What is it?
The performance view allows you to record and profile a session from your Dart application.
Start recording a CPU profile by clicking Record. When you are done recording, click Stop. At this point, CPU profiling data is pulled from the VM and displayed in the profiler views (Call Tree, Bottom Up, and Flame Chart).
The default rate at which the VM collects CPU samples is 1 sample / 250 μs. This is selected by default on the Performance view as “Profile granularity: medium”. This rate can be modified via the selector at the top of the page. The sampling rates for low, medium, and high granularity are 1 / 50 μs, 1 / 250 μs, and 1 / 1000 μs, respectively. It is important to know the trade-offs of modifying this setting.
A higher granularity profile has a higher sampling rate, and therefore yields a fine-grained CPU profile with more samples. This may also impact performance of your app since the VM is being interrupted more often to collect samples. This also causes the VM’s CPU sample buffer to overflow more quickly. The VM has limited space where it can store CPU sample information. At a higher sampling rate, the space fills up and begins to overflow sooner than it would have if a lower sampling rate was used. This means that you may not have access to CPU samples from the beginning of the recorded profile.
A lower granularity profile has a lower sampling rate, and therefore yields a coarse-grained CPU profile with fewer samples. However, this impacts your app’s performance less. The VM’s sample buffer also fills more slowly, so you can see CPU samples for a longer period of app run time. This means that you have a better chance of viewing CPU samples from the beginning of the recorded profile.
This tab of the profiler shows CPU samples for the recorded duration. This chart should be viewed as a top-down stack trace, where the top-most stack frame calls the one below it. The width of each stack frame represents the amount of time it consumed the CPU. Stack frames that consume a lot of CPU time may be a good place to look for possible performance improvements.
The call tree view shows the method trace for the CPU profile. This table is a top-down representation of the profile, meaning that a method can be expanded to show its callees.
- Total time
- Time the method spent executing its own code as well as the code for its callees.
- Self time
- Time the method spent executing only its own code.
- Name of the called method.
- File path for the method call site.
The bottom up view shows the method trace for the CPU profile but, as the name suggests, it’s a bottom-up representation of the profile. This means that each top-level method in the table is actually the last method in the call stack for a given CPU sample (in other words, it’s the leaf node for the sample).
In this table, a method can be expanded to show its callers.
- Total time
Time the method spent executing its own code as well as the code for its callee.
- Self time
For top-level methods in the bottom-up tree (leaf stack frames in the profile), this is the time the method spent executing only its own code. For sub nodes (the callers in the CPU profile), this is the self time of the callee when being called by the caller. In the following example, the self time of the caller
Element.updateSlotForChild.visit()is equal to the self time of the callee
[Stub] OneArgCheckInLineCachewhen being called by the caller.
Name of the called method.
File path for the method call site.