netdev - Re: [net-next 1/6] net/dcb: Add dcbnl buffer attribute

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Date:   Tue, 22 May 2018 10:36:17 -0500
From:   Huy Nguyen <huyn@...lanox.com>
To:     Jakub Kicinski <jakub.kicinski@...ronome.com>,
        Saeed Mahameed <saeedm@...lanox.com>
Cc:     "David S. Miller" <davem@...emloft.net>, netdev@...r.kernel.org,
        Jiri Pirko <jiri@...nulli.us>,
        Or Gerlitz <gerlitz.or@...il.com>,
        Parav Pandit <parav@...lanox.com>
Subject: Re: [net-next 1/6] net/dcb: Add dcbnl buffer attribute

On 5/22/2018 12:20 AM, Jakub Kicinski wrote:
> On Mon, 21 May 2018 14:04:57 -0700, Saeed Mahameed wrote:
>> From: Huy Nguyen <huyn@...lanox.com>
>>
>> In this patch, we add dcbnl buffer attribute to allow user
>> change the NIC's buffer configuration such as priority
>> to buffer mapping and buffer size of individual buffer.
>>
>> This attribute combined with pfc attribute allows advance user to
>> fine tune the qos setting for specific priority queue. For example,
>> user can give dedicated buffer for one or more prirorities or user
>> can give large buffer to certain priorities.
>>
>> We present an use case scenario where dcbnl buffer attribute configured
>> by advance user helps reduce the latency of messages of different sizes.
>>
>> Scenarios description:
>> On ConnectX-5, we run latency sensitive traffic with
>> small/medium message sizes ranging from 64B to 256KB and bandwidth sensitive
>> traffic with large messages sizes 512KB and 1MB. We group small, medium,
>> and large message sizes to their own pfc enables priorities as follow.
>>    Priorities 1 & 2 (64B, 256B and 1KB)
>>    Priorities 3 & 4 (4KB, 8KB, 16KB, 64KB, 128KB and 256KB)
>>    Priorities 5 & 6 (512KB and 1MB)
>>
>> By default, ConnectX-5 maps all pfc enabled priorities to a single
>> lossless fixed buffer size of 50% of total available buffer space. The
>> other 50% is assigned to lossy buffer. Using dcbnl buffer attribute,
>> we create three equal size lossless buffers. Each buffer has 25% of total
>> available buffer space. Thus, the lossy buffer size reduces to 25%. Priority
>> to lossless  buffer mappings are set as follow.
>>    Priorities 1 & 2 on lossless buffer #1
>>    Priorities 3 & 4 on lossless buffer #2
>>    Priorities 5 & 6 on lossless buffer #3
>>
>> We observe improvements in latency for small and medium message sizes
>> as follows. Please note that the large message sizes bandwidth performance is
>> reduced but the total bandwidth remains the same.
>>    256B message size (42 % latency reduction)
>>    4K message size (21% latency reduction)
>>    64K message size (16% latency reduction)
>>
>> Signed-off-by: Huy Nguyen <huyn@...lanox.com>
>> Signed-off-by: Saeed Mahameed <saeedm@...lanox.com>
> On a cursory look this bares a lot of resemblance to devlink shared
> buffer configuration ABI.  Did you look into using that?
>
> Just to be clear devlink shared buffer ABIs don't require representors
> and "switchdev mode".
> .
[HQN] Dear Jakub, there are several reasons that devlink shared buffer 
ABI cannot be used:
1. The devlink shared buffer ABI is written based on the switch cli 
which you can find out more
from this link https://community.mellanox.com/docs/DOC-2558.
2. The dcbnl interfaces have been used for QoS settings. In NIC, the 
buffer configuration are tied to
priority (ETS PFC). The buffer configuration are not tied to port like 
switch.
3. Shared buffer, alpha, threshold are switch specific terms.

Please let me know if you have any further question.
Regards,
Huy Nguyen