slab_alloc

// vi:set ft=cpp: -*- Mode: C++ -*-
/*
 * (c) 2008-2009 Alexander Warg <warg@os.inf.tu-dresden.de>,
 *               Alexander Warg <warg@os.inf.tu-dresden.de>
 *     economic rights: Technische Universität Dresden (Germany)
 *
 * License: see LICENSE.spdx (in this directory or the directories above)
 */
 
#pragma once
 
#include <l4/cxx/std_alloc>
#include <l4/cxx/hlist>
#include <l4/sys/consts.h>
 
namespace cxx {

template< int Obj_size, int Slab_size = L4_PAGESIZE,
  int Max_free = 2, template<typename A> class Alloc = New_allocator >
class Base_slab
{
private:
  struct Free_o
  {
    Free_o *next;
  };
 
protected:
  struct Slab_i;
 
private:
  struct Slab_head : public H_list_item
  {
    unsigned num_free;
    Free_o *free;
    Base_slab<Obj_size, Slab_size, Max_free, Alloc> *cache;
 
    inline Slab_head() noexcept : num_free(0), free(0), cache(0)
    {}
  };
 
  // In an empty or partially filled slab, each free object stores a pointer to
  // the next free object. Thus, the size of an object needs to be at least the
  // size of a pointer.
  static_assert(Obj_size >= sizeof(void *),
                "Object size must be at least the size of a pointer.");
  static_assert(Obj_size <= Slab_size - sizeof(Slab_head),
                "Object_size exceeds slab capability.");
 
public:
  enum
  {
    object_size      = Obj_size,
    slab_size        = Slab_size,
    objects_per_slab = (Slab_size - sizeof(Slab_head)) / object_size,
    max_free_slabs   = Max_free,
  };
 
protected:
  struct Slab_store
  {
    char _o[slab_size - sizeof(Slab_head)]; 
    Free_o *object(unsigned obj) noexcept
    { return reinterpret_cast<Free_o*>(_o + object_size * obj); }
  };

  struct Slab_i : public Slab_store, public Slab_head
  {};
 
public:
  typedef Alloc<Slab_i> Slab_alloc;
 
  typedef void Obj_type;
 
private:
  Slab_alloc _alloc;
  unsigned _num_free;
  unsigned _num_slabs;
  H_list<Slab_i> _full_slabs;
  H_list<Slab_i> _partial_slabs;
  H_list<Slab_i> _empty_slabs;

  void add_slab(Slab_i *s) noexcept
  {
    s->num_free = objects_per_slab;
    s->cache = this;
 
    //L4::cerr << "Slab: " << this << "->add_slab(" << s << ", size=" 
    //  << slab_size << "):" << " f=" << s->object(0) << '\n';
 
    // initialize free list
    Free_o *f = s->free = s->object(0);
    for (unsigned i = 1; i < objects_per_slab; ++i)
      {
  f->next = s->object(i);
  f = f->next;
      }
    f->next = 0;
 
    // insert slab into cache's list
    _empty_slabs.push_front(s);
    ++_num_slabs;
    ++_num_free;
  }

  bool grow() noexcept
  {
    Slab_i *s = _alloc.alloc();
    if (!s)
      return false;
 
    new (s, cxx::Nothrow()) Slab_i();
 
    add_slab(s);
    return true;
  }

  void shrink() noexcept
  {
    if (!_alloc.can_free)
      return;
 
    while (!_empty_slabs.empty() && _num_free > max_free_slabs)
      {
  Slab_i *s = _empty_slabs.front();
  _empty_slabs.remove(s);
  --_num_free;
  --_num_slabs;
  _alloc.free(s);
      }
  }
 
public:
  Base_slab(Slab_alloc const &alloc = Slab_alloc()) noexcept
    : _alloc(alloc), _num_free(0), _num_slabs(0), _full_slabs(),
      _partial_slabs(), _empty_slabs()
  {}
 
  ~Base_slab() noexcept
  {
    while (!_empty_slabs.empty())
      {
  Slab_i *o = _empty_slabs.front();
  _empty_slabs.remove(o);
  _alloc.free(o);
      }
    while (!_partial_slabs.empty())
      {
  Slab_i *o = _partial_slabs.front();
  _partial_slabs.remove(o);
  _alloc.free(o);
      }
    while (!_full_slabs.empty())
      {
  Slab_i *o = _full_slabs.front();
  _full_slabs.remove(o);
  _alloc.free(o);
      }
  }

  void *alloc() noexcept
  {
    H_list<Slab_i> *free = &_partial_slabs;
    if (free->empty())
      free = &_empty_slabs;
 
    if (free->empty() && !grow())
      return 0;
 
    Slab_i *s = free->front();
    Free_o *o = s->free;
    s->free = o->next;
 
    if (free == &_empty_slabs)
      {
  _empty_slabs.remove(s);
        --_num_free;
      }
 
    --(s->num_free);
 
    if (!s->free)
      {
  _partial_slabs.remove(s);
        _full_slabs.push_front(s);
      }
    else if (free == &_empty_slabs)
      _partial_slabs.push_front(s);
 
    //L4::cerr << this << "->alloc(): " << o << ", of " << s << '\n';
 
    return o;
  }

  void free(void *_o) noexcept
  {
    if (!_o)
      return;
 
    unsigned long addr = reinterpret_cast<unsigned long>(_o);
 
    // find out the slab the object is in
    addr = (addr / slab_size) * slab_size;
    Slab_i *s = reinterpret_cast<Slab_i*>(addr);
 
    if (s->cache != this)
      return;
 
    Free_o *o = reinterpret_cast<Free_o*>(_o);
 
    o->next = s->free;
    s->free = o;
 
    bool was_full = false;
 
    if (!s->num_free)
      {
  _full_slabs.remove(s);
  was_full = true;
      }
 
    ++(s->num_free);
 
    if (s->num_free == objects_per_slab)
      {
  if (!was_full)
    _partial_slabs.remove(s);
 
  _empty_slabs.push_front(s);
  ++_num_free;
  if (_num_free > max_free_slabs)
    shrink();
 
  was_full = false;
      }
    else if (was_full)
      _partial_slabs.push_front(s);
 
    //L4::cerr << this << "->free(" << _o << "): of " << s << '\n';
  }

  unsigned total_objects() const noexcept
  { return _num_slabs * objects_per_slab; }

  unsigned free_objects() const noexcept
  {
    unsigned count = 0;
 
    /* count partial slabs first */
    for (typename H_list<Slab_i>::Const_iterator s = _partial_slabs.begin();
         s != _partial_slabs.end(); ++s)
      count += s->num_free;
 
    /* add empty slabs */
    count += _num_free * objects_per_slab;
 
    return count;
  }
};

template<typename Type, int Slab_size = L4_PAGESIZE,
  int Max_free = 2, template<typename A> class Alloc = New_allocator > 
class Slab : public Base_slab<sizeof(Type), Slab_size, Max_free, Alloc>
{
private:
  typedef Base_slab<sizeof(Type), Slab_size, Max_free, Alloc> Base_type;
public:
 
  typedef Type Obj_type;
 
  Slab(typename Base_type::Slab_alloc const &alloc 
      = typename Base_type::Slab_alloc()) noexcept
    : Base_slab<sizeof(Type), Slab_size, Max_free, Alloc>(alloc) {}
 

  Type *alloc() noexcept
  {
    return reinterpret_cast<Type *>(Base_type::alloc());
  }

  void free(Type *o) noexcept
  { Base_slab<sizeof(Type), Slab_size, Max_free, Alloc>::free(o); }
};
 

template< int Obj_size, int Slab_size = L4_PAGESIZE,
  int Max_free = 2, template<typename A> class Alloc = New_allocator >
class Base_slab_static
{
private:
  typedef Base_slab<Obj_size, Slab_size, Max_free, Alloc> _A;
  static _A _a;
public:
  typedef void Obj_type;
  enum
  {
    object_size      = Obj_size,
    slab_size        = Slab_size,
    objects_per_slab = _A::objects_per_slab,
    max_free_slabs   = Max_free,
  };

  void *alloc() noexcept { return _a.alloc(); }

  void free(void *p) noexcept { _a.free(p); }

  unsigned total_objects() const noexcept { return _a.total_objects(); }

  unsigned free_objects() const noexcept { return _a.free_objects(); }
};
 
 
template< int _O, int _S, int _M, template<typename A> class Alloc >
typename Base_slab_static<_O,_S,_M,Alloc>::_A 
  Base_slab_static<_O,_S,_M,Alloc>::_a; 

template<typename Type, int Slab_size = L4_PAGESIZE,
  int Max_free = 2, template<typename A> class Alloc = New_allocator > 
class Slab_static 
: public Base_slab_static<sizeof(Type), Slab_size, Max_free, Alloc>
{
public:
 
  typedef Type Obj_type;
  Type *alloc() noexcept
  {
    return reinterpret_cast<Type *>(
      Base_slab_static<sizeof(Type), Slab_size, Max_free, Alloc>::alloc());
  }
};
 
}