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

Thu Feb 28 15:58:15 EST 2013

At least someone actually saw what I've been seeing for years now in Metro area HSPA and LTE deployments.

As you know, when I first reported this on the e2e list I was told it could not possibly be happening and that I didn't know what I was talking about.  No one in the phone companies was even interested in replicating my experiments, just dismissing them.  It was sad.

However, I had the same experience on the original Honeywell 6180 dual CPU Multics deployment in about 1973.  One day all my benchmarks were running about 5 times slower every other time I ran the code.  I suggested that one of the CPUs was running 5x slower, and it was probably due to the CPU cache being turned off.   The hardware engineer on site said that that was *impossible*.  After 4 more hours of testing, I was sure I was right.  That evening, I got him to take the system down, and we hauled out an oscilloscope.  Sure enough, the gate that received the "cache hit" signal had died in one of the processors.   The machine continued to run, since all that caused was for memory to be fetched every time, rather than using the cache.

Besides the value of finding the "root cause" of anomalies, the story points out that you really need to understand software and hardware sometimes.  The hardware engineer didn't understand the role of a cache, even though he fully understood timing margins, TTL logic, core memory (yes, this machine used core memory), etc.

We both understood oscilloscopes, fortunately.

In some ways this is like the LTE designers understanding TCP.   They don't.  But sometimes you need to know about both in some depth.

Congratulations, Jim.  More Internet Plumbing Merit Badges for you.

-----Original Message-----
From: "Jim Gettys" <jg at freedesktop.org>
Sent: Thursday, February 28, 2013 3:03pm
To: "Dave Taht" <dave.taht at gmail.com>
Cc: "David P Reed" <dpreed at reed.com>, "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

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 <[mailto:dave.taht at gmail.com] dave.taht at gmail.com> wrote:

On Thu, Feb 28, 2013 at 1:57 PM,  <[mailto:dpreed at reed.com] 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" <[mailto:maciej at soltysiak.com] maciej at soltysiak.com>
Sent: Thursday, February 28, 2013 1:03pm
 To: [mailto:cerowrt-devel at lists.bufferbloat.net] 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] 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] 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] https://android.googlesource.com/kernel/common/+log/experimental/android-3.8
Regards,
Maciej_______________________________________________
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-- 
Dave Täht

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