[openib-general] NFS/RDMA client release for Linux 2.6.15

[Caitlin Bestler]

On 2/20/06, Roland Dreier <rdreier at cisco.com> wrote:
>
>     Thomas> And, this is only one of many memory registration
>     Thomas> modes. We would use memory windows, if only OpenIB
>     Thomas> provided them (yes I know the hardware currently sucks for
>     Thomas> them). We will add FMR support shortly. In both these
>     Thomas> modes we perform all addressing by the book via 1-1 OpenIB
>     Thomas> registration.
>
> Memory windows don't solve anything here, do they?  You still have to
> register the full region using bus addresses somehow.
>
> - R.
> _______________________________________________


Keep in mind that there are two problems: registering memory and exposing
memory.
Windows solves the latter problem. FMR work requests solves both.

A kernel based storage related client will frequently want to form logical
buffers
from scattered physical pages. The pages selected are not necessarily part
of an existing virtual memory map, and especially not a registered one.

You can solve the memory registration problem once by creating an
exportable memory region that covers all of physical memory. The
problem is that you do not want to advertise that regions RKey/STag.

Memory Windows solve that problem, by allowing you to bind windows
within the memory region. The problem is that if the buffer is not
physically
continquous then you still have to export a multi-element list in order to
have the peer to read from a discontiguous target.

A Fast Memory Region allows arbitrary sets of pages to form a single
logical window for the purposes of peer-to-peer interaction, and has a
life cycle that more naturally maps the duration when the pages have
to be iomapped.
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