On Thu, Jan 22 2009, Boaz Harrosh wrote: > Jens Axboe wrote: > > On Thu, Jan 22 2009, FUJITA Tomonori wrote: > >> On Wed, 21 Jan 2009 11:52:39 +0200 > >> Boaz Harrosh <bharrosh@xxxxxxxxxxx> wrote: > >> > >>> Currently inherited from sg.c bsg will submit asynchronous request > >>> at the head-of-the-queue, (using "at_head" set in the call to > >>> blk_execute_rq_nowait()). This is bad in situation where we want > >>> to keep the queues full but need the requests to execute in order. > >> As I wrote, I think that blk_execute_rq_nowait inserts a request and > >> plugs a queue. So how can you keep the queue full? On the completion > >> of blk_execute_rq_nowait, the queue is empty. > > > > That's not true at all. If you submit more than one request, request 2 > > and up would be queued according to the orientation given. It may even > > include request 1 as well, what if the queue is busy doing work for > > someone else already? > > > > I think the patch makes sense, I also wish that the default would have > > been reversed so that at_back would be the default. at_back is more > > complex though, since it impacts existing requests for the device (it > > drains the scheduler queue). > > > > bsg only sends BLOCK_PC commands, I think these are not held in the > scheduler queue, right? They are not, correct. But that queue may have other requests pending, which may be "normal" file system requests. Then an at_back bsg command would act almost like a barrier, draining the entire queue to dispatch. > I think like you at_back is the default to use, but this is historic > compatibility with SG that had problems with ABORT and such not. With > my patch I give the user a choice and be done with it. at_head is definitely useful and required for some situations, but that doesn't mean it's a good default :-). But yes, we are stuck with it. I think the patch makes sense. -- Jens Axboe -- 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
