qmckl/org/qmckl_memory.org

Memory management

• Memory data structure for the context
• Passing info to allocation routines
• Allocation/deallocation functions

We override the allocation functions to enable the possibility of optimized libraries to fine-tune the memory allocation.

Memory data structure for the context

Every time a new block of memory is allocated, the information relative to the allocation is stored in a new qmckl_memory_info_struct. A qmckl_memory_info_struct contains the pointer to the memory block, its size in bytes, and extra implementation-specific information such as alignment, pinning, if the memory should be allocated on CPU or GPU etc.

typedef struct qmckl_memory_info_struct {
size_t size;
void*  pointer;
} qmckl_memory_info_struct;

static const qmckl_memory_info_struct qmckl_memory_info_struct_zero =
{
 .size = (size_t) 0,
 .pointer = NULL
};

The memory element of the context is a data structure which contains an array of qmckl_memory_info_struct, the size of the array, and the number of allocated blocks.

typedef struct qmckl_memory_struct {
size_t                    n_allocated;
size_t                    array_size;
qmckl_memory_info_struct* element;
} qmckl_memory_struct;

Passing info to allocation routines

Passing information to the allocation routine should be done by passing an instance of a qmckl_memory_info_struct.

Allocation/deallocation functions

Memory allocation inside the library should be done with qmckl_malloc. It lets the library choose how the memory will be allocated, and a pointer is returned to the user. The context is passed to let the library store data related to the allocation inside the context. In this particular implementation of the library, we store a list of allocated pointers so that all the memory can be properly freed when the library is de-initialized. If the allocation failed, the NULL pointer is returned.

void* qmckl_malloc(qmckl_context context,
                 const qmckl_memory_info_struct info);

void* qmckl_malloc(qmckl_context context, const qmckl_memory_info_struct info) {

assert (qmckl_context_check(context) != QMCKL_NULL_CONTEXT);

qmckl_context_struct* const ctx = (qmckl_context_struct* const) context;

/* Allocate memory and zero it */
void * pointer = malloc(info.size);
if (pointer == NULL) {
  return NULL;
}
memset(pointer, 0, info.size);

qmckl_lock(context);
{
  /* If qmckl_memory_struct is full, reallocate a larger one */
  if (ctx->memory.n_allocated == ctx->memory.array_size) {
    const size_t old_size = ctx->memory.array_size;
    qmckl_memory_info_struct * new_array = realloc(ctx->memory.element,
                                                        2L * old_size *
                                                        sizeof(qmckl_memory_info_struct));
    if (new_array == NULL) {
      qmckl_unlock(context);
      free(pointer);
      return NULL;
    }

    memset( &(new_array[old_size]), 0, old_size * sizeof(qmckl_memory_info_struct) );
    ctx->memory.element = new_array;
    ctx->memory.array_size = 2L * old_size;
  }

  /* Find first NULL entry */
  size_t pos = (size_t) 0;
  while ( pos < ctx->memory.array_size && ctx->memory.element[pos].size > (size_t) 0) {
    pos += (size_t) 1;
  }
  assert (ctx->memory.element[pos].size == (size_t) 0);

  /* Copy info at the new location */
  ctx->memory.element[pos].size    = info.size;
  ctx->memory.element[pos].pointer = pointer;
  ctx->memory.n_allocated += (size_t) 1;
}
qmckl_unlock(context);

return pointer;
}

/* Create a context */
qmckl_context context = qmckl_context_create();

qmckl_memory_info_struct info = qmckl_memory_info_struct_zero;
info.size = (size_t) 3;

/* Allocate an array of ints */
int *a = (int*) qmckl_malloc(context, info);

/* Check that array of ints is OK */
assert(a != NULL);
a[0] = 1;  assert(a[0] == 1);
a[1] = 2;  assert(a[1] == 2);
a[2] = 3;  assert(a[2] == 3);

/* Allocate another array of ints */
int *b = (int*) qmckl_malloc(context, info);

/* Check that array of ints is OK */
assert(b != NULL);
b[0] = 1;  assert(b[0] == 1);
b[1] = 2;  assert(b[1] == 2);
b[2] = 3;  assert(b[2] == 3);

When freeing the memory with qmckl_free, the context is passed, in case some important information has been stored related to memory allocation and needs to be updated.

qmckl_exit_code qmckl_free(qmckl_context context,
                         void * const ptr);

qmckl_exit_code qmckl_free(qmckl_context context, void * const ptr) {

if (qmckl_context_check(context) == QMCKL_NULL_CONTEXT) {
   return qmckl_failwith(context,
                         QMCKL_INVALID_CONTEXT,
                         "qmckl_free",
                         NULL);
}

if (ptr == NULL) {
 return qmckl_failwith(context,
                       QMCKL_INVALID_ARG_2,
                       "qmckl_free",
                       "NULL pointer");
}

qmckl_context_struct* const ctx = (qmckl_context_struct* const) context;

qmckl_lock(context);
{
 /* Find pointer in array of saved pointers */
 size_t pos = (size_t) 0;
 while ( pos < ctx->memory.array_size && ctx->memory.element[pos].pointer != ptr) {
   pos += (size_t) 1;
 }

 if (pos >= ctx->memory.array_size) {
   /* Not found */
   qmckl_unlock(context);
   return qmckl_failwith(context,
                         QMCKL_FAILURE,
                         "qmckl_free",
                         "Pointer not found in context");
 }

 free(ptr);

 memset( &(ctx->memory.element[pos]), 0, sizeof(qmckl_memory_info_struct) );
 ctx->memory.n_allocated -= (size_t) 1;
}
qmckl_unlock(context);

return QMCKL_SUCCESS;
}