[webkit-gtk] dns prefetching knob

[Evan Martin]

On Wed, Mar 9, 2011 at 6:18 AM, David Hill <dhill at mindcry.org> wrote:
>> Prefetch isn't mentioned in the HTTP spec at all. And I don't think
>> anyone was doing it back in the modem age. AFAIK, Google started doing
>> it with Chrome, and other people followed.
>
> If it is not mentioned in the spec, I think it should be optionized
> then, not hardcoded.  Google started doing it with Chrome, but was it a
> performance booster, or (imo) a way to track users even more?

Your conspiracy theory is impressive, but as far as I know Google
doesn't get to observe the DNS traffic of every person on the
internet.  Maybe you know something about DNS that I don't.  ;)

We did measure the benefit of DNS prefetching.  When we prefetch a
hostname, we record the time that we needed to wait for the resolution
to complete.  When we later do a real fetch for that host, we record
that the time we spent prefetching was a gain.  You can see these
numbers in Chrome by visiting
  about:histograms/DNS.ResolveSuccess

For users who have opted in to reporting anonymous statistics, we send
those numbers (no hostnames, just the millisecond counts) to Google.
Looking at that data right now, I see that 10% of users' requests were
sped up by 400ms or more, which is huge in terms of web speed.
(Consider that you can't even start fetching subresources until you've
successfully made the initial connection the host.)
There's a screenshot of some older data here:
http://blog.chromium.org/2008/09/dns-prefetching-or-pre-resolving.html

> I have tested this patch and the performance boost is unbelievable.

I guess on IRC Marco was saying that DNS on OpenBSD is single
threaded?  Maybe I misunderstood.  It would sound like a bug in
libsoup to me if so.  Certainly if you can't resolve DNS in parallel I
would expect it's better to do fewer resolutions.

For the record, I fully support you adding the option to disable
prefetching.  I just hope that you want to do it for a good reason.