Re: [PATCH] bsg: Add support for submitting requests at tail of queue

[Jens Axboe <jens.axboe@oracle.com>]

On Thu, Jan 22 2009, FUJITA Tomonori wrote:
> On Wed, 21 Jan 2009 11:52:39 +0200
> Boaz Harrosh <bharrosh@panasas.com> 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).

-- 
Jens Axboe