Wikipedia and lunatic charlatans

▼ I just love this story on Ars Technica by Megan Geuss: Wikipedia founder calls alt-medicine practitioners “lunatic charlatans”. It's about a bunch of alternative medicine proponents petitioning Wikipedia to change its rules so they can have more extensive coverage of their pseudoscience. But Wikipedia-founder Jimmy Wales won't have any of it: "No, you have to be kidding me." Trust me, you need to read the whole thing.

An interesting aspect of Wikipedia is that it wants to be an encyclopedia. As such, the rules are that all information in Wikipedia must be supported by secondary sources. This means that Philip Roth doesn't get to put information about his own book in Wikipedia, which seems counterintuitive to say the least. However, how useful would Wikipedia be if it were full of statements that are supported by nothing more than the authority of the author of those statements? By requiring secondary sources, you can always follow the trail back to the source of the information, which allows both Wikipedia users and Wikipedia itself to determine whether something is incorrect and fix the problem. Sources also have to be credible, which actually makes it hard for "lunatic charlatans" to get their stuff into Wikipedia.

It's amazing how well the whole system works—I don't think anyone would have guessed Wikipedia could grow to its current size and maintain any semblance of quality. But it did.

Permalink - posted 2014-03-26

The Nikon D7100 is useless for video

▼ Earlier, I posted a video filmed with my Nikon D7100 DSLR.

The video came out pretty good, although it's rather boring and I really should be even more careful making sure my tripod is level. However, the D7100 is pretty useless for video. First of all, the autofocus is unusable. All my lenses are pretty noisy when autofocussing. I guess that can be solved with an external microphone, though. However, the autofocus is also slow and hunts a lot to find the right focus distance. So I just used an f/11 aperture and set the focus to 5 meters or so, which seemed to have worked fairly well.

I guess the focus issues are unavoidable considering hardware limitations, but that's not true for another big problem: although you can use any shutter speed above the video framerate, you either have to set both the video shutter speed as well as the ISO manually, or leave both on auto. What I want is manually setting the shutter speed to the equivalent of 180 degrees (= 1/50 at 25 frames per second) so movement looks the way it should, and then let auto ISO take care of proper exposure. But no dice, this can't be done.

Permalink - posted 2014-03-19

Ethernet energy efficiency

▼ How power-hungry are various permutations of Ethernet on modern MacBook Pros? Tests performed and written up by Jeroen van der Ham and Iljitsch van Beijnum.

For some reason, we thought it would be a good idea to make the students of the System and Network Engineering master program test whether various permutations of IPv4/IPv6, energy-efficient Ethernet (eee) and jumboframes use more or less power. While the students were busy measuring, we were bored so we decided to run the same tests ourselves using our MacBook Pros (with Retina Display®)—Jeroen the 15" mid-2012 model and Iljitsch the 13" late 2013 model. These computers are too thin to house an Ethernet port, so we used Apple's Thunderbolt - Gigabit Ethernet adapter. We looked at the number of milliamps taken from the battery to see how much power the entire system used during various tests.

We didn't realize it at first, but the most surprising result was actually the first one, looking at system power use with the adapter plugged into the computer, but without attaching a network cable. This increased system power use by 300 mA, or about 3.4 Watts. On the 15" MBP this is already significant, increasing power use from 1250 to 1550 mA. But the 13" MBP only used 370 mA without the adapter plugged in, so its power use almost doubled after plugging in the adapter!

Could it be that the secret to the long battery life of Apple's recent laptops is the fact that they come without an Ethernet port?

We continued our testing trying different possibilities with Energy Efficient Ethernet (eee) turned on or off. Students in 2012 already looked at the general impact of eee on energy usage for clusters. They found that it provides an energy saving of up to 40% in switches, depending on the amount of traffic. Energy usage goes up when traffic goes up, but it already maxes out when you are using roughly 40% of the bandwidth. We confirmed this in our testing: there is not much difference with eee when the interface is idle, or with full traffic.

More significant energy savings started to appear when we enabled jumbo-frames (MTU 9000). The amount of data that goes through the adapter remains virtually identical, but with only one-sixth as many packets per second, the impact of per-packet overhead is greatly reduced, and this shows in the energy consumption. With full traffic in one direction this saves you over 10%. These savings also add up if you have traffic in both directions, in our experiments we saw a savings 16-19% for a bidirectional fully-utilized link.

Not surprisingly, the biggest difference in energy usage is to scale down the interface speed. When we forced the interface down to 10Mbps we saw a saving of up to 49% with full utilisation. Then again, if you're sending a large file, it is going to take a lot longer, and you basically use a lot more energy, only more distributed over time.

The results of our measurements are in the table below, but note that our measurements (using the System Profiler battery current figures) weren't very precise. The numbers give a pretty rough indication. If you want to do something serious with this data, perform your own experiments.

Total system power use	MBP15	MBP13
Interface attached vs baseline	+24%	+92%
Baseline vs idle eee	+27%	+110%
Idle eee vs no eee	-6%	-1%
Idle eee vs iperf 1 Gbps TCP eee	+74%	+97%
iperf 1 Gbps TCP eee vs no eee	-3%	-1%
iperf 1 Gbps TCP jumbo eee vs no jumbo eee	-13%	-12%
iperf 0.5 Gbps UDP eee vs no eee	+1%	+1%
iperf 1 Gbps TCP bi-di eee vs unidirectional eee	-0%	-3%
iperf 1 Gbps TCP bi-di jumbo eee vs no jumbo bi-di eee	-19%	-16%
iperf 10 Mbps TCP no eee vs 1 Gbps	-38%	-49%

We did some extra tests to look at the impact of jumboframes on CPU use. Please note that our Thunderbolt-to-Gigabit Ethernet adapters don't support TCP segmentation offloading (TSO). With 1500-byte packets the CPU utilization on the MBP13 was about 16%, with 9000-byte packets about 90%. Despite being idle more than 80% of the time, the CPU still ran at its advertised 2.4 GHz clock speed:

So the difference in CPU power use was 18% (7.9 W for 1500 bytes vs 6.4 W for 9000 bytes), despite the difference in CPU utilization only being 5%.

Permalink - posted 2014-03-03

Beware of the DFS band if you want gigabit Wi-Fi

▼ I got an Apple Time Capsule 802.11ac "gigabit Wi-Fi" base station this summer, and then a new 802.11ac-capable MacBook Pro (read my review) a few months ago. Those two can communicate wirelessly at a blistering 1.3 Gbps. Sweet!

However, after my initial testing I would never see speeds faster than 878 Mbps when clicking on the Wi-Fi symbol in the menu bar while holding the alt/option key. For a while I thought there was a software issue and I even waited to see if the MacOS 10.9.1 update would resolve the issue. It didn't.

But then I noticed something. I had my Time Capsule set for automatic channel selection, and it selected channel 100 (DFS). Then at some point it switched to one of the lower channels. And suddenly I was back to 1300 Mbps! (Although fallback to 1053 Mbps still happens regularly.)

I then tried switching channels a few more times, with fairly conclusive results: I would never get more than 878 Mbps on a DFS channel, but on channels 36 - 48 I'd often get 1053 or 1300 Mbps. Also, on DFS I would sometimes have really poor network throughput, which disappeared after switching to 36 - 48.

DFS is dynamic frequency selection. As you can see here, in the US, on all available channels except 36, 40, 44 and 48, dynamic frequency selection is required so the Wi-Fi equipment changes channels if it detects other stuff, most likely radar, getting in the way. Apparently Apple enforces this in Europe, too. So it could be that some radar installation is keeping me from getting my full 1.3 Gbps. Or perhaps the antennas or radios are less efficient at these higher frequencies.

In any event, I'm using channels 36 - 48 from now on, even though two of them are also used by neighbors.

Bonus tip: if you alt-click on the Wi-Fi icon in the menu bar and then hover over a network name, you get to see additional information:

(Yikes, I wrote "Time Machine" instead of "Time Capsule" twice originally!)

Permalink - posted 2014-02-14