Some of the SPARC servers from Sun support an IO memory mapping
mechanism for marking pages for weak or strong ordering. It has no
dependency on the HCA. Marking memory pages for data buffers as weak
ordered delivers significantly better throughput on IB.
I assumed that there are many more applications using the user verbs
that don't care about ordering for data buffers, but would need to be
modified to set a "weak ordering" flag.
Nonetheless, I'm somewhat flexible and willing to go with bit definition
that that verbs consumers prefer.
-David
Roland Dreier wrote:
> I decided to minimize the impact of an API change on the class of
> applications that use the current verbs interface because those
> applications can safely run on platforms that deliver optimal
> performance using weak ordering for data buffers. New binaries aren't
> required for this class of application.
>
> I thought it would be more appropriate to put the burden of added
> complexity on the class of applications that bypass the verbs to
> access special features in the hardware. In fact, those applications
> are selective about memory regions that need this special handling and
> would register lots of memory without the "strong ordering' bit. How
> applications determine that the platform is capable of performing the
> request would be beyond the scope of the verbs, however, I suppose
> that the verbs framework could check and return an error.
>
> If there are applications that expect the hardware to support "strong
> ordering" and don't check the hardware, then these might be a problem.
> Do any of these exists?
>
> By the way, if I had proposed this bit several years ago, then I would
> have chosen a "weak ordering" flag. Instead, I decided to try
> protecting the existing base of verbs-based software.
I can't really follow this. Right now Open MPI et al assume that if
they see a Mellanox adapter, they get the "last byte of RDMA becomes
visible last" behavior. And there is not a way that I know of to turn
this off at all, let alone get any performance difference. The
exception being the Cell processor system that started the previous
discussion, where weak ordering at the platform level helped things.
But given that current software does seem to rely on ordering, it seems
that opting into weak ordering would break fewer applications.
- R.
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