Memcached Memory Management algorithm

Simple writing, after reading this part of the code memcached and I feel that the ccache is still very similar.

1) Distribution

All memory in memcached is saved by the type slabclass_t structure

typedef　struct　{
unsigned　int　size;　　　　　　/*　sizes　of　items　*/
unsigned　int　perslab;　　　/*　how　many　items　per　slab　*/

void　**slots;　　　　　　　　　　　/*　list　of　item　ptrs　*/
unsigned　int　sl_total;　　/*　size　of　previous　array　*/
unsigned　int　sl_curr;　　　/*　first　free　slot　*/

void　*end_page_ptr;　　　　　　　　　/*　pointer　to　next　free　item　at　end　of　page,　or　0　*/
unsigned　int　end_page_free;　/*　number　of　items　remaining　at　end　of　last　alloced　page　*/

unsigned　int　slabs;　　　　　/*　how　many　slabs　were　allocated　for　this　class　*/

void　**slab_list;　　　　　　　/*　array　of　slab　pointers　*/
unsigned　int　list_size;　/*　size　of　prev　array　*/

unsigned　int　killing;　　/*　index+1　of　dying　slab,　or　zero　if　none　*/
}　slabclass_t;

There is a global slabclass_t array, and the size field in slabclass_t holds the amount of data that each slab can hold. In this slabclass_t array, the Size field is incremented and the incremented factor is determined by the Slabs_ The second parameter in the INIT function is specified by the factor parameter. For example, if factor is 2, then if the size of the first slabclass_t is unsigned int size = sizeof (item) + SETTINGS.CHUNK_ Size (also the statement in the Slabs_init function), the size of the next slabclass_t is Size*factor (this ignores the alignment factor).

So, assuming the first slab can save 8byte of data, Factor is 2, then the next slab size is 16byte,32byte ...

Each time you need to allocate memory, you need to find a size larger than the minimum size of the slab, for example, or the previous slab model, if you need to allocate 30byte of space now, find the minimum slab size greater than 30byte is 32byte, So, look up the item from this slab and assign it to it.

But here is a problem, that is, the waste of surplus resources, the previous 30byte is just a waste of 2byte, but if you want to allocate 17byte, then wasted 15byte, wasted nearly 50%! So there's the reason why you need to specify factor. Users can specify different growth factor as needed to reduce the waste of resources.

2) elimination

Eliminated is the LRU algorithm, all the most recently used item is saved in the static item *tails[largest_id];(ITEM.C), the allocated memory will be stored in the form of a linked list in this array, if the corresponding slab has not allocated enough memory, To this list of inquiries, the elimination is based on the item structure of the Exptime field.

Simple analysis to this, need a more detailed explanation to see the code, in memcached with this part of the code in Slab.h (. c)/item.h (. c), two key structures are item and slabclass_t.