[Cerowrt-devel] Google working on experimental 3.8 Linux kernel for Android

Jim Gettys jg at freedesktop.org
Fri Mar 1 11:09:36 EST 2013 

On Fri, Mar 1, 2013 at 10:40 AM, <dpreed at reed.com> wrote:

> One wonders why all this complexity is necessary, and how likely it is to
> be "well tuned" by operators and their contract installers.
>
>
>
> I'm willing to bet $1000 that all the testing that is done is "Can you
> hear me now" and a "speed test".  Not even something as simple and
> effective as RRUL.
>

Actually, at least some the the carriers do much more extensive testing;
but not with the test tools we would like to see used (yet).

An example is AT&T, where in research, KK Ramakrishnan has a van with 20 or
so laptops so he can go driving around and load up a cell in the middle of
the night and get data.   And he's research; the operations guys do lots of
testing I gather, but more at the radio level.

Next up, is to educate KK to run RRUL.

And in my own company, I've seen data, but it is too high level: e.g.
performance of "web" video: e.g. siverlight, flash, youtube, etc.

A common disease that has complicated all this is the propensity for
companies to use Windows XP internally for everything: since window scaling
is turned off, you can't saturate a LTE link the way you might like to do
with a single TCP connection.
                                              - Jim




>
>
> -----Original Message-----
> From: "Ketan Kulkarni" <ketkulka at gmail.com>
> Sent: Friday, March 1, 2013 3:00am
> To: "Jim Gettys" <jg at freedesktop.org>
> Cc: "cerowrt-devel at lists.bufferbloat.net" <
> cerowrt-devel at lists.bufferbloat.net>
> Subject: Re: [Cerowrt-devel] Google working on experimental 3.8 Linux
> kernel for Android
>
>
>
> On Fri, Mar 1, 2013 at 1:33 AM, Jim Gettys <jg at freedesktop.org> wrote:
>
>> I've got a bit more insight into LTE than I did in the past, courtesy of
>> the last couple days.
>> To begin with, LTE runs with several classes of service (the call them
>> bearers).  Your VOIP traffic goes into one of them.
>> And I think there is another as well that is for guaranteed bit rate
>> traffic.  One transmit opportunity may have a bunch of chunks of data, and
>> that data may be destined for more than one device (IIRC).  It's
>> substantially different than WiFi.
>>
>  Just thought to put more light on bearer stuff:
>
> There are two ways bearers are setup:
> 1. UE initiated - where User Equipment sets-up the "parameters" for bearer
> 2. Network initiated - where node like PCRF and PGW sets-up the
> "parameters".
> Parameters include the Guaranteed bit-rates, maximum bit-rates. Something
> called QCI is associated with bearers. The QCI parameters are authorized at
> PCRF (policy control rule function) and there is certain mapping maintained
> at either PCRF or PGW between QCI values and DSCP and MBRs.
> These parameters enforcing is done at PGW (in such case it is termed as
> PCEF - policy and rule enforcement function). So PGWs depending on bearers
> can certainly modify dscp bits. Though these can be modified by other nodes
> in the network.
>
> There are two types of bearers: 1. Dedicated bearers - to carry traffic
> which need "special" treatment 2. Default or general pupose bearers - to
> carry all general purpose data.
> So generally the voip, streaming videos are passed over dedicated bearers
> and apply (generally) higher GBRs, MBRs and correct dscp markings.
> And other non-latency sensitive traffic generally follows the default
> bearer.
>
> Theoretical limit on maximum bearers is 11 though practically most of the
> deployments use upto 3 bearers max.
>
> Note that these parameters may very well very based on the subscriber
> profiles. Premium/Corporate subscribers can well have more GBRs and MBRs.
> ISPs are generally very much sensitive to the correct markings at gateways
> for obvious reasons.
>
>  But most of what we think of as Internet stuff (web surfing, dns, etc)
>> all gets dumped into a single best effort ("BE"), class.
>> The BE class is definitely badly bloated; I can't say how much because I
>> don't really know yet; the test my colleague ran wasn't run long enough to
>> be confident it filled the buffers).  But I will say worse than most cable
>> modems I've seen.  I expect this will be true to different degrees on
>> different hardware.  The other traffic classes haven't been tested yet for
>> bufferbloat, though I suspect they will have it too.  I was told that those
>> classes have much shorter queues, and when the grow, they dump the whole
>> queues (because delivering late real time traffic is useless).  But trust
>> *and* verify....  Verification hasn't been done for anything but BE
>> traffic, and that hasn't been quantified.
>> But each device gets a "fair" shot at bandwidth in the cell (or sector of
>> a cell; they run 3 radios in each cell), where fair is basically time
>> based; if you are at the edge of a cell, you'll get a lot less bandwidth
>> than someone near a tower; and this fairness is guaranteed by a scheduler
>> than runs in the base station (called a b-nodeb, IIIRC).  So the base
>> station guarantees some sort of "fairness" between devices (a place where
>> Linux's wifi stack today fails utterly, since there is a single queue per
>> device, rather than one per station).
>> Whether there are bloat problems at the link level in LTE due to error
>> correction I don't know yet; but it wouldn't surprise me; I know there was
>> in 3g.  The people I talked to this morning aren't familiar with the HARQ
>> layer in the system.
>> The base stations are complicated beasts; they have both a linux system
>> in them as well as a real time operating system based device inside  We
>> don't know where the bottle neck(s) are yet.  I spent lunch upping their
>> paranoia and getting them through some conceptual hurdles (e.g. multiple
>> bottlenecks that may move, and the like).  They will try to get me some of
>> the data so I can help them figure it out.  I don't know if the data flow
>> goes through the linux system in the bnodeb or not, for example.
>> Most carriers are now trying to ensure that their backhauls from the base
>> station are never congested, though that is another known source of
>> problems.  And then there is the lack of AQM at peering point routers....
>>  You'd think they might run WRED there, but many/most do not.
>>  - Jim
>>
>>
>> On Thu, Feb 28, 2013 at 2:08 PM, Dave Taht <dave.taht at gmail.com> wrote:
>>
>>>
>>>
>>>  On Thu, Feb 28, 2013 at 1:57 PM, <dpreed at reed.com> wrote:
>>>
>>>> Doesn't fq_codel need an estimate of link capacity?
>>>>
>>>  No, it just measures delay. Since so far as I know the outgoing
>>> portion of LTE is not soft-rate limited, but sensitive to the actual
>>> available link bandwidth, fq_codel should work pretty good (if the
>>> underlying interfaces weren't horribly overbuffired) in that direction.
>>> I'm looking forward to some measurements of actual buffering at the
>>> device driver/device levels.
>>> I don't know how inbound to the handset is managed via LTE.
>>>  Still quite a few assumptions left to smash in the above.
>>> ...
>>> in the home router case....
>>> ...
>>> When there are artificial rate limits in play (in, for example, a cable
>>> modem/CMTS, hooked up via gigE yet rate limiting to 24up/4mbit down), then
>>> a rate limiter (tbf,htb,hfsc) needs to be applied locally to move that rate
>>> limiter/queue management into the local device, se we can manage it better.
>>> I'd like to be rid of the need to use htb and come up with a rate
>>> limiter that could be adjusted dynamically from a daemon in userspace,
>>> probing for short all bandwidth fluctuations while monitoring the load. It
>>> needent send that much data very often, to come up with a stable result....
>>> You've described one soft-rate sensing scheme (piggybacking on TCP), and
>>> I've thought up a few others, that could feed back from a daemon some
>>> samples into a a soft(er) rate limiter that would keep control of the
>>> queues in the home router. I am thinking it's going to take way too long to
>>> fix the CPE and far easier to fix the home router via this method, and
>>> certainly it's too painful and inaccurate to merely measure the bandwidth
>>> once, then set a hard rate, when
>>> So far as I know the gargoyle project was experimenting with this
>>> approach.
>>>  A problem is in places that connect more than one device to the cable
>>> modem... then you end up with those needing to communicate their perception
>>> of the actual bandwidth beyond the link.
>>>
>>>>  Where will it get that from the 4G or 3G uplink?
>>>>
>>>>
>>>>
>>>> -----Original Message-----
>>>> From: "Maciej Soltysiak" <maciej at soltysiak.com>
>>>> Sent: Thursday, February 28, 2013 1:03pm
>>>> To: cerowrt-devel at lists.bufferbloat.net
>>>> Subject: [Cerowrt-devel] Google working on experimental 3.8 Linux
>>>> kernel for Android
>>>>
>>>>  Hiya,
>>>> Looks like Google's experimenting with 3.8 for Android:
>>>> https://android.googlesource.com/kernel/common/+/experimental/android-3.8
>>>> Sounds great if this means they will utilize fq_codel, TFO, BQL, etc.
>>>> Anyway my nexus 7 says it has 3.1.10 and this 3.8 will probably go to
>>>> Android 5.0 so I hope Nexus 7 will get it too some day or at least 3.3+
>>>> Phoronix coverage:
>>>> http://www.phoronix.com/scan.php?page=news_item&px=MTMxMzc
>>>> Their 3.8 changelog:
>>>> https://android.googlesource.com/kernel/common/+log/experimental/android-3.8
>>>> Regards,
>>>> Maciej
>>>>    _______________________________________________
>>>> Cerowrt-devel mailing list
>>>> Cerowrt-devel at lists.bufferbloat.net
>>>> https://lists.bufferbloat.net/listinfo/cerowrt-devel
>>>>
>>>>
>>>
>>> --
>>> Dave Täht
>>>
>>> Fixing bufferbloat with cerowrt:
>>> http://www.teklibre.com/cerowrt/subscribe.html
>>> _______________________________________________
>>> Cerowrt-devel mailing list
>>> Cerowrt-devel at lists.bufferbloat.net
>>> https://lists.bufferbloat.net/listinfo/cerowrt-devel
>>>
>>>
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