"The OS will only use memory for cache when there is no other demand for that memory"
Ok, I'm not going to bother and read the smart people. I'm going to go straight to my point.
If you are using nearly all available RAM for disk cache, then EVERY REQUEST FOR RAM WILL REQUIRE CACHE DUMP.
It's like this;
If you have 4GB RAM and are using, say, 1.5GB for applications and system, and you use 2.2GB RAM for cache, then you are left with 300MB approx for any new demand. So any demand in excess is going to make your system dump cache, and it does take time. How much is an interesting discussion. And yes, we can consider if the request is going to be satisfied out of cache, but let's also assume if it is than that cache will not be dumped. That would be unfortunate.
Here's my beef with this much more aggressive caching in 7 v XP:
What the ^&)$ do you need 2.5G cache for?
What are the likely demands on caching? Office 2007? My XP machine at work rarely shows Office 07 components using more than 500MB RAM. 2G caching? Pagefile getting you down, you don't want to use that?
More to the point, how often would a real-world user be USING 2GB RAM, much less 4GB?
Well, I do. But real-world? I have 2-4 virtual machines running fairly often, and in XP I rarely get up to 3GB. Win7 would maybe get me into 4GB+, sure, and the pagefile would go crazy.
When Smartdrive first came out, we used to tune it down quite a bit to avoid hogging RAM. I know it's improved, and I've never worried about it since Win98, but the more I read about Win7's caching, the more I think it is there for some strategy here that has nothing to do with user demands, and everything to do with OS performance. Vista and 7 both cannot be descirbed as lightweight, so maybe this is really to keep the OS light on its feet.
Fine. But MS can't say so.
Another resource hog. Hopefully it won't hog anything we need, right?
I just don't get it. Obviously.