netdev - Re: [PATCH net-next v2] ipv6: ioam: Support for Queue depth data field

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Date:   Sun, 26 Dec 2021 13:59:08 +0100 (CET)
From:   Justin Iurman <justin.iurman@...ege.be>
To:     Ido Schimmel <idosch@...sch.org>
Cc:     netdev@...r.kernel.org, davem@...emloft.net, kuba@...nel.org,
        dsahern@...nel.org, yoshfuji@...ux-ipv6.org
Subject: Re: [PATCH net-next v2] ipv6: ioam: Support for Queue depth data
 field

On Dec 26, 2021, at 1:40 PM, Ido Schimmel idosch@...sch.org wrote:
> On Sun, Dec 26, 2021 at 12:47:51PM +0100, Justin Iurman wrote:
>> On Dec 24, 2021, at 6:53 PM, Ido Schimmel idosch@...sch.org wrote:
>> > Why 'qlen' is used and not 'backlog'? From the paragraph you quoted it
>> > seems that queue depth needs to take into account the size of the
>> > enqueued packets, not only their number.
>> 
>> The quoted paragraph contains the following sentence:
>> 
>>    "The queue depth is expressed as the current amount of memory
>>     buffers used by the queue"
>> 
>> So my understanding is that we need their number, not their size.
> 
> It also says "a packet could consume one or more memory buffers,
> depending on its size". If, for example, you define tc-red limit as 1M,
> then it makes a lot of difference if the 1,000 packets you have in the
> queue are 9,000 bytes in size or 64 bytes.

Agree. We probably could use 'backlog' instead, regarding this
statement:

  "It should be noted that the semantics of some of the node data fields
   that are defined below, such as the queue depth and buffer occupancy,
   are implementation specific.  This approach is intended to allow IOAM
   nodes with various different architectures."

It would indeed make more sense, based on your example. However, the
limit (32 bits) could be reached faster using 'backlog' rather than
'qlen'. But I guess this tradeoff is the price to pay to be as close
as possible to the spec.