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

[Boaz Harrosh <bharrosh@panasas.com>]

Jens Axboe wrote:
> 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).
> 

bsg only sends BLOCK_PC commands, I think these are not held in the
scheduler queue, right? 

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.

Thanks
Boaz