CPU usage in top, when looking at an individual process, is relative to one CPU's worth of power. So 100% means it's using one CPU core to the fullest. Therefore a quad core CPU can be running happily at 400% CPU usage according to top, when looking at the individual process. Multi-threaded processes will often do this, in fact, and report a "CPU Usage" of 3-400% or higher, depending on how many cores they're consuming.
Windows Task Manager shows values relative to the total power of all CPU cores.
In other words, if you had a dual core CPU, and one process consuming one complete core, top would show 100% CPU usage for that process, while Windows TM would show 50%.
I have a 12-core machine here with hyper threading (24 virtual cores). When it's really loaded up, you'll often see 10-12 processes in top each consuming 100% CPU. Doing the same in Windows would only show about 4% CPU for each process.
Last edited by suicidaleggroll; 07-31-2012 at 04:41 PM.