On Tue, May 24, 2011 at 1:16 AM, Pallav Bose <pallavbose@xxxxxxxxx> wrote: > Another question that I have is regarding my second experiment. It's as follows: > > Create a bio structure in my driver and call the __make_request() > function of the lower level driver. Only one 2560 bytes write request > is sent from my driver. > > Once this write is serviced, an interrupt is generated which is > intercepted by do_IRQ(). Finally, the function blk_complete_request() > is called. Keep in mind that we are still in the top half of the > interrupt handler. Now, we compose another struct bio in > blk_complete_request() and call the > __make_request() function. We record a timestamp at this point > (say T0). When the request completion callback is obtained, we > record another timestamp (call it T1). The difference - T1 - T0 - is > always above 1 millisec. This experiment was repeated numerous times, > and each time, the destination sector affected this difference - T1 - T0. > It was observed that if the destination sectors are separated by > approximately 350 sectors, the time difference is about 1.2 millisec > (never below > 1 millisec) for requests of size 2560 bytes. > > My understanding is that since the destination sectors of consecutive > requests have been separated by a fairly large amount, by the time the > next request is issued, the requested sector would be almost below the > disk head and thus the write should happen immediately and T1 - T0 > should be small (at least < 1 millisec). > > The Serial ATA AHCI 1.3 Specification (page 114) states that: > > When a software specified number of commands have completed or **a > software specified timeout has expired, an interrupt is generated by > hardware to allow software to process completed commands.** > > I assumed that this timer maybe the reason why the latency of each > request is above 1 millisec; that's why I wanted to disable CCC. > > Now since CCC is out of the picture, what could be the reason why the > latency is above 1 millisec always? Can we do better? > > Regards, > Pallav Any further thoughts on why the latency is above 1 millisec in the experiment that I have described above? Thanks, Pallav -- To unsubscribe from this list: send the line "unsubscribe linux-ide" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
