On 10/14/2013 04:16 PM, Brian Anderson wrote:
On 10/12/2013 05:01 PM, Lars Bergstrom wrote:
This was a great example and great find! After talking with Brian,
I've opened Rust issue 9831
(https://github.com/mozilla/rust/issues/9831 ) to track the scheduler
changes to make this code work.
What we can do in Servo right now is to start from a normal task
instead and spawn the constellation from there, spawning the special
compositor thread separately (instead of as the root task). We
believe that should work around the problem.
The reason we get any parallelism with code running inside an iframe
is that the task creation stuff I added to handle supervision and
unlink failure also caused those to get run on a separate scheduler
instead of blocking inside of the root scheduler associated with the
main task.
OK, I looked at this closer and there is another problem. Besides the
one I mentioned previously about tasks spawned from the main thread
getting serialized to another thread, there's another problem with the
main thread trying to steal work. The main scheduler, which is a
single-threaded (non-work-stealing) scheduler is advertising itself as
eligible to steal work; this happens through the 'sleeper list', which
is a stack of schedulers that are currently sleeping, with no work to
do. What is taking place is: the main thread pushes itself to the
sleeper list, another scheduler wakes it up to steal, the main thread
steals a task, finds it can't run it, sends it to another scheduler,
pushes itself to the sleeper list, repeat. Essentially, most of the
task stealing that's taking place is getting serialized through the
main thread that can't actually run the task. This PR should fix the
problem: https://github.com/mozilla/rust/pull/9857
Oh, this still probably needs the workaround of spawning all the
subtasks from a task that is not the main task - that is still an issue,
though it really only should affect startup.
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