Re: [PATCH rdma-next 0/2] Introduce mlx5 data direct placement (DDP)

[Edward Srouji <edwards@nvidia.com>]

On 9/4/2024 2:53 PM, Zhu Yanjun wrote:
> External email: Use caution opening links or attachments
>
>
> 在 2024/9/4 16:27, Edward Srouji 写道:
>>
>> On 9/4/2024 9:02 AM, Zhu Yanjun wrote:
>>> External email: Use caution opening links or attachments
>>>
>>>
>>> 在 2024/9/3 19:37, Leon Romanovsky 写道:
>>>> From: Leon Romanovsky <leonro@nvidia.com>
>>>>
>>>> Hi,
>>>>
>>>> This series from Edward introduces mlx5 data direct placement (DDP)
>>>> feature.
>>>>
>>>> This feature allows WRs on the receiver side of the QP to be consumed
>>>> out of order, permitting the sender side to transmit messages without
>>>> guaranteeing arrival order on the receiver side.
>>>>
>>>> When enabled, the completion ordering of WRs remains in-order,
>>>> regardless of the Receive WRs consumption order.
>>>>
>>>> RDMA Read and RDMA Atomic operations on the responder side continue to
>>>> be executed in-order, while the ordering of data placement for RDMA
>>>> Write and Send operations is not guaranteed.
>>>
>>> It is an interesting feature. If I got this feature correctly, this
>>> feature permits the user consumes the data out of order when RDMA Write
>>> and Send operations. But its completiong ordering is still in order.
>>>
>> Correct.
>>> Any scenario that this feature can be applied and what benefits will be
>>> got from this feature?
>>>
>>> I am just curious about this. Normally the users will consume the data
>>> in order. In what scenario, the user will consume the data out of 
>>> order?
>>>
>> One of the main benefits of this feature is achieving higher bandwidth
>> (BW) by allowing
>> responders to receive packets out of order (OOO).
>>
>> For example, this can be utilized in devices that support multi-plane
>> functionality,
>> as introduced in the "Multi-plane support for mlx5" series [1]. When
>> mlx5 multi-plane
>> is supported, a single logical mlx5 port aggregates multiple physical
>> plane ports.
>> In this scenario, the requester can "spray" packets across the
>> multiple physical
>> plane ports without guaranteeing packet order, either on the wire or
>> on the receiver
>> (responder) side.
>>
>> With this approach, no barriers or fences are required to ensure
>> in-order packet
>> reception, which optimizes the data path for performance. This can
>> result in better
>> BW, theoretically achieving line-rate performance equivalent to the
>> sum of
>> the maximum BW of all physical plane ports, with only one QP.
>
> Thanks a lot for your quick reply. Without ensuring in-order packet
> reception, this does optimize the data path for performance.
>
> I agree with you.
>
> But how does the receiver get the correct packets from the out-of-order
> packets efficiently?
>
> The method is implemented in Software or Hardware?


The packets have new field that is used by the HW to understand the 
correct message order (similar to PSN).

Once the packets arrive OOO to the receiver side, the data is scattered 
directly (hence the DDP - "Direct Data Placement" name) by the HW.

So the efficiency is achieved by the HW, as it also saves the required 
context and metadata so it can deliver the correct completion to the 
user (in-order) once we have some WQEs that can be considered an 
"in-order window" and be delivered to the user.

The SW/Applications may receive OOO WR_IDs though (because the first CQE 
may have consumed Recv WQE of any index on the receiver side), and it's 
their responsibility to handle it from this point, if it's required.

>
> I am just interested in this feature and want to know more about this.
>
> Thanks,
>
> Zhu Yanjun
>
>>
>> [1] https://lore.kernel.org/lkml/cover.1718553901.git.leon@kernel.org/
>>> Thanks,
>>> Zhu Yanjun
>>>
>>>>
>>>> Thanks
>>>>
>>>> Edward Srouji (2):
>>>>    net/mlx5: Introduce data placement ordering bits
>>>>    RDMA/mlx5: Support OOO RX WQE consumption
>>>>
>>>>   drivers/infiniband/hw/mlx5/main.c    |  8 +++++
>>>>   drivers/infiniband/hw/mlx5/mlx5_ib.h |  1 +
>>>>   drivers/infiniband/hw/mlx5/qp.c      | 51
>>>> +++++++++++++++++++++++++---
>>>>   include/linux/mlx5/mlx5_ifc.h        | 24 +++++++++----
>>>>   include/uapi/rdma/mlx5-abi.h         |  5 +++
>>>>   5 files changed, 78 insertions(+), 11 deletions(-)
>>>>
>>>
> -- 
> Best Regards,
> Yanjun.Zhu
>