Re: [RFC] libibverbs IB Device Memory support

[Jason Gunthorpe <jgunthorpe-ePGOBjL8dl3ta4EC/59zMFaTQe2KTcn/@public.gmane.org>]

On Mon, Jun 05, 2017 at 08:08:25PM +0300, Leon Romanovsky wrote:
> On Mon, Jun 05, 2017 at 11:44:00AM -0500, Christoph Lameter wrote:
> > On Wed, 10 May 2017, ahmad omary wrote:
> >
> > > We have considered using mmap(), but As the size of device memory may be limited
> > > ,the way to access it from host cpu may differ from vendor to vendor, due to
> > > the 4K (page) aligment limitation of mmap() and the need not to directly
> > > allow user to access the device memory, there is a need for a wrapper access
> > > methods API that allows allocating and managing chunks that are smaller than
> > > 4KB and not necessarily aligned to 4KB (page size).
> >
> > Why are 4k sized chunks a problem given that there are megabytes of memory
> > in these devices? We are using various adapters already with an mmapped
> > solution here.
> 
> Ahmad presented use case where he needs access to small objects
> (semaphores) in large scale (MPI). 1MB in the granularity of 4k will give us
> 256 chunks only, and it is definitely not enough.

Is 256 chunks per machine not enough? A single process could carve out
smaller regions from the 4k kernel allocation.

Jason
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