[webkit-dev] Parallel CSS styling

[Kulanthaivel Palanichamy]

Hi All,

At Qualcomm Innovation Center we have been working on a parallel algorithm
for CSS styling and wanted to see if there is any interest in the
community to see it implemented in WebKit. The overall idea is that we
replace CSS matching and styling with a parallel implementation assuming
there is a barrier before and after the computation. CSS style application
will be performed by the main thread, such that we avoid the need to make
thread safe data structures accessed in other passes. The algorithm is
task-based, so we would need to implement a thread pool and a simple task
scheduler (or maybe use an existing one).

In particular, our algorithm requires modifying Element::recalcStyle() and
some of the methods it invokes. Code that calls Element::recalcStyle()
will not have to be changed. By the time Element::recalStyle() returns,
all threads involved on the parallel CSS styling have completed their
execution.  Effectively, there is a barrier when Element::recalcStyle()
begins and another before it returns.

Our experiments show that our CSS computation for complex websites scales
rather well. For example, we observed that, for  flickr.com, Webkit spends
75% of its time in CSS doing CSS matching. Thus, our algorithm would give
a maximum speedup of 1.6X on 2 cores and 2.3X on 4 cores.

Please let us know whether this would be of interest to the community.


-Kulanthaivel