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Revision as of 12:37, 10 April 2006
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12:37, 10 April 2006
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Here are a few of the most obvious development issues/questions/comments:
* Will the laptops and/or an emulator be available to developers interested in direct contibutions to OLPC?
* memory footprint of applications
* memory leaks
* required flash footprint of the application; if it is big, there may not be space. However, JFFS2 does data compression, so don't presume that the size of a file in a conventional file system corresponds to the actual amount of flash used. As a *rule of thumb*, expect a factor of 2 compression for text and typical data, expect no compression on already compressed content, but take actual measurements to understand what your actual flash usage will be.
* your application should be engineered and packaged to make it easy to strip unneeded functionality. For example, GAIM supports just about every IM protocol in existance, but we can only anticipate 3 being common. So the fact it uses plugins that we can choose to not package saves greatly on its footprint, whereas if everything were built in, both the memory and flash footprint would be much larger.
* ability to operate applications when running in black/white mode
* the 640x480 effective resolution in color mode: some applications will find this hard, even though the frame buffer will always be in high resolution. If your application does not honor the font resolution change-on-the-fly mechanisms, or hardwires fonts by pixel size, your application won't be able to switch modes gracefully and may require manual intervention to be usable. Please fix.
* the hardware does support alpha blending (Porter-Duff compositing), so we expect to eventually support this well
* no 3D hardware: we don't have shaded triangles or a geometry pipe; so any OpenGL used will be based on Mesa, and without hardware, will be very slow
* choice of libraries and required applications: you may not have the dependencies you might need, or those dependencies might come at too high a memory cost. We will inventory what you can "count on" in the basic system as it becomes clear.
* graphical activity that does not turn itself off quickly; we want to save power
* CPU performance: the system is slow relative to current desktops, though fast relative to desktops at the turn of the millenium.
* power consumption: if your application is CPU bound for long periods, or routinely requires itself to be run (can't be suspended well), this isn't good
* while we are a great fan of interpreted languages, key CPU bound kernels had better be in compiled code, or your performance and power consumption will be poor
* your slow code has a direct impact on power consumption, and its usability. Applications that run CPU bound will be very, very unpopular on battery powered machines that may be powered by children having to run a generator.
* the file system is a flash file system, so its write performance is slow, while random access is actually very good. The performance is glacial if the file system is low on space and has to continually erase freed blocks before writing (jffs2 attempts to do this in the background, but if it can't....). Programs that continually write to the file system without need are anti-social; wear leveling helps flash longevity, but it certainly doesn't help, and burns power.
* looping waiting for events eats power; don't do it. Poll and select with timeouts are your friends. Don't gratuitously wake up at frequent intervals just to test if something has happened; design your hardware and software to be completely idle between events they have to respond to.
* presuming that you can contact a server for service will often not be possible. Design applications to be peer-to-peer, or to self elect servers in a local cloud of people working together. We can presume at least some technology like mDNS (e.g. Avahi) is available for discovery.
* applications should be localizable. Do not embedd any user readable text in your code.
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