netdev - Re: [Patch net-next 3/4] sch_htb: update backlog as well

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Date:	Wed, 14 Oct 2015 21:21:56 -0700
From:	Cong Wang <xiyou.wangcong@...il.com>
To:	Jamal Hadi Salim <jhs@...atatu.com>
Cc:	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	Cong Wang <cwang@...pensource.com>
Subject: Re: [Patch net-next 3/4] sch_htb: update backlog as well

On Wed, Oct 14, 2015 at 5:25 AM, Jamal Hadi Salim <jhs@...atatu.com> wrote:
> On 10/12/15 14:38, Cong Wang wrote:
>>
>> It is odd to see qlen!=0 but backlog==0, for a real example:
>>
>
> Backlog is a transient stat so a lot of times it should be 0. Only when
> the CPU is sending faster than the link can handle should you see
> the backlog grow (and eventually drain to 0).

Of course. But in my case, we were sending a burst of traffic while
with a lower HTB bw limit, so we can consistently see backlog!=0
for many seconds.

>
> Even though your explanation above is inaccurate I think the spirit
> of the patch looks reasonable. i.e keeping track of all additions to
> the queue and removals from the queue in the backlog stats is useful.
> However, you need to be extremely careful: This should only be done
> at exactly the spot the packet is enqueued (and not by a parent's
> enqueue asking for hierarchical enques).

The reason why I care about backlog and qlen is I want to know
the average length of each packet in backlog, to check if it is a
GSO packet at least.

>
> I think some more work is needed Cong for this general patchset.
>

Sure, I could miss something somewhere, just point it out. :)

Thanks.
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