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

[Bart Van Assche]

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".

Bart.