What's New in Mac OS X 10.3 Panther
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Process Management: the new Activity Monitor
/Applications/Utilities/ is a new utility called Activity Monitor, which combines the functionality of the old CPU Monitor and Process Viewer utilities and displays some additional infomation that was previously only available in the command-line utility
Below the process list are five tabs: CPU, System Memory, Disk Activity, Disk Usage, and Network. The CPU tab is what the old CPU Monitor used to display: a rolling history of CPU usage, and current CPU usage statistics.
The System Memory tab shows the current allocation of your physical real memory, as well as the size of your swap files and how many times the system has had to page something out to disk. Too many page outs means your system is thrashing, and you probably need to buy more memory.
Double-clicking on a process, or selecting "Inspect" from the Process menu or from the toolbar, opens a window that displays more details about the process.
The Statistics tab within the Inspect window displays how many threads the process has created, how many mach ports it uses, the total CPU time it's sucked up since starting, context switches, page faults, how many times the process has paged in stored memory, messages to the mach kernel, and the number of UNIX system calls it has made.
From the "Sample" button in the Inspect window, or from the Process menu in the main menu, you can take a sample of a process. This is a way for developers to find out where an unresponsive program is spending its time. The output is not terribly useful unless you have the source code to the application you're sampling, but it can be very useful to developers. (Thanks to Jens and Quinn for explaining this feature to me.)
There are several options for the dock icon of the Activity Monitor application. It can display current CPU usage (one bar per processor), or a rolling history of CPU, memory, disk, or network activity (similar to the graphs in the tabs below the process list).