Although the core of prerendering is a fairly simple change to Chrome, there are a number of issues which make the implementation more complex:
- Minimizing resource contention.
- Handling dynamic media [video, audio, plugins, canvas]
- Cancellation of pages on certain corner cases.
- Minimizing server side effects.
- Mutations to shared local storage [cookies, sessionStorage, etc.]
Note that this document is not intended to be a comprehensive list of issues.
Creating a Prerendered Page
Prerender is initiated when a page contains a <link rel=”prerender”> resource. The ResourceDispatcherHost receives a resource request with ResourceType::Prerender - but this request will never be sent out to the network. Instead, it is used as a signal to create a PrerenderContents, and the request itself is cancelled.
The PrerenderContents is stored in the PrerenderManager, which maintains a directory of all PrerenderContents created by the same profile. A small number of recently created PrerenderContents are allowed. The current implementation only keeps one page around for a maximum of 30 seconds, but this may change in the future. Older pages are pruned, and a Least-Recently-Created eviction algorithm is used if capacity has been reached.
There are two cases where a prerendered page may be used instead of a new page load:
- In a call to TabContents::NavigateToPending entry, which is triggered e.g. when a user types in a new URL to visit.
- In a call to TabContents::DidNavigate, which is, among other cases, exercised when a user clicked on a link on the current page.
In both cases, the current profile’s PrerenderManager is checked to see if a valid PrerenderContents matches the destination URL. If a match is found, then:
- The PrerenderContents is removed from the PrerenderManager, so it can only be used once.
- The PrerenderContents’ TabContentsWrapper is swapped in for the existing TabContentsWrapper using the TabContents delegate ReplaceAt mechanism.
- The old TabContentsWrapper is kept alive until unload handlers complete running, and then is destroyed.
A page visibility API has been added to expose the current visibility state of a web page, such as whether it is prerendered, hidden, or potentially visibile.
This can be used for a variety of purposes, such as lowering the volume of a game while it’s not visible, or pausing an intro sequence until the page transitions out of the prerender state.
Minimizing Resource Contention
Prerendering runs the risk of negatively impacting page load time of other pages due to resource contention. Although some of these issues are also tickled with the existing prefetch support, prerendering makes the potential for problems more severe: more resources will be fetched because the subresources are also retrieved, not just the top level page; and CPU and memory consumption will likely be higher.
To minimize network contention, all resources in a prerendered page are retrieved at the lowest priority level. Currently the priority is only used to order pending requests for a particular domain: the scheduling may need to change for this priority to only allow idle requests if there are no active network requests at all. There is currently no way to cancel active connections when a higher priority request comes along--this may be needed so long-lived speculative requests for prerender do not block requests for a page that the user is actively visiting on the same domain. Finally, there is no way to change resource priorities after a request has started--this may be needed to bump up the priority of requests after a page has transitioned from prerender to visible.
Memory utilization is being handled by restricting the number of prerendered pages to only 3 (At most 2 per page), and is restricted if there is not enough available RAM on the system at the time of the prerender. Additionally, if the memory for the page exceeds 100 MB then the prerendering is cancelled and the memory returned to the system.
CPU utilization is being handled by lowering the priority of the render process which contains the prerendered page. A prerendered page is only created if it can be assigned to a unique process, so this minimizes the chance that we will decrease the priority of a render process containing an active tab.
Minimizing GPU utilization is currently not handled. One problem is that GPU usage is measured per render process rather than per RenderView.
Minimizing disk cache utilization is currently not handled. The disk cache is not currently priority based. Also, the cache hit rate may decrease over time for users with prerender enabled due to more unused resources being inserted. The eviction algorithm for the disk cache may also need to change to more aggressively evict resources which were speculatively retrieved but never used.
If a prerendered page tries to prerender another page, the requested prerender is deferred until and unless the first prerender is navigate to.
Prerenders are destroyed if not used within 5 minutes. The source page can cancel the prerender earlier if desired.
Cancellation on Corner Cases
While a page is in the prerendered state, plugin instantiation (and loading) will be deferred until the page has been activated. The main rationale is to prevent audio or video media from playing prior to the user actually viewing the page, as well as to minimize the possible exploit surface.
A BlockedPlugin instance is created for each plugin element on the original page. This is the same instance used by the Click-to-Play feature. It places a simple shim plugin in the place of the originally intended plugin, and retains the parameters that are needed to correctly create and initialize the originally intended plugin. When the page transitions out of the prerendered state, all BlockedPlugin instances created for prerendering purposes will swap in the originally intended plugins.
Playback is deferred the page completes, similar to plugins.
Pages are canceled if any of the conditions happen:
- The top-level page is not an HTTP/HTTPS scheme, either on the initial link or during any server-side or client-side redirects. For example, both ftp are canceled. Content scripts are allowed to run on prerendered pages.
- window.opener would be non-null when the page is navigated to.
- A download is triggered. The download is cancelled before it starts.
- A request is issued which is not a GET, HEAD, or OPTIONS. The most common case is POST XmlHttpRequest's.
- A authentication prompt would appear.
- An SSL Client Certificate is requested and requires the user to select a certificate.
- A script tries to open a new window.
- alert() is called.
- window.print() is called.
- Any of the resources on the page are flagged by Safe Browsing as malware or phishing.
- The fragment on the page does not match the navigated-to location.
When a problem is detected, the cancellation is done synchronously and the offending behavior is typically stopped. For example, an XmlHttpRequest POST will cancel the request before it goes over the network, and prevent the page from being swapped in. The prerendered page may live for a little while longer due to the asynchronous nature of RenderView cancellation, but it will not be swapped in.
Additional behavior may cause cancellation in the future, and some of the existing cancellation causes may be relaxed in the future.
Mutations to shared local storage
Shared local storage such as DOM storage, IndexedDB, and HTTP cookies present challenges for prerendering. Ideally mutations made by the prerendered page should not be visible to other tabs until after the user has activated the page. Mutations made by other pages should be reflected in the prerendered page, which may have already read from local storage before the other pages made the changes.
One option is to not worry about these issues. Since the prerendered pages are only retained for a short period of time after their creation, the window for race-like conditions is fairly short. However, this may result in confusion for users (for example, if a prerender starts for a page, the user logs out on the main page, and then the prerendered page becomes active with the user’s old credentials). Additionally, the Visibility API provides ways for pages to defer any mutating behavior until after the page becomes visibile. This is the current approach taken in Chrome.
A second option is to cancel the prerender any time local storage is accessed. This may be feasible for more recent versions of local storage [such as IndexedDB] but is not an option for more commonplace schemes, particularly HTTP cookies. Long-term, the cancel prerendering approach will also lead to resistance of adoption of new forms of local storage.
Minimizing server side effects
Prerendering is only triggered when the top level page is retrieved with a GET, and is assumed to be idempotent. Additionally, any non-GET, HEAD, or OPTION requests from XmlHttpRequests will not be sent over the network and the page will be cancelled.
If the server sends a redirect response for a subresource with a "Follow-Only-When-Prerender-Shown: 1" header, Chrome will hold off on following the redirect and on fetching the respective subresource until the prerender is shown to the user.