[PATCH v4, under testing] nvme-rdma: support devices with queue size < 32

[Doug Ledford]

On 5/3/2017 12:24 PM, Bart Van Assche wrote:
> On Wed, 2017-05-03 at 12:17 -0400, Doug Ledford wrote:
>> On 5/3/2017 11:58 AM, Bart Van Assche wrote:
>>> On Wed, 2017-05-03 at 09:53 -0600, Jason Gunthorpe wrote:
>>>> On Wed, May 03, 2017 at 05:19:27PM +0200, Marta Rybczynska wrote:
>>>>
>>>>>> where
>>>>>> 	queue->sig_limit = max(queue->queue_size / 2, 1);
>>>>>
>>>>> I tried to avoid that because this adds a division in the fast path Bart
>>>>> was unhappy about in v2.
>>>>
>>>> The compiler switches divide/multiply by powers of two into fast bit shifts.
>>>
>>> Hello Jason,
>>>
>>> As far as I know the compiler only does that for compile-time constants. In
>>> this case the divisor (max(queue_size / 2, 1)) is not a compile-time constant.
>>
>> Sure it is.  The only thing that needs to be constant for the compiler
>> to do the right thing is the '/ 2' part.  queue_size need not be
>> constant, and the max is performed after the division.  I would fully
>> expect the compiler to get this right and convert it internally to the
>> equivalent bit shift, but if it didn't you could always just write it
>> that way in the first place:
>>
>> queue->sig_limit = max(queue->queue_size >> 1, 1);
> 
> Hello Doug,
> 
> In my comment I was referring to "% max(queue_size / 2, 1)" and not to
> "queue_size / 2".

Sorry, too much context was cut for that to come through.


-- 
Doug Ledford <dledford at redhat.com>
    GPG Key ID: B826A3330E572FDD
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