[webkit-dev] PreloadScanner aggressiveness

Joe Mason jmason at rim.com
Thu Jan 7 12:52:08 PST 2010

I don't think every port should be required to implement prioritization
and throttling itself - that's just duplication of effort.  Maybe
there's a good middle-ground, where PreloadScanner is run more often but
still does the priority sorting?




From: webkit-dev-bounces at lists.webkit.org
[mailto:webkit-dev-bounces at lists.webkit.org] On Behalf Of Mike Belshe
Sent: Thursday, January 07, 2010 3:09 PM
To: webkit-dev at lists.webkit.org
Subject: [webkit-dev] PreloadScanner aggressiveness


Hi -

I've been working on SPDY, but I think I may have found a good
performance win for HTTP.  Specifically, if the PreloadScanner, which is
responsible for scanning ahead within an HTML document to find
subresources, is throttled today.  The throttling is intentional and
probably sometimes necessary.  Nonetheless, un-throttling it may lead to
a 5-10% performance boost in some configurations.  I believe Antti is no
longer working on this?  Is there anyone else working in this area that
might have data on how aggressive the PreloadScanner should be?  Below
I'll describe some of my tests.


The PreloadScanner throttling happens in a couple of ways.  First, the
PreloadScanner only runs when we're blocked on JavaScript (see
HTMLTokenizer.cpp).  But further, as it discovers resources to be
fetched, it may delay or reject loading the subresource at all due to
throttling in loader.cpp and DocLoader.cpp.  The throttling is very
important, depending on the implementation of the HTTP networking stack,
because throwing too many resources (or the low-priority ones) into the
network stack could adversely affect HTTP load performance.  This latter
problem does not impact my Chromium tests, because the Chromium network
stack does its own prioritization and throttling (not too dissimilar
from the work done by loader.cpp).



The theory I'm working under is that when the RTT of the network is
sufficiently high, the *best* thing the browser can do is to discover
resources as quickly as possible and pass them to the network layer so
that we can get started with fetching.  This is not speculative - these
are resources which will be required to render the full page.   The SPDY
protocol is designed around this concept - allowing the browser to
schedule all resources it needs to the network (rather than being
throttled by connection limits).  However, even with SPDY enabled,
WebKit itself prevents resource requests from fully flowing to the
network layer in 3 ways:

   a) loader.cpp orders requests and defers requests based on the state
of the page load and a number of criteria.

   b) HTMLTokenizer.cpp only looks for resources further in the body
when we're blocked on JS

   c) "preload" requests are treated specially (docloader.cpp); if they
are discovered too early by the tokenizer, then they are either queued
or discarded.


Test Case

Can aggressive preloadscanning (e.g. always preload scan before parsing
an HTML Document) improve page load time?


To test this, I'm calling the PreloadScanner basically as the first part
of HTMLTokenizer::write().  I've then removed all throttling from
loader.cpp and DocLoader.cpp.  I've also instrumented the PreloadScanner
to measure its effectiveness.


Benchmark Setup

Windows client (chromium).

Simulated network with 4Mbps download, 1Mbps upload, 100ms RTT, 0%
packet loss.

I run through a set of 25 URLs, loading each 30 times; not recycling any
connections and clearing the cache between each page.

These are running over HTTP; there is no SPDY involved here.



(without my changes)



Average PLT



+5.8% latency redux.

Time spent in the PreloadScanner



As expected, we spend about 4x more time in the PreloadScanner. In this
test, we loaded 750 pages, so it is about 6ms per page. My machine is
fast, though.

Preload Scripts discovered



4x more scripts discovered

Preload CSS discovered



3x more CSS discovered

Preload Images discovered



3x more images discovered

Preload items throttled



Preload Complete hits



This is the count of items which were completely preloaded before WebKit
even tried to look them up in the cache. This is pure goodness.

Preload Partial hits



These are partial hits, where the item had already started loading, but
not finished, before WebKit tried to look them up.

Preload Unreferenced



These are bad and the count should be zero. I'll try to find them and
see if there isn't a fix - the PreloadScanner is just sometimes finding
resources that are never used. It is likely due to clever JS which
changes the DOM.





For this network speed/client processor, more aggressive PreloadScanning
clearly is a win.   More testing is needed for slower machines and other
network types.  I've tested many network types; the aggressive preload
scanning seems to always be either a win or a wash; for very slow
network connections, where we're already at capacity, the extra CPU
burning is basically free.  For super fast networks, with very low RTT,
it also appears to be a wash.  The networks in the middle (including
mobile simulations) see nice gains.


Next Steps and Questions:

I'd like to land my changes so that we can continue to gather data.  I
can enable these via macro definitions or I can enable these via dynamic
settings.  I can then try to do more A/B testing.


Are there any existing web pages which the WebKit team would like tested
under these configurations?  I don't see a lot of testing that I can
leverage from the initial great work Antti did for verifying that I'm
not breaking anything.


Is there any other information or data from the original PreloadScanner
work which I should read?







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