Question about `timer base'

[Lee David]

Hi, all

When I reading the Cascading Timer Wheel code, I found there is a
`base' field and so does in hrtimers.

struct timer_list {
struct list_head entry;
unsigned long expires;
void (*function)(unsigned long);
unsigned long data;
struct timer_base_s *base; /* timer base (1) */
};

struct hrtimer {
struct rb_node node;
ktime_t expires;
enum hrtimer_state state;
int (*function)(void *);
void *data;
struct hrtimer_base *base; /* timer base (2) */
};

Can somebody pls tell me what does 'timer base' exact mean?
And what purpose does the two `base' field above used for?

Thanks in advance,
D.L.

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