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Message-ID: <a50bd6bd-5cae-49d9-8bd8-172f88035d18@nvidia.com>
Date: Fri, 6 Sep 2024 15:17:50 +0300
From: Edward Srouji <edwards@...dia.com>
To: Zhu Yanjun <yanjun.zhu@...ux.dev>, Leon Romanovsky <leon@...nel.org>,
 Jason Gunthorpe <jgg@...dia.com>
Cc: Leon Romanovsky <leonro@...dia.com>, linux-kernel@...r.kernel.org,
 linux-rdma@...r.kernel.org, netdev@...r.kernel.org,
 Saeed Mahameed <saeedm@...dia.com>, Tariq Toukan <tariqt@...dia.com>,
 Yishai Hadas <yishaih@...dia.com>
Subject: Re: [PATCH rdma-next 0/2] Introduce mlx5 data direct placement (DDP)


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@...dia.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 :).


>
> 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
>>>
>

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