I just can't bring myself to give the whole run down on what you're seeing here ... I've given the same spiel so many times before.
Suffice to say that everything you describe is pretty 'normal' for this game.
With your rig you should probably run Mafia 2 with one card for rendering, and one card for PhysX. Just trust me on this one
Ok, an abbreviated explanation, just because I'm a compulsive ...
1) The only time you see PhysX usage is when you're looking at it on a dedicated card. If the card is also rendering, the only usage you'll see is the rendering usage. So if 30% of the cards power is going to physX, you'll only see 70% usage on the AB graphs.
2) PhysX will always run on only one card. However, when two cards are in SLI, they must be running at the same speed. Therefore, while the entire 'hit' from PhysX occurs on one card, the other card slows down to match it in terms of rendering. Thus, if one card is occupied 30% w/PhysX, both your cards in SLI will show (up to) 70% GPU usage ... that's the rendering work happening on each card.
3) In Mafia 2, if you do not have a DEDICATED PhysX card, the cloth PhysX will run on the CPU. There's parts in the game where this will totally slam your CPU's capabilities.
These three things should explain to you the observations that you've made, and why I'm suggesting that you split your SLI set and run one card as dedicated PhysX card for this game if you want to run PhysX.Edited by brettjv - 2/28/11 at 12:05am