forked from wezm/wezm.net
81 lines
3.9 KiB
Markdown
81 lines
3.9 KiB
Markdown
Whilst developing the [Radiopaedia iOS app][radiopaedia-app] I ran
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into the problem of very slow download speeds within the app. The app
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has packs of content available for purchase and download. The download
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speed was fine in the simulator but on the device it was painfully
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slow<sup>1</sup>. Both the Mac running the simulator and device were
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connected to the same Wi-Fi network. Profiling the code didn't reveal
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any issues either.
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[radiopaedia-app]: http://radiopaedia.org/articles/radiopaedia-ios-radiology-app
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Some searching revealed people complaining about YouTube videos
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downloading faster over 3G than Wi-Fi. There were a number of theories
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and proposed fixes, including [people seeing faster rates when
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downgrading their access point to 802.11b][80211b]<sup>2</sup>. I
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read one particular article (that I can no longer find) that said the
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power management in the iOS Wi-Fi stack interfered with download speeds
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when communicating to a high latency destination over a high speed Wi-Fi
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connection. For example YouTube servers in the US from AU<sup>3</sup>.
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[80211b]: http://web.archive.org/web/20120511020134/http://qelix.com/blog/2008/08/31/get-better-wifi-speeds-on-iphone-3g
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With this in mind I tested hosting the content packs on an Australian
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server, instead of in Amazon S3 accessed via CloudFront. This showed a huge
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improvement in download speed. At the time CloudFront didn't
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have an Australian presence so we switched to hosting the packs on
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RackSpace's CloudFiles, which uses the much more extensive Akamai CDN
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and ensured that users of the app would have the best possible download
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speeds no matter where they were.
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<div class="seperator"><hr class="left">✦<hr class="right"></div>
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Fast forward a year or so and I started managing podcasts on my phone
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instead of via iTunes. I noticed that some of the podcasts I was
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subscribed to were horrendously slow to download. Some would take hours
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to download, particularly ones hosted a long way away, such as in the
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UK or The Netherlands. I also noted that podcasts that used a CDN
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downloaded quickly.
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This seemed to be the same problem I had encountered with the Radiopaedia app.
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I wondered if telling the iPhone to use a proxy on the local
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network would work around the issue.
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To test my theory I set up [Squid][squid]<sup>4</sup> on the Mac mini
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that's connected to my TV. I configured the iPhone to use the proxy and
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compared the speed with and without the proxy in use. You can see this
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in action in the screenshots below.
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[squid]: http://www.squid-cache.org/
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In the first screenshot the phone isn't using the proxy. I have let
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it download a third of the file to be well clear of [TCP's slow
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start][slow-start] and it is estimating 57 minutes to download the
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remaining 176.1 Mb (53 kb/sec). The second screenshot (only a minute
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later) shows the result after I paused the download, changed the proxy
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settings and then again let the download warm up a little. The estimated
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time to complete the remaining 143.8 Mb is now only 5 mins (491 kb/sec)
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-- a huge improvement.
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[slow-start]: https://en.wikipedia.org/wiki/Slow-start
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<figure class="screenshot">
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<img src="/images/2013/01/podcast-download-without-proxy.png" alt="Podcast download without proxy" />
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<figcaption>Podcast download without proxy, 53 kb/sec</figcaption>
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</figure>
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<figure class="screenshot">
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<img src="/images/2013/01/podcast-download-with-proxy.png" alt="Podcast download with proxy" />
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<figcaption>Podcast download with proxy, 491 kb/sec</figcaption>
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</figure>
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After the successful experiment I left the phone using the proxy and configured
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launchd to start squid on boot:
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<script src="https://gist.github.com/4521952.js"></script>
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----
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<sup>1</sup> Tested on iPhone 3GS and iPhone 4S.
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<sup>2</sup> Original article linked from [Apple support discussion](https://discussions.apple.com/message/9564410#9564410).
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<sup>3</sup> It appears there's YouTube end-points in AU these days.
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<sup>4</sup> I installed squid using [Homebrew](http://mxcl.github.com/homebrew/).
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