Profiling with Instruments

Instruments can be used for memory profiling and for statistical profiling.

Official Apple documentation

Basic Usage

  • Select “Time Profiler” from the “Choose a profiling template for:” dialog.

  • In the top left, next to the record and pause button, there will be a “[machine name] > All Processes”. Click “All Processes” and select “firefox” from the “Running Applications” section.

  • Click the record button (red circle in top left)

  • Wait for the amount of time that you want to profile

  • Click the stop button

Command line tools

There is instruments and iprofiler.

How do we monitor performance counters (cache miss etc.)? Instruments has a “Counters” instrument that can do this.

Memory profiling

Instruments will record a call stack at each allocation point. The call tree view can be quite helpful here. Switch from “Statistics”. This malloc profiling is done using the malloc_logger infrastructure (similar to MallocStackLogging). Currently this means you need to build with jemalloc disabled (ac_add_options --disable-jemalloc). You also need the fix to Bug 719427

Kernel stacks

Under “File” -> “Recording Options” you can enable “Record Kernel Callstacks”. To get full symbols and not just the exported ones, you’ll to install the matching Kernel Debug Kit. Make sure you install the one whose macOS version exactly matches your version, including the identifier at the end (e.g. “12.3.1 (21E258)”).

Allow Instruments to find kernel symbols

Installing the KDK is often not enough for Instruments to find the symbols. Instruments uses Spotlight to find the dSYMs with the matching UUID, so you need to put the dSYM in a place where Spotlight will index it.

First, check the UUID of your macOS installation’s kernel. To do so, run the following:

% dwarfdump --uuid /System/Library/Kernels/kernel.release.t6000
UUID: C342869F-FFB9-3CCE-A5A3-EA711C1E87F6 (arm64e) /System/Library/Kernels/kernel.release.t6000

Then, find the corresponding dSYM file in the KDK that you installed, and run mdls on it. For example:

% mdls /Library/Developer/KDKs/KDK_12.3.1_21E258.kdk/System/Library/Kernels/kernel.release.t6000.dSYM

(Make sure you use the .release variant, not the .development variant or any of the others.)

If the output from mdls contains the string com_apple_xcode_dsym_uuids and the UUID matches, you’re done.

Otherwise, try copying the kernel.release.t6000.dSYM bundle to your home directory, and then run mdls on the copied file. For example:

% cp -r /Library/Developer/KDKs/KDK_12.3.1_21E258.kdk/System/Library/Kernels/kernel.release.t6000.dSYM ~/
% mdls ~/kernel.release.t6000.dSYM
_kMDItemDisplayNameWithExtensions      = "kernel.release.t6000.dSYM"
com_apple_xcode_dsym_paths             = (
com_apple_xcode_dsym_uuids             = (
kMDItemContentCreationDate             = 2022-03-21 15:25:57 +0000

Now Instruments should be able to pick up the kernel symbols.


The DTPerformanceSession api can be used to control profiling from applications like the old CHUD API we use in Shark builds. Bug 667036

System Trace might be useful.