Re: strong ordering for data registered memory

[Richard Frank <richard.frank-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org>]

Would anyone like to through out the list of HCAs that do this... I can 
guess at a few...
and can ask the vendors directly.. if not.. .
 
It would be much nicer to not hardcode names of adapters.. but that won't
stop us.. :)

David Brean wrote:
> Yes, there are HCAs that provide strong ordering.  And an application 
> such as OpenMPI checks the HCA model and if appropriate enables a 
> mechanism called "eager RDMA" that depends on it.
>
> -David
>
> Richard Frank wrote:
>> Today apps are forced to assume that all transports can not provide 
>> strong ordering..
>> and hence must implement solutions to work around this.
>>
>> There are specific optimizations an app might make if it knows the 
>> underpinning
>> transport can make these guarantees..
>>
>> It would be useful if "strong ordering" were exposed as attribute 
>> from a transport..
>> and as well - have the ability to provide a hint to enable "strong 
>> ordering" on either
>> registration or per operation or at the qp level .
>>
>> Are there any HCAs that provide "strong ordering" today ?
>>
>> Roland Dreier wrote:
>>>  > Some time ago there was an email sent to this group with the subject
>>>  > "weak ordering for data registered memory".  I don't recall any 
>>> action
>>>  > resulting from this thread.  So, I have a question.  If a bit were
>>>  > defined to specify "strong ordering", perhaps as a "access" flag 
>>> (see
>>>  > ibv_access_flags) and used with ibv_reg_mr(), would that be 
>>> sufficient
>>>  > for (1) client applications that need a HW "guarantee" of writing 
>>> the
>>>  > last byte of an RDMA last and (2) platform implementations that need
>>>  > to deliver that feature?
>>>
>>> What would happen if an application asked for strong ordering and the
>>> adapter and/or platform is not capable of that?
>>>
>>> Weak ordering is a bit easier to handle -- the app is saying "if you 
>>> can
>>> make things go faster, don't worry about ordering here" and a platform
>>> where it doesn't matter can just ignore it.
>>>
>>>  - R.
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