object expiry tracking memroy usage

Bitrot daemon tracks objects for expiry in a data structure known as "timer-wheel" (after which the object is signed). It's a well known data structure for tracking million of objects of expiry. Let's see the memory usage involved when tracking 1 million objects (per brick).

Bitrot daemon uses "br_object" structure to hold information needed for signing. An instance of this structure is allocated for each object that needs to be signed.

            struct br_object {
                    xlator_t *this;

                    br_child_t *child;

                    void *data;
                    uuid_t gfid;
                    unsigned long signedversion;

                    struct list_head list;

Timer-wheel requires an instance of the structure below per object that needs to be tracked for expiry.

            struct gf_tw_timer_list {
                    void *data;
                    unsigned long expires;

                    /** callback routine */
                    void (*function)(struct gf_tw_timer_list *, void *, unsigned long);

                    struct list_head entry;

Structure sizes:

  • sizeof (struct br_object): 64 bytes
  • sizeof (struct gf_tw_timer_list): 40 bytes

Together, these structures take up 104 bytes. To track all 1 million objects at the same time, the amount of memory taken up would be:

1,000,000 * 104 bytes: ~100MB

Not so bad, I think.