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

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Date:   Sun, 26 Dec 2021 14:40:41 +0200
From:   Ido Schimmel <idosch@...sch.org>
To:     Justin Iurman <justin.iurman@...ege.be>
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 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.

> 
> > Did you check what other IOAM implementations (SW/HW) report for queue
> > depth? I would assume that they report bytes.
> 
> Unfortunately, IOAM is quite new, and so IOAM implementations don't
> grow on trees. The Linux kernel implementation is one of the first,
> except for VPP and IOS (Cisco) which did not implement the queue
> depth data field.

At least on Mellanox/Nvidia switches, queue depth (not necessarily for
IOAM) is always reported in bytes. I have a colleague who authored a few
IOAM IETF drafts, I will ask for his input on this and share.