On Thu, Apr 12 2007, Ming Zhang wrote: > On Thu, 2007-04-12 at 23:16 +0200, Jens Axboe wrote: > > On Thu, Apr 12 2007, Ming Zhang wrote: > > > On Thu, 2007-04-12 at 13:29 +0200, Jens Axboe wrote: > > > > On Wed, Apr 11 2007, Ming Zhang wrote: > > > > > Hi All > > > > > > > > > > For this ISSUE event, currently it is in elv_next_request(), any idea > > > > > why it is not in elv_dequeue_request() which is where the request > > > > > marked > > > > > as on-the-fly and send to lower level? > > > > > > > > elv_next_request() is the driver hand-off point, so should be pretty > > > > close to the issue time unless the request gets requeued due to some > > > > busy condition (which will also be logged). elv_dequeue_request() may > > > > happen much later, some drivers do it right before calling the io > > > > completion handler - IDE does this - since it leaves the request on the > > > > queue list for the duration of the operation. So moving the ISSUE event > > > > to elv_dequeue_request() would not be correct. > > > > > > > > > > ic. i assumed all requests will be removed from queue before llDD handle > > > it. > > > > > > in 2.6.20 ele_dequeue_request > > > > > > 771 > > > 772 /* > > > 773 * the time frame between a request being removed from the > > > lists > > > 774 * and to it is freed is accounted as io that is in progress > > > at > > > 775 * the driver side. > > > 776 */ > > > 777 if (blk_account_rq(rq)) > > > 778 q->in_flight++; > > > > > > then this in_flight counter is more likely to be how many outstanding > > > requests that not in the queue and before it is free. and it might be > > > less than how many undergoing IOs? > > > > It's good enough for what ->in_flight is used for. Your assumption on > > that all low level drivers dequeue before handling a request is wrong. > > Usually only drivers that do queueing do this. > > so u meant most driver will leave request in queue unless they do > internal queuing?
Not internal queuing, queuing at the device level. No drivers should do internal queuing, they should always push requests back if they cannot handle them. > does this mean the queue can be unnecessary long and travel the list > will go through some requests that under io already? Typically no, because such devices will only have 1 request active at the time. -- Jens Axboe - To unsubscribe from this list: send the line "unsubscribe linux-btrace" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
