Re: [Flightgear-devel] Re: code optimisation
-BEGIN PGP SIGNED MESSAGE- Hash: SHA1 Jim Campbell schrieb: > Hi, > One comment and some documentation has indicted that FDM doesnt consume > much CPU. > I ask myself why? > The modelling of a generalised rigid body with six degrees of freedom in > a rotating frame of > reference should max out anyones and everyones CPU! Calculating the updating 6 DOF is a real simple task. It only takes a few operations. It can be far more challenging to calculate change in the 6 DOFs. This can be done cheaply on the basis of functions / lookup tables (like the FDMs we use). Or it can be done by solving navier stokes equations at each timestep. This would max out the CPUs for quite a while (especially when it comes to direct numerical simulation - there are even current supercomputers maxed out for simpler tasks than the flow around an air foil) Oh, btw, we are currently updating our 6 DOF much more often than the image gets redrawn... > I notice that JSBSim uses a simple single step method for integration > whereas LaRCSim uses > a multi-step method. Spare CPU could be utilised in doing a multi-step > predictor/corrector integration > with variable step size especially for more esoteric aircraft types with > high speeds and high altitudes > and even near orbital capability in JSBSim. I doubt that changing the integration method will cause any performance problem. So you might try it. But making delta t smaller also helps, so that might give you more precision for the time spend tweaking the code. And as we've got an interactive application I guess that the absolute error in the integration doesn't bite us. Any turbulence in the real world will change the position more than precision errors will (assuming perfect FDM parameters) CU, Christian -BEGIN PGP SIGNATURE- Version: GnuPG v1.4.0 (MingW32) iD8DBQFCzC7xlhWtxOxWNFcRAstrAJ9F5IodbEMqjrRUQxvOjDAEJAXR/QCgnxCD yavFP/qrqy1BScpH8FFzOEE= =saYF -END PGP SIGNATURE- ___ Flightgear-devel mailing list Flightgear-devel@flightgear.org http://mail.flightgear.org/mailman/listinfo/flightgear-devel 2f585eeea02e2c79d7b1d8c4963bae2d
Re: [Flightgear-devel] Re: code optimisation
Jim Campbell wrote: > The modelling of a generalised rigid body with six degrees of > freedom in a rotating frame of reference should max out anyones and > everyones CPU! People were doing that in real time with VAX 11/780's in 1981. :) Seriously, no: the FDM takes up comparatively little CPU. Of the two main ones, YASim is the slowest, because it breaks the fuselage down into a bunch of discrete units that need to be individually handled. The last time I looked at this YASim took up about 5-7% of so of the total CPU budget for the flight model I was looking at. I honestly forget when this was, on what computer, or what plane I was using. Sorry, but the 3D graphics are the lions share of the performance budget, and always will be. > Of course any model is only as good as the approximations used so we > apply symmetry to an aircraft (what happens if an engine drops off!) > and various other restrictions. YASim is perfectly happy with asymmetric aircraft, FWIW. Andy ___ Flightgear-devel mailing list Flightgear-devel@flightgear.org http://mail.flightgear.org/mailman/listinfo/flightgear-devel 2f585eeea02e2c79d7b1d8c4963bae2d
[Flightgear-devel] Re: code optimisation
Hi, One comment and some documentation has indicted that FDM doesnt consume much CPU. I ask myself why? The modelling of a generalised rigid body with six degrees of freedom in a rotating frame of reference should max out anyones and everyones CPU! Of course any model is only as good as the approximations used so we apply symmetry to an aircraft (what happens if an engine drops off!) and various other restrictions. From my point of view I would rather devote spare CPU to the accuracy of the FDM. I notice that JSBSim uses a simple single step method for integration whereas LaRCSim uses a multi-step method. Spare CPU could be utilised in doing a multi-step predictor/corrector integration with variable step size especially for more esoteric aircraft types with high speeds and high altitudes and even near orbital capability in JSBSim. Anyone with any comments or advice or done anything similar? cheers Jim ___ Flightgear-devel mailing list Flightgear-devel@flightgear.org http://mail.flightgear.org/mailman/listinfo/flightgear-devel 2f585eeea02e2c79d7b1d8c4963bae2d
