[Flightgear-devel] Memory issues

[Renk T. <tho...@jy...>]

Of late, I have increasingly been running into some segfaults which seem to have to do with peak memory usage (I don't think memory leaks are a major issue for what I've been seeing, since some segfauls happened quite early on, and especially when I try pushing my limits of graphical goodies). Memory also appears to be an issue for other users, judging by forum response.

For me, running into swap space means basically either immediate or imminent crash. Sometimes I get a 30 seconds grace with framerates below 5 fps, sometimes Flightgear dies immediately. Apparently other binaries (the 64 bit Jenkins) seem to be a bit more tolerant - see for instance user experiences here:

http://www.flightgear.org/forums/viewtopic.php?f=68&t=17114

None of this is a problem on a fundamental level for me personally - I largely know my 'safe' limits for visibility  (250 km for default scenery and bare terrain on short suborbital hops, 120 km for long-distance flights in bare default scenery, 60 km in bare custom scenery or default scenery with buildings, 40 km in custum scenery with buildings, subtract another 10% for trees...), so I check a range of options I know by experience to be safe before the flight and am not usually troubled by crashes unless I try something new.

But I know this stuff because I do a lot of benchmark testing, am active on this list and have some understanding of the inner workings of Flightgear. But there's nothing to inform the casual user what he should do. In fact, by chance the combination of options we offer is quite dangerous.

* before the new edition of random buildings came out, I've played a lot with opening up visibility at high altitude. Even for a ground visibility of less than 8 km, you'd get 120 km at airliner cruise altitude if 'realistic visibility' is checked and the visibility limit shifted to max. in Advanced Weather

* at the same time, random buildings (even in a rather unpopulated area like the Alps and with a density around 0.8) account for half of my memory consumption (I tested 1.0 GB for loading a 40 km radius without buildings against 1.9 GB for loading the same scene with buildings). That's before trees and/or higher densities. Hitting a major urban area changes the picture drastically.

The combination of the two means instant death at high altitude, even when all you do on the ground looks sane - you can see a nice collection of random buildings nearby, have a safe visibility range, good momory consumption - and once you are up, the tile manager starts loading a city 120 km away you don't even see yet and populates it with buildings, memory consumption explodes and you are dead.

Both options, random buildings and a realistic view range, are in my view extremely cool as long as you know what you are doing with them, and I don't really want to abandon either. But I suspect their combination (combined with other memory-hungry goodies) is not something we want to confront the casual user with.

Do we have any ideas addressing these issues?

For instance, should I agressively limit the maximally available visibility range in Advanced Weather if random buildings and trees are selected? Is it technically feasible to let Flightgear probe the remaining physical memory and if we run into trouble initiate an agressive unloading of terrain, e.g. by forcing visibility down (since most memory-hungry stuff scales with the square of the visibility range, I think adjusting that is most effective)? I know this is not realistic, but I'd rather have sudden haze than sudden segfault? Should we code a few warnings into the GUI?

And other ideas? Or is this not an issue for the majority?

* Thorsten