Interrupts : request_percpu_irq function

I was reading about request_irq() function. We have another variant of this function known as request_percpu_irq(). Can any one explain me the usage of this function? and how it is different that the traditional request_irq(). I also couldn't find a single driver using this function though this has been extensively used in architecture specific files especially the timers and the watchdog. Also it would be great, if someone can point out any document/link where I can find the description. I failed to find it from Linux kernel documentation. Regards, Priyaranjan