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

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Message-ID: <751671897.247201108.1640614002305.JavaMail.zimbra@uliege.be>
Date:   Mon, 27 Dec 2021 15:06:42 +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 2:15 PM, Ido Schimmel idosch@...sch.org wrote:
> On Sun, Dec 26, 2021 at 01:59:08PM +0100, Justin Iurman wrote:
>> 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.
> 
> At least in Linux 'backlog' is 32 bits so we are OK :)
> We don't have such big buffers in hardware and I'm not sure what
> insights an operator will get from a queue depth larger than 4GB...

Indeed :-)

> I just got an OOO auto-reply from my colleague so I'm not sure I will be
> able to share his input before next week. Anyway, reporting 'backlog'
> makes sense to me, FWIW.

Right. I read that Linus is planning to release a -rc8 so I think I can
wait another week before posting -v3.