CPU Load Comparisons and Testing

People often want to meaningfully compare CPU use in order to determine of efficient different plugins or synths are or to determine which iPad or iPhone model will give the most processing power. The CPU load indicators in many apps often give a misleading picture of how much CPU is actually being used unless the device is running at near full speed.

Apple to Apples Comparisons - each app uses its own method for calculating CPU load. So, don't draw too many conclusions from looking at one app's CPU numbers without doing a load test as described below.

The CPU load indicators you find in apps are rough indicators of how much of the CPU is being used in its current state. However, because the OS does CPU-throttling to optimize power-consumption; the CPU may not be running at its full speed.

Throttling. Modern CPUs can have their speeds reduced dynamically to reduce power consumption. Because power consumption is critical on battery-powered mobile devices, the OS uses sophisticated algorithms to determine how much CPU capacity is needed at any given moment. The OS dynamically adjusts the CPU speed to what it thinks you need. On multi-core devices, core allocation may also shift to optimize power consumption.

For these reasons, you don't know how much of the CPU's full capacity you are using until you start to overload it – because that it is the only time that you are assured that the CPU is at full speed.

The short version: set up a test running from a fresh boot in which you incrementally add iterations of a known process (for example, instances of a particular plugin with a particular preset) until CPU overload is detected.

For example, you could use a process like this:

Launch your preferred AU host or DAW
Load in an instance of an AU that is not memory intensive (for example, Zeeon)
Load in a particular preset which we will call the test preset
Have a MIDI source set to play notes through all loaded AU instances
Play notes, if the CPU use is high and you hear crackling, you have probably maxed out your device.
Continue adding instances (being sure to use your test preset) until your test fails.

Repeat this test on other devices and compare the number of instances that could be loaded to get a sense of how the CPUs compare.