got problem with timer interrupt

A timer has been added into my system, and it  is used to generate
continuous interrupt every 1 ms.

This hw is register by request_irq(19, handler_1ms)

Howeve I found that , it is longer than 1 ms between two interrupts,
for I used get_cycles everytime we entered

do_irq :

  if(irq==19)
     {
        delta = get_cycle()-old;
     }

  but to find that delta is always larger than cpu cycles between 1ms.


Does anyone know why this happened?


regards,