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Monitoring CPU and CPU Core Usage

Last updated 2009-06-01 - Send Feedback
Related: how-to, Optimization

Note: Activity Monitor in OS X Mavericks has changed in appearance and certain terminology. See Using Activity Monitor in OS X Mavericks.

The graphical displays provided by Activity Monitor are excellent.

Activity Monitor is found in the Utilities folder under Applications. Drag it into the dock so that it’s always readily available. Especially on dual display systems, it can be left open on the 2nd monitor for conveniently monitoring system performance.

Click to go to Activity Monitor in the Finder

For more on CPU cores and how well programs use them, see CPU Cores and Application support for multiple CPU cores.

Percentage CPU usage

Sometimes you want to know which program is using the most CPU time. The main Activity Monitor window shows processes and various information about them.

Choose All Processes in the popup menu at center. Control-click or right-click on column headers to choose which columns to display. Note that the column names are abbreviated when the window size is reduced.

Quad-core CPU history in Activity Monitor (mostly idle)

Sort by percentage CPU usage by clicking on the CPU column (triangle should point down as shown). This makes it easy to see which applications are using CPU resources. The example below shows a MemoryTester (dlt) test in progress, taking 364.7% of the available CPU cycles. The remainder is being used by WindowServer, DreamWeaver, etc. One CPU core represents 100%, 2 CPU cores is 200%, etc.

You might find that some “vampire” programs waste CPU time when doing nothing useful— these are programs you don’t want to leave running when you’re not using them!

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Disk and network activity

Observe the tabs at bottom (CPU / System Memory / Disk Activity / etc). These show other useful information. Unfortunately, they can be shown only one tab at a time (you can’t watch Disk Activity and Network at the same time).

CPU history graph

The history graph (Window => CPU History) can be sized wider to show CPU history over quite some time (View => Update Frequency). Black areas indicate idle CPU cores. Green represents CPU utilization by user applications, red represents CPU utilization by Mac OS X itself, and blue indicates low-priority tasks. The example below shows a mostly idle system.

Make sure Windows => Keep CPU Windows on Top is disabled (unchecked).

Quad-core CPU history in Activity Monitor (mostly idle)

The display updates as specified, scrolling left at each interval. Watch this display for your favorite programs, and you can see how well (or poorly) they utilize the CPU cores while doing some hard work, like Unsharp Mask in Photoshop.

For a dual-core system, you’ll see two layers, four layers for 4-core-, eight for 8-core, etc. When we have 32 cores, let’s wait and see what Apple does!


Command line tools for monitoring CPU and memory usage

Monitoring performance in Terminal (a plain-text display) can be useful; it is mentioned here for completeness.

The top and vm_stat tools (and iostat for disk I/O) can come in handy. You must use these tools in a Terminal window. The most useful tool is top, which can be left running continually, refreshing the window at regular intervals.

Show top 10 processes by CPU usage every 2 seconds: top -o cpu -s 2 10
Show top 10 processes by real memory every 2 seconds: top -o rsize -s 2 10

'top' display by CPU usage

The vm_stat tool also provides some useful statistics:

vm_stat display by CPU usage


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