINE_FUNCTION MemoryPool()
    : m_tracker()
    , m_sb_state_array(0)
    , m_sb_state_size(0)
    , m_sb_size_lg2(0)
    , m_max_block_size_lg2(0)
    , m_min_block_size_lg2(0)
    , m_sb_count(0)
    , m_hint_offset(0)
    , m_data_offset(0)
    , m_unused_padding(0)
    {}

  MemoryPool( const base_memory_space & memspace
            , const size_t min_total_alloc_size
            , size_t min_block_alloc_size = 0
            , size_t max_block_alloc_size = 0
            , size_t min_superblock_size  = 0
            )
    : m_tracker()
    , m_sb_state_array(0)
    , m_sb_state_size(0)
    , m_sb_size_lg2(0)
    , m_max_block_size_lg2(0)
    , m_min_block_size_lg2(0)
    , m_sb_count(0)
    , m_hint_offset(0)
    , m_data_offset(0)
    , m_unused_padding(0)
    {
      const uint32_t int_align_lg2   = 3 ; /* align as int[8] */
      const uint32_t int_align_mask  = ( 1u << int_align_lg2 ) - 1 ;
      const uint32_t default_min_block_size       = 1u << 6  ; /* 64 bytes */
      const uint32_t default_max_block_size       = 1u << 12 ;/* 4k bytes */
      const uint32_t default_min_superblock_size  = 1u << 20 ;/* 1M bytes */

      //--------------------------------------------------
      // Default block and superblock sizes:

      if ( 0 == min_block_alloc_size ) {
        // Default all sizes:

        min_superblock_size =
          std::min( size_t(default_min_superblock_size)
                  , min_total_alloc_size );

        min_block_alloc_size =
          std::min( size_t(default_min_block_size)
                  , min_superblock_size );

        max_block_alloc_size =
          std::min( size_t(default_max_block_size)
                  , min_superblock_size );
      }
      else if ( 0 == min_superblock_size ) {

        // Choose superblock size as minimum of:
        //   max_block_per_superblock * min_block_size
        //   max_superblock_size
        //   min_total_alloc_size

        const size_t max_superblock =
          min_block_alloc_size * max_block_per_superblock ;

        min_superblock_size =
          std::min( max_superblock ,
          std::min( size_t(max_superblock_size)
                  , min_total_alloc_size ) );
      }

      if ( 0 == max_block_alloc_size ) {
        max_block_alloc_size = min_superblock_size ;
      }

      //--------------------------------------------------

      /* Enforce size constraints:
       *   min_block_alloc_size <= max_block_alloc_size
       *   max_block_alloc_size <= min_superblock_size 
       *   min_superblock_size  <= max_superblock_size
       *   min_superblock_size  <= min_total_alloc_size
       *   min_superblock_size  <= min_block_alloc_size * 
       *                           max_block_per_superblock
       */

      Kokkos::Impl::memory_pool_bounds_verification
        ( min_block_alloc_size
        , max_block_alloc_size
        , min_superblock_size
        , max_superblock_size
        , max_block_per_superblock
        , min_total_alloc_size
        );

      //--------------------------------------------------
      // Block and superblock size is power of two:
      // Maximum value is 'max_superblock_size'

      m_min_block_size_lg2 =
        Kokkos::Impl::integral_power_of_two_that_contains(min_block_alloc_size);

      m_max_block_size_lg2 =
        Kokkos::Impl::integral_power_of_two_that_contains(max_block_alloc_size);
  
      m_sb_size_lg2 =
        Kokkos::Impl::integral_power_of_two_that_contains(min_superblock_size);

      {
        // number of superblocks is multiple of superblock size that
        // can hold min_total_alloc_size.

        const uint64_t sb_size_mask = ( 1LU << m_sb_size_lg2 ) - 1 ;

        m_sb_count = ( min_total_alloc_size + sb_size_mask ) >> m_sb_size_lg2 ;
      }

      {
        // Any superblock can be assigned to the smallest size block
        // Size the block bitset to maximum number of blocks

        const uint32_t max_block_count_lg2 =
          m_sb_size_lg2 - m_min_block_size_lg2 ;

        m_sb_state_size =
          ( CB::buffer_bound_lg2( max_block_count_lg2 ) + int_align_mask ) & ~int_align_mask ;
      }

      // Array of all superblock states

      const size_t all_sb_state_size =
        ( m_sb_count * m_sb_state_size + int_align_mask ) & ~int_align_mask ;

      // Number of block sizes

      const int32_t number_block_sizes =
         1 + m_max_block_size_lg2 - m_min_block_size_lg2 ;

      // Array length for possible block sizes
      // Hint array is one uint32_t per block size

      const int32_t block_size_array_size =
        ( number_block_sizes + int_align_mask ) & ~int_align_mask ;

      m_hint_offset = all_sb_state_size ;
      m_data_offset = m_hint_offset +
                      block_size_array_size * HINT_PER_BLOCK_SIZE ;

      // Allocation:

      const size_t header_size = m_data_offset * sizeof(uint32_t);
      const size_t alloc_size  = header_size +
                                 ( size_t(m_sb_count) << m_sb_size_lg2 );

      Record * rec = Record::allocate( memspace , "MemoryPool" , alloc_size );

      m_tracker.assign_allocated_record_to_uninitialized( rec );

      m_sb_state_array = (uint32_t *) rec->data();

      Kokkos::HostSpace host ;

      uint32_t * const sb_state_array = 
        accessible ? m_sb_state_array
                   : (uint32_t *) host.allocate(header_size);

      for ( int32_t i = 0 ; i < m_data_offset ; ++i ) sb_state_array[i] = 0 ;

      // Initial assignment of empty superblocks to block sizes:

      for ( int32_t i = 0 ; i < number_block_sizes ; ++i ) {
        const uint32_t block_size_lg2  = i + m_min_block_size_lg2 ;
        const uint32_t block_count_lg2 = m_sb_size_lg2 - block_size_lg2 ;
        const uint32_t block_state     = block_count_lg2 << state_shift ;
        const uint32_t hint_begin = m_hint_offset + i * HINT_PER_BLOCK_SIZE ;

        // for block size index 'i':
        //   sb_id_hint  = sb_state_array[ hint_begin ];
        //   sb_id_begin = sb_state_array[ hint_begin + 1 ];

        const int32_t jbeg = ( i * m_sb_count ) / number_block_sizes ;
        const int32_t jend = ( ( i + 1 ) * m_sb_count ) / number_block_sizes ;

        sb_state_array[ hint_begin ] = uint32_t(jbeg);
        sb_state_array[ hint_begin + 1 ] = uint32_t(jbeg);

        for ( int32_t j = jbeg ; j < jend ; ++j ) {
          sb_state_array[ j * m_sb_state_size ] = block_state ;
        }
      }

      // Write out initialized state:

      if ( ! accessible ) {
        Kokkos::Impl::DeepCopy< base_memory_space , Kokkos::HostSpace >
          ( m_sb_state_array , sb_state_array , header_size );

        host.deallocate( sb_state_array, header_size );
      }
      else {
        Kokkos::memory_fence();
      }
    }

  //--------------------------------------------------------------------------

private:

  /* Given a size 'n' get the block size in which it can be allocated.
   * Restrict lower bound to minimum block size.
   */
  KOKKOS_FORCEINLINE_FUNCTION
  uint32_t get_block_size_lg2( uint32_t n ) const noexcept
    {
      const unsigned i = Kokkos::Impl::integral_power_of_two_that_contains( n );

      return i < m_min_block_size_lg2 ? m_min_block_size_lg2 : i ;
    }

public:

  /* Return 0 for invalid block size */
  KOKKOS_INLINE_FUNCTION
  uint32_t allocate_block_size( uint64_t alloc_size ) const noexcept
    {
      return alloc_size <= (1UL << m_max_block_size_lg2)
           ? ( 1UL << get_block_size_lg2( uint32_t(alloc_size) ) )
           : 0 ;
    }

  //--------------------------------------------------------------------------
  KOKKOS_FUNCTION
  void * allocate( size_t alloc_size
                 , int32_t attempt_limit = 1 ) const noexcept
    {
      if ( size_t(1LU << m_max_block_size_lg2) < alloc_size ) {
        Kokkos::abort("Kokkos MemoryPool allocation request exceeded specified maximum allocation size");
      }

      if ( 0 == alloc_size ) return (void*) 0 ;

      void * p = 0 ;

      const uint32_t block_size_lg2 = get_block_size_lg2( alloc_size );

      // Allocation will fit within a superblock
      // that has block sizes ( 1 << block_size_lg2 )

      const uint32_t block_count_lg2 = m_sb_size_lg2 - block_size_lg2 ;
      const uint32_t block_state     = block_count_lg2 << state_shift ;
      const uint32_t block_count     = 1u << block_count_lg2 ;

      // Superblock hints for this block size:
      //   hint_sb_id_ptr[0] is the dynamically changing hint
      //   hint_sb_id_ptr[1] is the static start point

      volatile uint32_t * const hint_sb_id_ptr
        = m_sb_state_array     /* memory pool state array */
        + m_hint_offset        /* offset to hint portion of array */
        + HINT_PER_BLOCK_SIZE  /* number of hints per block size */
          * ( block_size_lg2 - m_min_block_size_lg2 ); /* block size id */

      const int32_t sb_id_begin = int32_t( hint_sb_id_ptr[1] );

      // Fast query clock register 'tic' to pseudo-randomize
      // the guess for which block within a superblock should
      // be claimed.  If not available then a search occurs.

      const uint32_t block_id_hint =
        (uint32_t)( Kokkos::Impl::clock_tic()
#if defined( KOKKOS_ACTIVE_EXECUTION_MEMORY_SPACE_CUDA )
        // Spread out potentially concurrent access
        // by threads within a warp or thread block.
        + ( threadIdx.x + blockDim.x * threadIdx.y )
#endif
        );

      // expected state of superblock for allocation
      uint32_t sb_state = block_state ;

      int32_t sb_id = -1 ;

      volatile uint32_t * sb_state_array = 0 ;

      while ( attempt_limit ) {

        int32_t hint_sb_id = -1 ;

        if ( sb_id < 0 ) {

          // No superblock specified, try the hint for this block size

          sb_id = hint_sb_id = int32_t( *hint_sb_id_ptr );

          sb_state_array = m_sb_state_array + ( sb_id * m_sb_state_size );
        }

        // Require:
        //   0 <= sb_id
        //   sb_state_array == m_sb_state_array + m_sb_state_size * sb_id

        if ( sb_state == ( state_header_mask & *sb_state_array ) ) {

          // This superblock state is as expected, for the moment.
          // Attempt to claim a bit.  The attempt updates the state
          // so have already made sure the state header is as expected.

          const uint32_t count_lg2 = sb_state >> state_shift ;
          const uint32_t mask      = ( 1u << count_lg2 ) - 1 ;

          const Kokkos::pair<int,int> result =
            CB::acquire_bounded_lg2( sb_state_array
                                   , count_lg2
                                   , block_id_hint & mask
                                   , sb_state
                                   );

          // If result.first < 0 then failed to acquire
          // due to either full or buffer was wrong state.
          // Could be wrong state if a deallocation raced the
          // superblock to empty before the acquire could succeed.

          if ( 0 <= result.first ) { // acquired a bit

            const uint32_t size_lg2 = m_sb_size_lg2 - count_lg2 ;

            // Set the allocated block pointer

            p = ((char*)( m_sb_state_array + m_data_offset ))
              + ( uint64_t(sb_id) << m_sb_size_lg2 ) // superblock memory
              + ( uint64_t(result.first) << size_lg2 ); // block memory

#if 0
  printf( "  MemoryPool(0x%lx) pointer(0x%lx) allocate(%lu) sb_id(%d) sb_state(0x%x) block_size(%d) block_capacity(%d) block_id(%d) block_claimed(%d)\n"
        , (uintptr_t)m_sb_state_array
        , (uintptr_t)p
        , alloc_size
        , sb_id
        , sb_state 
        , (1u << size_lg2)
        , (1u << count_lg2)
        , result.first 
        , result.second );
#endif

            break ; // Success
          }
        }
        //------------------------------------------------------------------
        //  Arrive here if failed to acquire a block.
        //  Must find a new superblock.

        //  Start searching at designated index for this block size.
        //  Look for superblock that, in preferential order,
        //  1) part-full superblock of this block size
        //  2) empty superblock to claim for this block size
        //  3) part-full superblock of the next larger block size

        sb_state = block_state ; // Expect to find the desired state
        sb_id = -1 ;

        bool update_hint = false ;
        int32_t sb_id_empty = -1 ;
        int32_t sb_id_large = -1 ;
        uint32_t sb_state_large = 0 ;

        sb_state_array = m_sb_state_array + sb_id_begin * m_sb_state_size ;

        for ( int32_t i = 0 , id = sb_id_begin ; i < m_sb_count ; ++i ) {

          //  Query state of the candidate superblock.
          //  Note that the state may change at any moment
          //  as concurrent allocations and deallocations occur.
          
          const uint32_t full_state = *sb_state_array ;
          const uint32_t used       = full_state & state_used_mask ;
          const uint32_t state      = full_state & state_header_mask ;

          if ( state == block_state ) {

            //  Superblock is assigned to this block size

            if ( used < block_count ) {

              // There is room to allocate one block

              sb_id = id ;

              // Is there room to allocate more than one block?

              update_hint = used + 1 < block_count ;

              break ;
            }
          }
          else if ( 0 == used ) {

            // Superblock is empty

            if ( -1 == sb_id_empty ) {

              // Superblock is not assigned to this block size
              // and is the first empty superblock encountered.
              // Save this id to use if a partfull superblock is not found.

              sb_id_empty = id ;
            }
          }
          else if ( ( -1 == sb_id_empty /* have not found an empty */ ) &&
                    ( -1 == sb_id_large /* have not found a larger */ ) &&
                    ( state < block_state /* a larger block */ ) &&
                    // is not full:
                    ( used < ( 1u << ( state >> state_shift ) ) ) ) {
            //  First superblock encountered that is
            //  larger than this block size and
            //  has room for an allocation.
            //  Save this id to use of partfull or empty superblock not found
            sb_id_large    = id ;
            sb_state_large = state ;
          }

          // Iterate around the superblock array:

          if ( ++id < m_sb_count ) {
            sb_state_array += m_sb_state_size ;
          }
          else {
            id = 0 ;
            sb_state_array = m_sb_state_array ;
          }
        }

 // printf("  search m_sb_count(%d) sb_id(%d) sb_id_empty(%d) sb_id_large(%d)\n" , m_sb_count , sb_id , sb_id_empty , sb_id_large);

        if ( sb_id < 0 ) {

          //  Did not find a partfull superblock for this block size.

          if ( 0 <= sb_id_empty ) {

            //  Found first empty superblock following designated superblock
            //  Attempt to claim it for this block size.
            //  If the claim fails assume that another thread claimed it
            //  for this block size and try to use it anyway,
            //  but do not update hint.

            sb_id = sb_id_empty ;

            sb_state_array = m_sb_state_array + ( sb_id * m_sb_state_size );

            //  If successfully changed assignment of empty superblock 'sb_id'
            //  to this block_size then update the hint.

            const uint32_t state_empty = state_header_mask & *sb_state_array ;

            // If this thread claims the empty block then update the hint
            update_hint =
              state_empty ==
                Kokkos::atomic_compare_exchange
                  (sb_state_array,state_empty,block_state);
          }
          else if ( 0 <= sb_id_large ) {

            // Found a larger superblock with space available

            sb_id    = sb_id_large ;
            sb_state = sb_state_large ;

            sb_state_array = m_sb_state_array + ( sb_id * m_sb_state_size );
          }
          else {
            // Did not find a potentially usable superblock
            --attempt_limit ;
          }
        }

        if ( update_hint ) {
          Kokkos::atomic_compare_exchange
            ( hint_sb_id_ptr , uint32_t(hint_sb_id) , uint32_t(sb_id) );
        }
      } // end allocation attempt loop
      //--------------------------------------------------------------------

      return p ;
    }
  // end allocate
  //--------------------------------------------------------------------------

  KOKKOS_INLINE_FUNCTION
  void deallocate( void * p , size_t /* alloc_size */ ) const noexcept
    {
      if ( 0 == p ) return ;

      // Determine which superblock and block
      const ptrdiff_t d =
        ((char*)p) - ((char*)( m_sb_state_array + m_data_offset ));

      // Verify contained within the memory pool's superblocks:
      const int ok_contains =
        ( 0 <= d ) && ( size_t(d) < ( size_t(m_sb_count) << m_sb_size_lg2 ) );

      int ok_block_aligned = 0 ;
      int ok_dealloc_once  = 0 ;

      if ( ok_contains ) {

        const int sb_id = d >> m_sb_size_lg2 ;

        // State array for the superblock.
        volatile uint32_t * const sb_state_array =
          m_sb_state_array + ( sb_id * m_sb_state_size );

        const uint32_t block_state    = (*sb_state_array) & state_header_mask ;
        const uint32_t block_size_lg2 =
          m_sb_size_lg2 - ( block_state >> state_shift );

        ok_block_aligned = 0 == ( d & ( ( 1UL << block_size_lg2 ) - 1 ) );

        if ( ok_block_aligned ) {

          // Map address to block's bit
          // mask into superblock and then shift down for block index

          const uint32_t bit =
            ( d & ( ptrdiff_t( 1LU << m_sb_size_lg2 ) - 1 ) ) >> block_size_lg2 ;

          const int result =
            CB::release( sb_state_array , bit , block_state );

          ok_dealloc_once = 0 <= result ;

#if 0
  printf( "  MemoryPool(0x%lx) pointer(0x%lx) deallocate sb_id(%d) block_size(%d) block_capacity(%d) block_id(%d) block_claimed(%d)\n"
        , (uintptr_t)m_sb_state_array
        , (uintptr_t)p
        , sb_id
        , (1u << block_size_lg2)
        , (1u << (m_sb_size_lg2 - block_size_lg2))
        , bit
        , result );
#endif
        }
      }

      if ( ! ok_contains || ! ok_block_aligned || ! ok_dealloc_once ) {
#if 0
  printf( "  MemoryPool(0x%lx) pointer(0x%lx) deallocate ok_contains(%d) ok_block_aligned(%d) ok_dealloc_once(%d)\n"
        , (uintptr_t)m_sb_state_array
        , (uintptr_t)p
        , int(ok_contains)
        , int(ok_block_aligned)
        , int(ok_dealloc_once) );
#endif
        Kokkos::abort("Kokkos MemoryPool::deallocate given erroneous pointer");
      }
    }
  // end deallocate
  //--------------------------------------------------------------------------

  KOKKOS_INLINE_FUNCTION
  int number_of_superblocks() const noexcept { return m_sb_count ; }

  KOKKOS_INLINE_FUNCTION
  void superblock_state( int sb_id
                       , int & block_size
                       , int & block_count_capacity
                       , int & block_count_used ) const noexcept
    {
      block_size           = 0 ;
      block_count_capacity = 0 ;
      block_count_used     = 0 ;

      if ( Kokkos::Impl::MemorySpaceAccess
             < Kokkos::Impl::ActiveExecutionMemorySpace
             , base_memory_space >::accessible ) {
       // Can access the state array
       
        const uint32_t state =
          ((uint32_t volatile *)m_sb_state_array)[sb_id*m_sb_state_size];

        const uint32_t block_count_lg2 = state >> state_shift ;
        const uint32_t block_used      = state & state_used_mask ;

        block_size           = 1LU << ( m_sb_size_lg2 - block_count_lg2 );
        block_count_capacity = 1LU << block_count_lg2 ;
        block_count_used     = block_used ;
      }
    }
};

} // namespace Kokkos 

#endif /* #ifndef KOKKOS_MEMORYPOOL_HPP */