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

Jim Gettys jg at freedesktop.org
Thu Feb 28 15:03:07 EST 2013

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.

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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