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

[Edward Srouji <edwards@nvidia.com>]

On 9/6/2024 6:17 PM, Zhu Yanjun wrote:
> External email: Use caution opening links or attachments
>
>
> 在 2024/9/6 20:17, Edward Srouji 写道:
>>
>> On 9/6/2024 8:02 AM, Zhu Yanjun wrote:
>>> External email: Use caution opening links or attachments
>>>
>>>
>>> 在 2024/9/5 20:23, Edward Srouji 写道:
>>>>
>>>> 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.
>>>
>>> Got it. It seems that all the functionalities are implemented in HW. 
>>> The
>>> SW only receives OOO WR_IDs. Thanks a lot. Perhaps it is helpful to 
>>> RDMA
>>> LAG devices. It should enhance the performance^_^
>>>
>>> BTW, do you have any performance data with this feature?
>>
>> Not yet. We tested it functionality wise for now.
>>
>> But we should be able to measure its performance soon :).
>
> Thanks a lot. It is an interesting feature. If performance reports,
> please share them with us.
Sure, will do.
>
>
> IMO, perhaps this feature can be used in random read/write devices, for
> example, hard disk?
>
> Just my idea. Not sure if you have applied this feature with hard disk
> or not.
You're right, it can be used with storage and we're planning to do this 
integration and usage in the near future.
>
> Best Regards,
>
> Zhu Yanjun
>
>>
>>
>>>
>>> Best Regards,
>>> Zhu Yanjun
>>>
>>>>
>>>>>
>>>>> 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
>>>>>
>>>
> -- 
> Best Regards,
> Yanjun.Zhu
>