The Performance panel offers tools for profiling and analyzing the responsiveness of your UI during the course of user interaction. With it, you can:
- Measure execution times of the various components of your page
- Drill down to where you're spending the most CPU cycles to run your page and the resulting visual effect for your users
- Get a step-by-step breakdown of the processes consuming page execution time
Recording a profile
The first step to analyzing the performance of your page is to capture a profile as you perform a particular user scenario, such as the repro steps of a performance bug you're trying to fix, or a typical use case you want to optimize for a better user experience.
Use the Start / Stop buttons on the toolbar (or
Ctrl+E) to initiate and conclude your performance trace. A green indicator will apear on the Performance tab to indicate a recording is in progress.
A performance report will generate upon stopping the profile. You can choose to save it to disk (
Ctrl+S) and reload (
Ctrl+O) in DevTools at a later time. DevTools diagnostic sessions are saved with the .diagsession extension.
Here are some things to keep in mind when recording a profile:
Perform the fewest actions you need to capture the scenario you're trying to analyze. Extraneous actions with the page will produce extra data and clutter your results.
The profiler will automatically mark major app lifecycle events in the report, such as page navigation, DOMContentLoaded, and page load. You can add custom markers by calling the Performance.mark() method from within your code or the console.
If initial page load times are important to your analysis, make sure to clear your browser cache (from the Network panel) to ensure all page resources are loading from the network.
Sometimes it helps to record multiple sessions and/or sample the same scenario across different machines to better understand the performance issue in the wild.
You will see two types of markers on the ruler:
User marks are custom markers you can choose to add with calls to the Performance.mark() method from within your code or the DevTools Console. You can group start and end marks together as a single, named measure with the Performance.measure() method.
Once you have selected a time range, you can further Zoom in from the toolbar, or Reset zoom and Clear selection to return to the full view of the performance trace (with no time range selected). These controls are also available from the right-click context menu.
The CPU utilization % timeline graph describes the processing resources consumed by the various browser subsystems required to run the page, broken out by category:
Indicates time spent retrieving app resources and parsing HTML and CSS. This can include network requests. The following associated events are logged in the Timeline:
|CssParsing||New CSS content was encountered that needed to be parsed.|
|HtmlParsing||New HTML content was encountered that needed to be parsed into nodes and inserted into the DOM.|
|HttpRequest||A remote resource was encountered in the DOM or an XMLHttpRequest was created that required an HTTP request to be made.|
|HtmlSpeculativeDownloading||The page's HTML content was being searched for required resources so that the HTTP requests for them could be scheduled as quickly as possible.|
|DomEvent||An event was fired on a DOM object.|
|EventHandler||A registered event listener was triggered in response to a DOM event being fired.|
|Frame||While a new frame was being prepared a registered callback was triggered so that it could contribute visual changes.|
|Measure||An app-specific scenario was measured using the
|MediaQueryListener||A registered media query was invalidated which resulted in the execution of its associated listener(s).|
|MutationObserver||One or more observed DOM elements were modified which resulted in the execution of a MutationObserver's associated callback.|
|TimerFired||A scheduled timer elapsed which resulted in the execution of its associated callback.|
|WindowsRuntimeAsyncCallback||An async operation was completed by a Windows Runtime object which triggered a
|WindowsRuntimeEvent||An event was fired on a Windows Runtime object which triggered a registered listener.|
Indicates time spent collecting memory for objects that are no longer in use. The following associated events are logged in the Timeline:
Indicates time spent calculating element presentation and layout. The following associated events are logged in the Timeline:
|AlignedBeat||Pending visual changes that were made to the DOM were processed so that the app's display could be updated.|
|CssCalculation||Changes were made to the DOM or new CSS content was added, requiring the style properties of all affected elements to be recalculated.|
|Layout||Changes were made to the DOM that required the size and/or position of all affected elements to be computed.|
Indicates time spent in painting the screen. The following associated events are logged in the Timeline:
|Paint||Visual changes were made to the DOM that required all affected portions of the page to be redrawn.|
|RenderLayer||Visual changes were made to an independently rendered fragment of the DOM (called a layer) which required its respective portion of the page to be redrawn.|
Indicates time spent decompressing and decoding images. The following associated events are logged in the Timeline:
|ImageDecoded||An image was included into the DOM and needed be to decompressed from its original format into a bitmap.|
The Visual throughput (FPS) graph shows the estimated frames per second (FPS) during the course of the profiling scenario, where 60 FPS is the ideal display rate. Dips in the frame rate indicate performance bottlenecks and a frame rate of zero means that frames are getting dropped entirely.
Use the lowermost details pane to get the full breakdown of what happened on the page. The Timeline details tab provides a breakdown of events that occurred within the various browser subsystems.
Event list sort control
Group events by frame
Use the Group top level events by frames toggle to group top-level events (HTML parsing, Layout, DOM event, etc.) into their corresponding unit of work (or "frame") during periods of time where animations/visual updates were occurring. The frames are treated like other events, so they can be sorted/filtered and provide an Inclusive time summary when clicked in the Event list.
Event list filter controls
Use the Filter events menu to configure the types of events shown in the timeline details.
The following filters are available:
Image decoding: Show events which occurred on a background thread (e.g. Image decoding, GC).
Network traffic: Show HTTP requests which were network-bound.
UI activity: Show events which occurred on the UI thread and/or render thread (e.g. DOM event handlers, Layout).
User measures: Show custom events which indicate calls to the performance.measure() method.
You can further filter top-level events by their inclusive duration.
The Event list gives you a chronological list of browser subsystem events that occured during the selected span of time.
Click on any entry to populate the Selected event details chart for that item. Entries with nested events / functions will show their inclusive (time spent executing the function and any other functions it called) and exclusive (time spent only within the body of the calling function itself) times displayed in the chart.
Selected timeline details
The Selected timeline details provides a detailed bar graph of inclusive/exclusive event times during the selected time span. When you sort by Duration (inclusive) using the Event list sort control, the longest running events will visually stand out in this chart.
Selected event details
This report provides further information about the selected event, including Start time, the executing thread type (for example, Download, UI, Render), and other contextual details specific to the specific event type. For example, Event listener types provide debugger links to the Callback function and Scheduling call stack.
|Function name||Name of browser or user-defined function.|
|Inclusive CPU (%)||Percentage of selected CPU activity in this function and in functions called by this function.|
|Exclusive CPU (%)||Percentage of selected CPU activity in this function, excluding activity in functions called by this function.|
|Inclusive CPU (ms)||CPU time spent executing code in this function and in functions called by this function.|
|Exclusive CPU (ms)||CPU time spent executing code in this function, excluding time in functions called by this function.|
|URL||URL(s) where stack frame occurred. Function calls originating from the browser (standards-based web APIs) are labeled as [DOM].|
|Start / Stop profiling session||
|Import profiling session||
|Export profiling session||