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diff --git a/docs/performance/activity_monitor_and_top.md b/docs/performance/activity_monitor_and_top.md new file mode 100644 index 0000000000..4e687c4dfe --- /dev/null +++ b/docs/performance/activity_monitor_and_top.md @@ -0,0 +1,165 @@ +# Activity Monitor, Battery Status Menu and top + +This article describes the Activity Monitor, Battery Status Menu, and +`top` --- three related tools available on Mac OS X. + +**Note**: The [power profiling overview](power_profiling_overview.md) is +worth reading at this point if you haven't already. It may make parts +of this document easier to understand. + +## Activity Monitor + +This is a [built-in OS X tool](https://support.apple.com/en-au/HT201464) +that shows real-time process measurements. It is well-known and its +"Energy Impact" measure is likely to be consulted by users to compare +the power consumption of different programs. ([Apple support +documentation](https://support.apple.com/en-au/HT202776) specifically +recommends it for troubleshooting battery life problems.) +***Unfortunately "Energy Impact" is not a good measure for either +users or software developers and it should be avoided.*** Activity +Monitor can still be useful, however. + +### Power-related measurements + +Activity Monitor has several tabs. They can all be customized to show +any of the available measurements (by right-clicking on the column +strip) but only the "Energy" tab groups child processes with parent +processes, which is useful, so it's the best one to use. The following +screenshot shows a customized "Energy" tab. + + + +The power-related columns are as follows. + +- **Energy Impact** / **Avg Energy Impact**: See the separate section + below. +- **% CPU**: CPU usage percentage. This can exceed 100% if multiple + cores are being used. +- **Idle wake Ups**: + - In Mac OS 10.9 this measured "package idle exit" wakeups. This + is the same value as + [powermetrics](./powermetrics.md)' + "Pkg idle" measurement (i.e. + `task_power_info::task_platform_idle_wakeups` obtained from the + `task_info` function.) + - In Mac OS 10.10 it appears to have been changed to measure + interrupt-level wakeups (a superset of idle wakeups), which are + less interesting. This is the same value as + [powermetrics](./powermetrics.md)' + "Intr" measurement (i.e. + `task_power_info::task_interrupt_wakeups` obtained from the + `task_info` function.) +- **Requires High Perf GPU**: Many Macs have two GPUs: a low-power, + low-performance integrated GPU, and a high-power, high-performance + external GPU. Using the high-performance GPU can greatly increase + power consumption, and should be avoided whenever possible. This + column indicates which GPU is being used. + +Activity Monitor can be useful for cursory measurements, but for more +precise and detailed measurements other tools such as +[powermetrics](./powermetrics.md) are better. + +### What does "Energy Impact" measure? + +"Energy Impact" is a hybrid proxy measure of power consumption. +[Careful +investigation](https://blog.mozilla.org/nnethercote/2015/08/26/what-does-the-os-x-activity-monitors-energy-impact-actually-measure/) +indicates that on Mac OS 10.10 and 10.11 it is computed with a formula +that is machine model-specific, and includes the following factors: CPU +usage, wakeup frequency, [quality of service +class](https://developer.apple.com/library/prerelease/mac/documentation/Performance/Conceptual/power_efficiency_guidelines_osx/PrioritizeWorkAtTheTaskLevel.html) +usage, and disk, GPU, and network activity. The weightings of each +factor can be found in one of the the files in +`/usr/share/pmenergy/Mac-<id>.plist`, where `<id>` can be determined +with the following command. + + ioreg -l | grep board-id + +In contrast, on Mac OS 10.9 it is computed via a simpler machine +model-independent formula that only factors in CPU usage and wakeup +frequency. + +In both cases "Energy Impact" often correlates poorly with actual +power consumption and should be avoided in favour of direct measurements +that have clear physical meanings. + +### What does "Average Energy Impact" measure? + +When the Energy tab of Activity Monitor is first opened, the "Average +Energy Impact" column is empty and the title bar says "Activity +Monitor (Processing\...)". After 5--10 seconds, the "Average Energy +Impact" column is populated with values and the title bar changes to +"Activity Monitor (Applications in last 8 hours)". If you have `top` +open during those 5--10 seconds you'll see that `systemstats` is +running and using a lot of CPU, and so presumably the measurements are +obtained from it. + +`systemstats` is a program that runs continuously and periodically +measures, among other things, CPU usage and idle wakeups for each +running process. Tests indicate that it is almost certainly using the +same "Energy Impact" formula to compute the "Average Energy Impact", +using measurements from the past 8 hours of wake time (i.e. if a laptop +is closed for several hours and then reopened, those hours are not +included in the calculation.) + +## Battery status menu + +When you click on the battery icon in the OS X menu bar you get a +drop-down menu that includes a list of "Apps Using Significant Energy". +This is crude but prominent, and therefore worth understanding --- even +though it's not much use for profiling applications. + + + +When you open this menu for the first time in a while it says +"Collecting Power Usage Information" for a few seconds, and if you have +`top` open during that time you'll see that, once again, `systemstats` +is running and using a lot of CPU. Furthermore, if you click on an +application name in the menu Activity Monitor will be opened and that +application's entry will be highlighted. Based on these facts it seems +reasonable to assume that "Energy Impact" is again being used to +determine which applications are "using significant energy". + +Testing shows that once an energy-intensive application is started it +takes less than a minute for it to show up in the battery status menu. +And once the application stops using high amounts of energy it takes a +few minutes to disappear. The exception is when the application is +closed, in which case it disappears immediately. And it appears that a +program with an "Energy Impact" of roughly 20 or more will eventually +show up as significant, and programs that have much higher "Energy +Impact" values tend to show up more quickly. + +All of these battery status menu observations are difficult to make +reliably and so should be treated with caution. It is clear, however, +that the window used by the battery status menu is measured in seconds +or minutes, which is much less than the 8 hour window used for "Average +Energy Impact" in Activity Monitor. + +## `top` + +`top` is similar to Activity Monitor, but is a command-line utility. To +see power-related measurements, invoke it as follows. + +``` +top -stats pid,command,cpu,idlew,power -o power -d +``` + +**Note**: `-a` and `-e` can be used instead of `-d` to get different +counting modes. See the man page for details. + +It will show periodically-updating data like the following. + + PID COMMAND %CPU IDLEW POWER + 50300 firefox 12.9 278 26.6 + 76256 plugin-container 3.4 159 11.3 + 151 coreaudiod 0.9 68 4.3 + 76505 top 1.5 1 1.6 + 76354 Activity Monitor 1.0 0 1.0 + +- The **PID**, **COMMAND** and **%CPU** columns are self-explanatory. +- The **IDLEW** column is the number of "package idle exit" wakeups. +- The **POWER** column's value is computed by the same formula as the + one used for "Energy Impact" by Activity Monitor in Mac OS 10.9, + and should also be avoided. + +`top` is unlikely to be much use for power profiling. |