- From: Sigbjørn Vik <sigbjorn@opera.com>
- Date: Wed, 22 Sep 2010 10:47:15 +0200
- To: "public-web-perf@w3.org" <public-web-perf@w3.org>
On Wed, 22 Sep 2010 10:19:25 +0200, Zhiheng Wang <zhihengw@google.com> wrote: > On Thu, Sep 16, 2010 at 3:09 AM, Sigbjørn Vik <sigbjorn@opera.com> wrote: >> Does domainLookupStart need a clarification that if DNS lookup was >> started >> before fetchStart (in the case of DNS prefetching), domainLookupStart >> should >> still be set to fetchStart? > > Make sense. Out of curiosity, how does the user agent prefetch a DNS for > the root page before the fetch? Part of the fetching processing is to > formulate the address of the resource. > I can only imagine the user agent prefetch DNS for embedded > links/elements > on the current page. There are multiple ways DNS could be prefetched. Hovering a regular link to a new domain might trigger a DNS prefetch. The parser might request DNS as soon as it hits a URL, even before the code has been parsed to the point where it can be decided whether the resource needs to be fetched or not. Some ways can also happen in a blank, new tab: Highlighting or showing domains in the address bar history completion dropdown. DNS might randomly be requested immediately before a request due to autorenewal when the pinning expires, or by some unrelated action, e.g. a background RSS fetch, or another open tab requesting a new resource. Opening a bookmark folder, where two or more of the bookmarks have the same domain, or manually opening two tabs for the same domain very quickly. When starting with many open tabs, the user agent might prefetch DNS while parsing the saved tabs file, before starting to load the tabs. In all cases, there might be an ongoing DNS request for the domain in question at the time when the fetch process is actually started. In addition, if all sockets are in use, the user agent might prefetch DNS while waiting for an empty socket (though in this case, domainLookupStart does start after fetchStart, so that might not be relevant). -- Sigbjørn Vik Quality Assurance Opera Software
Received on Wednesday, 22 September 2010 08:47:42 UTC