Google DevTools Explanation【chrome浏览器中的Timing详情说明】

来源:  2018-08-03 14:49:26    评论:0点击:



DNS Lookup
时间执行DNS查找。每个新域pagerequires DNS查找一个完整的往返。 DNS查询的时间,当本地DNS缓存没有的时候,这个时间可能是有一段长度的,但是比如你一旦在host中设置了DNS,或者第二次访问,由于浏览器的DNS缓存还在,这个时间就为0了。
Initial connection

Request sent 

请求发出后,到收到响应的第一个字节所花费的时间(Time To First Byte),发送请求完毕到接收请求开始的时间;这个时间段就代表服务器处理和返回数据网络延时时间了。服务器优化的目的就是要让这个时间段尽可能短。

Content Download 

Evaluating network performance

The Network panel records information about each network operation in your application, including detailed timing data, HTTP request and response headers, cookies, WebSocket data, and more. The Network panel helps you answer questions about the network performance of your web application, such as:

  • Which resource had the slowest time to first byte?
  • Which resources took the longest time to load (duration)?
  • Who initiated a particular network request?
  • How much time was spent in the various network phases for a particular resource?

The Network panel uses the Resource Timing API, a JavaScript API that provides detailed network timing data for each loaded resource. For example, the API can tell you precisely when the HTTP request for an image started, and when the image's final byte was received. The following illustration shows the network timing data points that the Resource Timing API provides.

Resource timing overview

The API is available to any web page, not just DevTools. In Chrome, it's exposed as methods on the globalwindow.performance object. The performance.getEntries() method returns an array of "resource timing objects", one for each requested resource on the page.

Try this: open the JavaScript console on the current page, enter the following at the prompt, and hit Return:


This evaluates the first element in the array of resource timing objects and displays its properties in the console, as shown below.

Performance resource timing

Each timestamp is in microseconds, following the High ResolutionTime specification. This API is available inChrome as the window.performance.now() method.

The Network panel automatically records all network activity while DevTools is open. The first time you open the panel it may be empty. Reload the page to start recording, or simply wait for network activity to occur in your application.

Network overview

Each requested resource is added as a row to the Network table, which contains the columns listed below. Note the following about the Network table:

  • Not all columns listed below are visible by default; you can easily show or hide columns.
  • Some columns contain a primary field and a secondary field (Time and Latency, for example). When viewing the Network table with large resource rows both fields are shown; when using small resource rows only the primary field is shown.
  • You can sort the table by a column's value by clicking the column header. The the Timeline columnbehaves a bit differently: clicking its column header displays a menu of additional sort fields. See Waterfall view and Sorting and filtering for more information.
Name and Path
The name and URL path of the resource, respectively.
The HTTP method used for the request. For example: GET or POST.
Status and Text
The HTTP status code and text message.
The domain of the resource request.
The MIME type of the requested resource.
The object or process that initiated the request. It can have one of the following values:
Chrome's HTML parser initiated the request.
A HTTP redirect initiated the request.
A script initiated the request.
Some other process or action initiated the request, such as the user navigating to a page via a link, or by entering a URL in the address bar.
The number of cookies transferred in the request. These correspond to the cookies shown in the Cookies tab when viewing details for a given resource.
The number of cookies set in the HTTP request.
Size and Content
Size is the combined size of the response headers (usually a few hundred bytes) plus the response body, as delivered by the server. Content is the size of the resource's decoded content. If the resource was loaded from the browser's cache rather than over the network, this field will contain the text (from cache).
Time and Latency
Time is total duration, from the start of the request to the receipt of the final byte in the response. Latency is the time to load the first byte in the response.
The Timeline column displays a visual waterfall of all network requests. Clicking the header of this column reveals a menu of additional sorting fields.

By default, the current network record log is discarded when you navigate to another page, or reload the current page. To preserve the recording log in these scenarios, click the black Preserve log upon navigationbutton Don at the bottom of the Network panel; new records are appended to the bottom of the table. Click the same button again (now red Preserve resources on navigation) to disable log preservation.

By default, resources in the Network table are sorted by the start time of each request (the network "waterfall"). You can sort the table by another column value by clicking the column header. Click the header again to change the sort order (ascending or descending).

Sort by

The Timeline column is unique from the others in that, when clicked, it displays a menu of additional sort fields.

Timeline column

The menu contains the following sorting options:

  • Timeline — Sorts by the start time of each network request. This is the default sort, and is the same as sorting by the Start Time option).
  • Start Time — Sorts by the start time of each network request (same as sorting by the Timeline option).
  • Response Time — Sorts by each request's response time.
  • End Time — Sorts by the time when each request completed.
  • Duration — Sorts by the total time of each request.
  • Latency — Sorts by the time between the start of the request and the beginning of the response (also known as the "time to first byte").

To filter the Network table to only show certain types of resources, click one of the content types along the bottom of the panel: Documents, Stylesheets, Images, Scripts, XHR, Fonts, WebSockets, and Other. In the following screenshot only CSS resources are shown. To view all content types, click the All filter button.

Filter type

In addition to resource type filtering, you can narrow down resources by filter queries. For example, to find all resources which have a status code of 200, you can enter the query StatusCode:200 in the filter input field.

Advanced filtering in the Network Panel

Take note of the following behaviour: A filter query includes a type (StatusCode) and value (200). A filter query is case insensitive, so you can type uppercase or lowercase. The filter type gives you autocomplete suggestions. Use the arrows keys to form a selection and press Tab to select it. The filter value has autocomplete which shows you values present in the current network recording. Quickly preview the results of your query, use the Up / Down arrow keys to cycle through the autocomplete suggestions. The results appear immediately even you do not press enter or tab to complete the selection. To negate a filter query, prepend the query with a dash (-), for example -StatusCode:200.

Available filter types:

The domain portion from the URL of the resource. E.g. www.google-analytics.com.
Checks the resource has a response header, regardless of the value. E.g.Access-Control-Allow-Origin.
Shows requests running at the current point in time. Current available value:running
Shows requests with a transfer size larger than the specified amount. Units assumed in bytes, but kilobyte (k) & megabyte (m) units also allowed: E.g.larger-than:50, larger-than:150k, larger-than:2m
The HTTP method used. E.g. GET.
Also known as Content-type - the identifier for the type of resource. E.g.text/html.
The scheme portion in a URL. E.g. https.
The name of the cookie as set by the server. E.g. loggedIn (assuming a cookie like loggedIn=true).
The value of the cookie set by the server. E.g. true (assuming a cookie likeloggedIn=true).
The domain of the cookie as set by the server. E.g. foo.com (assuming a cookie like loggedIn=true; Domain=foo.com; Path=/; Expires=Wed, 13 Jan 2021 22:23:01 GMT; HttpOnly).
The status code in the HTTP response. E.g. 200.

To use a query shown in the above list, construct it in the format of: <Filter Type>:<Description>. You will almost always want to use the autocomplete suggestions which ensures your query is valid.

You can change the default set of columns displayed by the Network table. To show or hide a column, Right+click or Control+click (Mac only) in the table header and select or deselect column names from the list.

Add or remove columns

You can view the Network table with large resource rows (the default), or small resource rows. Click the blueUse small resource rows toggle button Small resource rows at the bottom of the panel to view small rows. Click the same button (now gray Large resource rows) to view large resource rows again. Large rows enable some columns to display two text fields: a primary field and a secondary field (Time and Latency, for instance). When viewing small rows only the primary field is displayed.

Resized resource rowsThe network table with small resource rows and just the timeline column.

The waterfall view in the network panel graphs the time it took to load each resource.From the start of the HTTP request to the receipt of the final byte of the response.

Each resource loading time is represented as a bar.This has color-coded information related to each resource.Each color specifies a different step needed to recieve the resource.The larger the bar grows represents more data being trasmitted for the request.

Network timeline viewThe network timeline for a simple web page.

Hovering your mouse over the bar itself will present the complete timing data.This is the same information that's presented in the Timing details tab for a given resource.

Timeline view on hoverNetwork timing details revealed on hover.

The waterfall highlights DOMContentLoaded and load events display with blue and red vertical lines. When the engine has completed parsing of the main document the DOMContentLoaded event fires. Upon retrieving all the page's resources the load event will fire.

DOM event linesThe event lines as displayed over the timeline.

You may enable in the network settings to view the timeline as color-coded by resource type.If you do the network timing information is still accessible through the tooltip.Waterfall bars are color-coded as follows:


Right-clicking or Ctrl + Click (Mac only) within the Network table a context menu appears with several actions. Some of these actions apply to the resource row under the mouse click (like copying HTTP request headers), while others apply to the entire network recording (such as saving a Network recording as a HAR file).

Right-click on Network

The following menu actions apply to the selected resource:

  • Open Link in New Tab — Opens the resource in a new tab. You can also double-click the resource name in the Network table.
  • Copy Link Address — Copies the resource URL to the system clipboard.
  • Copy Request Headers — Copies the HTTP request headers to the system clipboard.
  • Copy Response Headers — Copies the HTTP response headers to the system clipboard.
  • Copy as cURL — Copies the network request as a cURL command string to the system clipboard. SeeCopying requests as cURL commands.
  • Replay XHR — If the associated request is an XMLHTTPRequest, re-sends the original XHR.

cURL is a command line tool for making HTTP transactions. The Network panel's Copy as cURL command recreates an HTTP request (including HTTP headers, SSL certificates, and query string parameters) and copies it as a cURL command string to the clipboard. You can then paste the string into a terminal window (on a system with cURL) to execute the same request.

Below is an example cURL command line string taken from a XHR request on the Google News home page.

curl 'http://news.google.com/news/xhrd=us' -H 'Accept-Encoding: gzip,deflate,:sdch' -H 'Host: news.google.com' -H 'Accept-Language: en-US,en;q=0.8' -H 'User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_8_3) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/29.0.1510.0 Safari/537.36' -H 'Accept: */*' -H 'Referer: http://news.google.com/nwshp?hl=en&tab=wn' -H 'Cookie: NID=67=eruHSUtoIQA-HldQn7U7G5meGuvZOcY32ixQktdgU1qSz7StUDIjC_Knit2xEcWRa-e8CuvmADminmn6h2_IRpk9rWgWMdRj4np3-DM_ssgfeshItriiKsiEXJVfra4n; PREF=ID=a38f960566524d92:U=af866b8c07132db6:FF=0:TM=1369068317:LM=1369068321:S=vVkfXySFmOcAom1K' -H 'Connection: keep-alive' --compressed

You can save the data from a network recording as a HAR (HTTP Archive) file, or copy the records as a HAR data structure to your clipboard. A HAR file contains a JSON data structure that describes the network "waterfall". Several third-party tools can reconstruct the network waterfall from the data in the HAR file.

To save a recording:

  1. Right+click or Control+click on the Network table.
  2. In the context menu that appears, choose one of the following actions:
    • Copy All as HAR — Copies the network recording to the system clipboard in the HAR format.
    • Save as HAR with Content — Saves all network data to a HAR file along with each page resource. Binary resources, including images, are encoded as Base64-encoded text.

For more information, Web Performance Power Tool: HTTP Archive (HAR).

When you click a resource name in the Network table a tabbed window appears that contains the following additional details:

The Headers tab displays the resource's request URL, HTTP method, and response status code. Additionally, it lists the HTTP response and request headers and their values, and any query string parameters. You can view HTTP headers parsed and formatted, or in their source form by clicking the View parsed/View source toggle button, respectively, located next to each header's section. You can also view parameter values in their decoded or URL encoded forms by clicking the View decoded/View URL encoded toggle button next to each query string section.

Network headers

You can also copy request and response headers to your clipboard.

The Preview tab displays a preview of the resource, when available. Previews are currently displayed for image and JSON resources, as shown below.

Resource JSON preview

Resource image preview

You can view the resource's unformatted response on the Responsetab.

The Response tab contains the resource's unformatted content. Below is a screenshot of a JSON data structure that was returned as the response for a request.

Resource response preview

You can also view formatted previews of some resource types, including JSON data structures and images.

The Cookies tab displays a table of all the cookies transmitted in theresource's HTTP request and response headers. You can also clear all cookies.

Resource cookies

The Cookies table contain the following columns:

The cookie's name.
The value of the cookie.
The domain the cookie belongs to.
The URL path the cookie came from.
Expires / Max-Age
The value of the cookie's expires or max-age properties.
The size of the cookie in bytes.
This indicates that the cookie should only be set by the browser in the HTTP request, and cannot be accessed with JavaScript.
The presence of this attribute indicates that the cookie should only be transmitted over a secure connection.

The Frames tab shows messages sent or received over a WebSocket connection. This tab is only visible when the selected resource initiated a WebSocket connection. The table contains the following columns:

The message payload. If the message is plain text, it's displayed here. For binary opcodes, this field displays the opcode's name and code. The following opcodes are supported:
  • Continuation Frame
  • Binary Frame
  • Connection Close Frame
  • Ping Frame
  • Pong Frame
The length of the message payload in bytes.
The time stamp when the message was created.

Messages are color-coded according to their type. Outgoing text messages are color-coded light-green; incoming text messages are white:

Websocket text

WebSocket opcodes are light-yellow:

Websocket opcodes

Errors are light-red.

Notes about current implementation:

  • To refresh the Frames table after new messages arrive, click the resource name on the left.
  • Only the last 100 WebSocket messages are preserved by the Frames table.

The Timing tab graphs the time spent on the various network phases involved loading the resource. This is the same data displayed when you hover over a resource bar in the waterfall view.

Resource network timing graph

Time the request spent waiting before it could be sent. This time is inclusive of any time spent in proxy negotiation. Additionally, this time will include when the browser is waiting for an already established connection to become available for re-use, obeying Chrome's maximum six TCP connection per origin rule.
Proxy Negotiation
Time spent negotiating with a proxy server connection.
DNS Lookup
Time spent performing the DNS lookup. Every new domain on a page requires a full roundtrip to do the DNS lookup.
Initial Connection / Connecting
Time it took to establish a connection, including TCP handshakes/retries and negotiating a SSL.
Time spent completing a SSL handshake.
Request Sent / Sending
Time spent issuing the network request. Typically a fraction of a millisecond.
Waiting (TTFB)
Time spent waiting for the initial response, also known as the Time To First Byte. This time captures the latency of a round trip to the server in addition to the time spent waiting for the server to deliver the response.
Content Download / Downloading
Time spent receiving the response data.

To learn more optimizing the network performance of your application, see the following resources:





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