On Fri, Dec 14, 2012 at 05:15:37PM +0100, Bart Van Assche wrote: > On 12/14/12 17:44, scameron@xxxxxxxxxxxxxxxxxx wrote: > >I expect the flash devices re-order requests as well, simply because > >to feed requests to the things at a sufficient rate, you have to pump > >requests into them concurrently on multiple hardware queues -- a single > >cpu jamming requests into them as fast as it can is still not fast enough > >to keep them busy. Consequently, they *can't* care about ordering, as the > >relative order requests on different hardware queues are submitted into > >them > >is not even really controlled, so the OS *can't* count on concurrent > >requests > >not to be essentially "re-ordered", just because of the nature of the way > >requests get into the device. > > Why should a flash device have to reorder write requests ? These devices > typically use a log-structured file system internally. It's not so much that they are re-ordered as that there is no controlled ordering to begin with because multiple cpus are submitting to multiple hardware queues concurrently. If you have 12 requests coming in on 12 cpus to 12 hardware queues to the device, it's going to be racy as to which request is processed first by the device -- and this is fine, the hardware queues are independent of one another and do not need to worry about each other. This is all to provide a means of getting enough commands on the device to actually keep it busy. A single cpu can't do it, the device is too fast. If you have ordering dependencies such that request A must complete before request B completes, then don't submit A and B concurrently, because if you do submit them concurrently, you cannot tell whether A or B will arrive into the device first because they may go into it via different hardware queues. Note, in case it isn't obvious, the hardware queues I'm talking about here are not the struct scsi_device, sdev->request_queue queues, they are typically ring buffers in host memory from which the device DMAs commands/responses to/from depending on if it's a submit queue or a completion queue and with producer/consumer indexes one of which is in host memory and one of which is a register on the device (which is which depends on the direction of the queue, from device (pi = host memory, ci = device register), or to device (pi = device register, ci = host memory)) -- steve -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
