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

From: Jiri Pirko <[email-masked]>
Date: Wed, 23 May 2018 11:43:31 +0200

Tue, May 22, 2018 at 07:20:26AM CEST, [email-masked] wrote:
>On Mon, 21 May 2018 14:04:57 -0700, Saeed Mahameed wrote:
>> From: Huy Nguyen <[email-masked]>
>> 
>> 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 <[email-masked]>
>> Signed-off-by: Saeed Mahameed <[email-masked]>
>
>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".

If the CX5 buffer they are trying to utilize here is per port and not a
shared one, it would seem ok for me to not have it in "devlink sb".