Here's how this game works when you enable PhysX in the game:
No Nvidia GPU(s):
Cloth and Particle physX both run on the CPU. Any CPU in the world will get crushed by this, and you will get crap framerates wherever physX effects occur.
Any nV setup w/o dedicated physX card (single or SLI):
Particle physX runs on one of your GPU's, cloth physX (mainly that's everyone's clothes) runs on your CPU. Cloth physX is very demanding and you'll get significant FPS reduction wherever there's a lot of cloth physX calcs to be done.
Note that it appears to me that for any cloth (basically that means NPC's) that's appears within a certain radius of your character, even if you can't see it, the cloth physX calcs will be performed.
A lot of the indoor scenes get horrible framerates and I believe it's because of this, as does the area outside the Maltese Falcon where there's almost always a bunch of people walking around.
Someone above mentioned modding the game files to remove all cloth physX except on Vito, and this def. works and is a good way to deal w/FPS issues when you have no dedicated physX card.
Single or SLI setup + plus nV card with 'dedicated' physX running on it:
Particle and Cloth physX will both run entirely on the dedicated card. Anything from a 9800GT and up is adequate for these calcs, but if you have like 4/580's in SLI, there can be some additional FPS benefit by running something more powerful like a GTX260.
About PhysX on an SLI set:
Note that in an SLI set, you take a 'double-hit' from the physX load. This is because physX can only be run on one card, and both cards in an SLI set have to be synchronized in how fast they render. So the non-physX card slows down it's rendering to match the speed of the rendering on the physX card.
So if physX requires 30% of one card's power, you will effectively reduce the rendering power of BOTH cards by 30%.
This isn't a Mafia 2-specific situation, it's just a downside of running physX on an SLI set. With pretty much any other physX game it doesn't matter, but Mafia 2 is demanding enough when maxed out that it's important to keep in mind.
Note that this effect makes it so that SLI scaling always looks poor on physX games when it's running on one of the cards in the SLI set.
My Theory About PhysX and GPU Usage:
When a card is doing physX AND rendering, it *appears to me* that the gpu usage you see (such as with AB OSD) reflects *only* the rendering portion of the GPU's workload. With physX running on a dedicated card, the gpu usage will reflect the actual physX load.
Edited by brettjv - 4/14/11 at 4:35pm