Date:	Fri, 23 Aug 2013 17:53:24 +0200
From:	Teco Boot <teco@...-net.nl>
To:	Stephen Hemminger <stephen@...workplumber.org>
Cc:	Eric Dumazet <eric.dumazet@...il.com>,
	Ferry Huberts <mailings@...ie.com>,
	Netem <netem@...ts.linux-foundation.org>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"core-users@...itd.nrl.navy.mil" <core-users@...itd.nrl.navy.mil>
Subject: Re: netem: the reorder discussion


Op 23 aug. 2013, om 17:44 heeft Stephen Hemminger <stephen@...workplumber.org> het volgende geschreven:

> On Fri, 23 Aug 2013 08:12:36 -0700
> Eric Dumazet <eric.dumazet@...il.com> wrote:
> 
>> On Fri, 2013-08-23 at 16:16 +0200, Teco Boot wrote:
>>> I had reorder problems with netem qdisc. I make use of the NRL CORE
>>> network emulator, which sets up virtual routers and links, build with
>>> network namespaces and netem. Typical CORE config for a n1<-->n2 link
>>> with rate (1024Kbps), delay (50ms) and no jitter:
>>>  qdisc tbf 1: dev n1.eth0.222 root refcnt 2 rate 1024Kbit burst 2999b
>>> lat 488.6ms 
>>>  qdisc netem 10: dev n1.eth0.222 parent 1:1 limit 1000 delay 50.0ms
>>>  qdisc tbf 1: dev n2.eth0.222 root refcnt 2 rate 1024Kbit burst 2999b
>>> lat 488.6ms 
>>>  qdisc netem 10: dev n2.eth0.222 parent 1:1 limit 1000 delay 50.0ms
>>> 
>>> I added jitter to this bi-directional link, e.g. 20ms. Now the delay
>>> for each packet is 50ms +/- 20ms is 30ms to 70ms.
>>> However, this has some unexpected results: packets may be reordered.
>>> That is because the actual delay is calculated for each packet. Some
>>> packets have a larger delay, some have a smaller. If the inter-packet
>>> spacing is smaller than the time differences set by netem (up to 2x
>>> configured jitter), packets are reordered. In this example with
>>> +/20ms, this is the case with packet rate larger than 25pps.
>>> Reordering has bad effects on transport protocol throughputs.
>>> Reordering is less common on the Internet, so I don't want to emulate
>>> such. I don't say there is no reordering, I just say I don't want this
>>> netem behavior.
>>> 
>>> The netem guide mentions this unexpected results. It was caused by a
>>> change in version 1.1 (2.6.15). For people like me that do not want
>>> this, there is a work-around mentioned. However, this doesn't work
>>> anymore since somewhere before 2.6.31.
>>> http://www.linuxfoundation.org/collaborate/workgroups/networking/netem
>>> https://lists.linux-foundation.org/pipermail/netem/2011-April/001507.html
>>> 
>>> So what to do? The recent patch from Eric Dumazet eliminates
>>> jitter_reordering when netem rate is configured. Maybe not the clean
>>> approach, but acceptable. Ferry his patch eliminates
>>> jitter_reordering. Some people may use reorder by jitter, so this
>>> "feature" should not get removed. So Ferry his patch is not accepted.
>>> On the other hand, people like me are very confused by current
>>> behavior.
>>> 
>>> What we could do is keep existing features and describe what netem
>>> currently does. That is:
>>> - netem with delay and jitter may reorder packets, if inter packet
>>> spacing is smaller than jitter
>>> - reordering caused by delay and jitter can be turned off by using
>>> netem rate. rate can be set to very high value is no shaping is
>>> wanted.
>> 
>> As long as one can define the expected behavior, you can add whatever
>> new netem parameter.
>> 
>> One could envision adding flow separation (skb->sk or rxhashing), so
>> that each flow can have his own local queue, to guarantee no reorders
>> per flow _if_ this is needed, even if per flow delays/jitter is/are
>> configured.
>> 
>> We also use netem to test on large scale, where the reordering stuff
>> needs fixes in transport stacks (And yes, we are working on TCP stack
>> to permit higher levels of reorders)
> 
> I am happy with any solution that is:
>  * allows both always ordering and reordering based on random jitter of delay
>  * documented
> 
> I do get worried that people's tests get different results because of
> netem behavior changes. Researchers like to have repeatable results.

Questions that pops out of of my head: What is more important, understandable configuration or keep existing behavior for unmodified configuration? Do we want to reorder, even with configured rate?

My preference: make in understandable, document well and make all sensible options available. 

Teco



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