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PODrastic performance inprovement in .NET CF after app gets moved out of the foreground. Why?
 

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  1. PODrastic performance inprovement in .NET CF after app gets moved out of the foreground. Why?
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    <p>I have a large (500K lines) .NET CF (C#) program, running on CE6/.NET CF 3.5 (v.3.5.10181.0). This is running on a FreeScale i.Mx31 (ARM) @ 400MHz. It has 128MB RAM, with ~80MB available to applications. My app is the only significant one running (this is a dedicated, embedded system). Managed memory in use (as reported by GC.Collect) is about 18MB. To give a better idea of the app size, here's some stats culled from .NET CF Remote Performance Monitor after staring up the application:</p> <pre> GC: Garbage Collections 131 Bytes Collected by GC 97,919,260 Managed Bytes in use after GC 17,774,992 Total Bytes in use after GC 24,117,424 GC Compactions 41 JIT: Native Bytes Jitted: 10,274,820 Loader: Classes Loaded 7,393 Methods Loaded 27,691 </pre> <p>Recently, I have been trying to track down a performance problem. I found that my benchmark after running the app in two different startup configurations would run in approximately 2 seconds (slow case) vs. 1 second (fast case). In the slow case, the time for the benchmark could change randomly from EXE run to EXE run from 1.1 to 2 seconds, but for any given EXE run, would not change for the life of the application. In other words, you could re-run the benchmark and the time for the test stays the same until you restart the EXE, at which point a new time is established and consistent. </p> <p>I could not explain the 1.1 to 2x slowdown via any conventional mechanism, or by narrowing the slowdown to any particular part of the benchmark code. It appeared that the overall process was just running slower, almost like a thread was spinning and taking away some of "my" CPU.</p> <p>Then, I randomly discovered that just by switching away from my app (the GUI loses the foreground) to another app, my performance issue disappears. It stays gone even after returning my app to the foreground. I now have a tentative workaround where my app after startup launches an auxiliary app with a 1x1 size window that kills itself after 5ms. Thus the aux app takes the foreground, then relinquishes it.</p> <p>The question is, why does this speed up my application?</p> <p>I know that code gets pitched when a .NET CF app loses the foreground. I also notice that when performing a "GC Heap" capture with .NET CF Remote Performance Monitor, a Code Pitch is logged -- and this also triggers the performance improvement in my app. So I suspect somehow that code pitching is related or even responsible for fixing performance. But I'm at a loss as to figure out how to determine if that is really the case, or even to explain why pitching code could help in this way. Does pitching out lots of code somehow significantly help locality of reference of code pages (that are re-JITted, presumably near each other in memory) enough to help this much? (My benchmark spans probably 3 dozen routines and hundreds of lines of code.)</p> <p>Most importantly, what can I do in my app to reliably avoid this slower condition. Any pointers to relevant .NET CF / JIT / Code pitching information would be greatly appreciated.</p>
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