[Cerowrt-devel] Equivocal results with using 3.10.28-14
Sebastian Moeller
moeller0 at gmx.de
Tue Feb 25 08:29:16 PST 2014
Hi Dave,
On Feb 25, 2014, at 16:54 , Dave Taht <dave.taht at gmail.com> wrote:
> On Tue, Feb 25, 2014 at 5:37 AM, Sebastian Moeller <moeller0 at gmx.de> wrote:
>> Hi Rich,
>>
>>
>> On Feb 25, 2014, at 14:09 , Rich Brown <richb.hanover at gmail.com> wrote:
>>
>>> Thanks everyone for all the good advice. I will summarize my responses to all your notes now, then I'll go away and run more tests.
>>>
>>> - Yes, I am using netperf 2.6.0 and netperf-wrapper from Toke's github repo.
>>>
>>> - The "sync rate" is the speed with which the DSL modem sends bits to/from my house. I got this by going into the modem's admin interface and poking around. (It turns out that I have a very clean line, high SNR, low attenuation. I'm much less than a km from the central office.) So actual speed should approach this, except...
>>
>> I would think of this as the theoretical upper limit ;)
>>
>>>
>>> - Of course, I have to subtract all those overheads that Sebastian described - ATM 48-in-53, which knocks off 10%; ATM frame overhead which could add up to 47 bytes padding to any packet, etc.)
>>>
>>> - I looked at the target calculation in Dave's Home Gateway best practices. (http://snapon.lab.bufferbloat.net/~d/draft-taht-home-gateway-best-practices-00.html) Am I correct that it sets the target to five 1500-byte packet transmission time or 5 msec, whichever is greater?
>>
>> Note, the auto target implementation in ceropackages-3.10 sqm-scripts uses the following:
>>
>> adapt_target_to_slow_link() {
>> CUR_LINK_KBPS=$1
>> CUR_EXTENDED_TARGET_US=
>> MAX_PAKET_DELAY_IN_US_AT_1KBPS=$(( 1000 * 1000 *1540 * 8 / 1000 ))
>> CUR_EXTENDED_TARGET_US=$(( ${MAX_PAKET_DELAY_IN_US_AT_1KBPS} / ${CUR_LINK_KBPS} )) # note this truncates the decimals
>> # do not change anything for fast links
>> [ "$CUR_EXTENDED_TARGET_US" -lt 5000 ] && CUR_EXTENDED_TARGET_US=5000
>> case ${QDISC} in
>> *codel|pie)
>> echo "${CUR_EXTENDED_TARGET_US}"
>> ;;
>> esac
>> }
>>
>> This is modeled after the shell code Dave sent around, and does not exactly match the free version, because I could not make heads and tails out of the free version. (Happy to discuss change this in SQM if anybody has a better idea)
>
> Really the target calculation doesn't matter so much so long as it's
> larger than the MTU. This is somewhat an artifact of htb which buffers
> up an extra packet, and the cpu scheduler…
Ah, so we just leave the implementation as is? Great!
>
> It is becoming clearer (with the recent description of the pie + rate
> limiter + mac compensation stuff in future cablemodems) that we can do
> much work to improve the rate limiters, and should probably intertwine
> them like what the cable folk just did, in order to get best
> performance.
>
>>>
>>> - I was astonished by the calculation of the bandwidth consumed by acks in the reverse direction. In a 7mbps/768kbps setting, I'm going to lose one quarter of the reverse bandwidth? Wow!
>
> Yes. TCP's sending ack requirement was actually the main driver for
> having any bandwidth at all on the upstream. Back in the 90s all cable
> providers wanted to provide was sufficient bandwidth for a "buy"
> button. But, due to
> this behavior of TCP, a ratio between down/up of somewhere between 6
> and 12 to 1 was what was needed to make TCP work well. (and even then
> they tried hard to make it worse and ack compression is still part of
> many cable modem's provisioning)
Interesting!
>
> IPv6 has much larger acks than ipv4….
Hopefully mostly delayed/stretched?
>
> I did a lot of work on various forms of hybrid network compression
> back in the day (early 90s), back when we had a single 10Mbit radio
> feeding hundreds of subscribers, and a 14.4 modem sending back acks…
I assume one modem per customer?
> and data. It turned out that a substantial percentage of subscribers
> actually wanted to upload stuff... and that we couldn't achieve 10Mbit
> in the lab with a 14.4 return…
So automatic "fair-ish" queueing, no single customer could hog all 10Mbit ;)
> and that you can only slice up 10Mbit
> so many ways before you ran out of bandwidth, and you run out of
> bandwidth long before you can turn a profit...
>
> (and while I remember details of the radio setup and all the crazy
> stuff we did to get more data through the modems, I can't remember the
> name of the company)
>
> At the time I was happy, sorta, in that we'd proven that future ISPs
> HAD to provide some level of upstream bandwidth bigger than a buy
> button, and the original e2e internet was going to be at least
> somewhat preserved...
>
> I didn't grok at the time that NAT was going to be widely deployed...
> I mean, at the time, you sold a connection, and asked how big a
> network did you want, we defaulted to a /28, and we were still handing
> out class C's to everyone that asked.
>
>> Well, so was I (first time I did that) but here is the kicker with classical non0-delayed ACKs this actually doubles since each data packet gets acknowledged (I assume this puts a lower bound on how asymmetrical a loin an ISP can sell ;) ). But since I figured out that macosx seems to default to 1 Ack for every 4 packets, so only half the traffic. And note any truly bi-directional traffic shouldbe able to piggyback many of those ACKs into normal data packets.
>
> I doubt your '4'. Take a capture for a few hundred ms.
Ah, I took this from a description about current behavior of macosx that I found on the internet, it sounded about right. I guess what wanted to say that the ACK imprint on the upload in the given example could range from ~400kbps (1 ACK per packet) to 66.7 kbps (6 ACKs per packet), but even that is a considerable fraction of the available upload bandwidth…
Silly idea, it would be great if cerowrt could have a real-time display of the number/size of ACK packets (lightly smoothed in time) going in and out the WAN link, that way it would be quite easy to better estimate the traffic components hidden from RRUL. But that is m playing arm-chair software architect ;) not that I am going to implement that...
>
> My understanding of how macos works is that after a stream is
> sustained for a while, it switches from one ack every 2 packets into
> "stretch acks" - to one every 6 (or so).
According to http://rolande.wordpress.com/2010/12/30/performance-tuning-the-network-stack-on-mac-osx-10-6/
net.inet.tcp.delayed_ack has the following meaning:
• delayed_ack=0 responds after every packet (OFF)
• delayed_ack=1 always employs delayed ack, 6 packets can get 1 ack
• delayed_ack=2 immediate ack after 2nd packet, 2 packets per ack (Compatibility Mode)
• delayed_ack=3 should auto detect when to employ delayed ack, 4 packets per ack. (DEFAULT)
I have it set at three, so this is why I argued with 4 (just hoping it would not opt for one ACK per packet), and I am still on 10.8.5 (just in case someone in the know should happen to read this; what is the true behavior?)
Best Regards
Sebastian
>
> there are some interesting bugs associated with stretch acks, and also
> TSO - in one case we observed a full TSO of TCP RSTs being sent
> instead of one.
>
>
>
>>>
>>> - I wasn't entirely clear how to set the target in the SQM GUI. I believe that "target ##msec" is an acceptable format. Is that correct?
>>
>> In the new version with the dedicated target field, "40ms" will work as will "40000us", "40 ms". In the most recent version "auto" will set the auto adjustment of target (it will also extend interval by the new-target - 5ms) to avoid the situation where Target gets larger than interval.
>> In the older versions you would put "target 40ms" into the egress advanced option string. Note I used double quotes in my examples for clarity, the GUI does not want those...
>>
>>
>>>
>>> - There's also a discussion of setting the target with "auto", but I'm not sure I understand the syntax.
>>
>> just type in auto. You can check with log read and "tc -d qdsic"
>>
>> Best Regards
>> Sebastian
>>
>>>
>>> Now to find some time to go back into the measurement lab! I'll report again when I have more data. Thanks again.
>>>
>>> Rich
>>>
>>>
>>>
>>> On Feb 24, 2014, at 9:56 AM, Aaron Wood <woody77 at gmail.com> wrote:
>>>
>>>> Do you have the latest (head) version of netperf and netperf-wrapper? some changes were made to both that give better UDP results.
>>>>
>>>> -Aaron
>>>>
>>>>
>>>> On Mon, Feb 24, 2014 at 3:36 PM, Rich Brown <richb.hanover at gmail.com> wrote:
>>>>
>>>> CeroWrt 3.10.28-14 is doing a good job of keeping latency low. But... it has two other effects:
>>>>
>>>> - I don't get the full "7 mbps down, 768 kbps up" as touted by my DSL provider (Fairpoint). In fact, CeroWrt struggles to get above 6.0/0.6 mbps.
>>>>
>>>> - When I adjust the SQM parameters to get close to those numbers, I get increasing levels of packet loss (5-8%) during a concurrent ping test.
>>>>
>>>> So my question to the group is whether this behavior makes sense: that we can have low latency while losing ~10% of the link capacity, or that getting close to the link capacity should induce large packet loss...
>>>>
>>>> Experimental setup:
>>>>
>>>> I'm using a Comtrend 583-U DSL modem, that has a sync rate of 7616 kbps down, 864 kbps up. Theoretically, I should be able to tell SQM to use numbers a bit lower than those values, with an ATM plus header overhead with default settings.
>>>>
>>>> I have posted the results of my netperf-wrapper trials at http://richb-hanover.com - There are a number of RRUL charts, taken with different link rates configured, and with different link layers.
>>>>
>>>> I welcome people's thoughts for other tests/adjustments/etc.
>>>>
>>>> Rich Brown
>>>> Hanover, NH USA
>>>>
>>>> PS I did try the 3.10.28-16, but ran into troubles with wifi and ethernet connectivity. I must have screwed up my local configuration - I was doing it quickly - so I rolled back to 3.10.28.14.
>>>> _______________________________________________
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>>>> Cerowrt-devel at lists.bufferbloat.net
>>>> https://lists.bufferbloat.net/listinfo/cerowrt-devel
>>>>
>>>
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>>
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>
>
>
> --
> Dave Täht
>
> Fixing bufferbloat with cerowrt: http://www.teklibre.com/cerowrt/subscribe.html
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