On 2014-08-29 8:27 AM, Lars Bergstrom wrote:
On Aug 28, 2014, at 9:26 PM, Cameron Zwarich <zwar...@mozilla.com> wrote:
As a side point, why is there a 7 us overhead here for message-passing between
green threads? Is it really that bad? Are script and layout currently green
tasks, or did something land causing this to not be the case?
Yes, the current perf issue is a bug due to a workaround landed to move script
onto a native task to avoid some bugs related to Servo's very-old SpiderMonkey
but very-new Web Workers support:
https://github.com/servo/servo/pull/2842
That only affected workers, not the main script task.
That architecture will probably persist until we get a SpiderMonkey upgrade
that will allow us to switch script back to also being on a green thread, per
the discussion in this thread:
https://github.com/servo/servo/pull/2915#issuecomment-50355651
We should not assume that anybody on the SpiderMonkey is doing that
work. If we're going to bet on this, we're going to need to push on
them/do it ourselves.
At that point, I'd expect script to layout calls via message passing to be much
cheaper than a mutex acquisition, but would be very interested to see numbers
comparing them!
In general, I'd be a little wary of adding global locks into Servo unless it's really
necessary (i.e., if it turns out we can't architect away the race condition Cameron
brought up in any other way). Since we haven't found a good way to do any concurrent
protocol verification yet for Servo, it's really easy to end up writing deadlocky or racy
code (mainly w.r.t. underlying native resource allocation/destruction). For example, I
spent way too much time getting shutdown "cleaned up" so that we don't
intermittently crash due to attempting to render to graphics contexts that had been
destroyed too early, and that was just tracking through our spaghetti message passing
code, with no locks to reason about.
- Lars
Cheers,
Josh
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