Interleaving small reads of multiple files – why World of Tanks 1.0 has abysmal loading times on HDDs

TL;DR: If you have WoT (1.0) installed on an HDD, you may experience loading times so long that you might even miss the start of your battles. Fortunately, we can mitigate this.

World of Tanks update 1.0 was released recently, introducing a new game engine with lots of eye candy and better physics (amongst other improvements). I have been playing and enjoying this game since 2011 (when it was still in beta), as it has really solid gameplay mechanics, and I love World War 2 machinery in general1.

I was really excited for this major update, but right after installing it and playing a few battles, I noticed something very peculiar: loading into a battle was really slow, and my HDD sounded like an A-10 firing (almost). In fact, it was so slow that I arrived to battles about 30-40 seconds after they have started (and a minute late for the first battle of the day). That’s unfortunate, because:

  • The fraction of my free time that I’m spending on playing video games, I prefer to actually play the games, and not wait for disk IO instead
  • On open maps, spending a ~minute at the spawn location is a disaster
  • With certain types of tanks, you have to contest key positions on the map, and you need to make a run for it right at the start of the battle

Moving the installation to my SSD would have “solved”2 this issue, but I don’t have enough space on it and I was curious anyway, so I decided to try and investigate.

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Compressing ETL (ETW output) files

TL;DR: General purpose compression algorithms are pretty good. Can we beat them with a low effort optimization in compressing ETL files? Even if we could, it doesn’t mean we should.

Event Tracing for Windows is outright amazing. If you are a developer on Windows (native, managed, or even web) and have never used or (God forbid) heard of it, I can tell you that you are missing out on a very useful technology. I’m not an industry veteran nor an ETW specialist, but I already solved many incredibly complex customer issues using it (and its GUI analyzer, called the “Windows Performance Analyzer”) throughout my career. If you are not familiar with ETW, I suggest you start reading some of Bruce Dawson’s blog posts about investigations he made using it.

Anyways, this blog post will not be about ETW in general, but its binary output files of extension “.etl”. If you’ve ever taken a trace with the Windows Performace Recorder (or xperf), you know that these files can end up being pretty huge. Run system-wide tracing with sampled profiling and context switch recording for a few minutes, and we are already talking about gigabytes. Let’s see if we can beat general purpose compression algorithms using semantic knowledge about the content of ETL files.

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