How much memory does FG use?
The answer depends almost exclusively on the settings - what aircraft do you run, what scenery do you want to see, how many scenery objects are to be displayed, what is the selected visibility. In numbers, it can range anywhere from 470 MB (ufo over ocean) to >12 GB (modern airliner, 150 km visibility range in hires scenery).
Most numbers given here are taken from Rebecca Palmer's test with the 3.4 RC posted on the mailing list.
What drives memory consumption?
Roughly, it can be understood as three components as
Memory = Base + Aircraft + Terrain * visibility^2
Here, base is about 450 MB, i.e. what the FG core uses. This contribution can not reduced by any options. Aircraft is specific for the aircraft you select - more complex and more richly textured aircraft need more memory. A few examples:
- c172p: 250 MB
- Citation-X: 100 MB
- 777-300: 650 MB
Typically, the sum of Base and Aircraft is less than a GB, so the rest is terrain - the combination of terrain mesh, objects and visibility is the single most influential factor for memory consumption.
How does the terrain affect memory usage?
The terrain mesh has a certain vertex density which determines how finely it resolves features such as roads, landclass boundaries or elevations. All these vertices have to be available to the graphics card to be rendered, i.e. all visible terrain mesh needs to reside in memory. Hence, if you know the memory consumption for one visibility, you can estimate that it will increase fourfold if you double visibility and decrease by a factor four if you half visibility.
The vertex density multiplies the visibility factor. The old World Scenery 1.0 has a fairly low vertex density, and hence hardly affects memory at all for visibilities < 30 km. The new World Scenery 2.0 has a much higher vertex density - in Europe almost a factor 100 more data - and does affect memory quite a bit.
The difference is (KSFO runway, default settings, default visibility of 16 km - scenery-only memory consumption)
- new World Scenery 2.0: 1.9 GB
- old World Scenery 1.0: 300 MB
(i.e. in order to get the same memory occupancy with the new scenery than with the old scenery, you'd have to reduce visibility to ~6.5 km - the situation in Europe is even more drastic).
What should I do to run FG with a low memory footprint?
From the above considerations, in decreasing order of importance follow:
- disable terrasync and revert to the 1.0 World Scenery (this can be manually downloaded from the FG website)
- use settings to limit visibility (setting a low LOD bare also works, but this destroys the visible horizon and looks artificial - setting/limiting the visibility directly works equally well and keeps a natural-looking hazy horizon)
- do not use random objects, trees or buildings (which have their own memory footprint)
- avoid highly detailed planes
- manually resize textures to make them smaller (note that this gains you a few tens of MB only - hardly worth the effort in most cases)
Using all of these measures, it should be possible to run FG with under 2 GB of memory footprint.