The memory profiler relies heavily on passing channels over channels.
The memory profiler makes measurement requests of threads and passes
them a channel through which they can return the measurements.
One example from paint_task.rs:
impl Reporter for PaintChan {
// Just injects an appropriate event into the paint task's queue.
fn collect_reports(&self, reports_chan: ReportsChan) -> bool {
let PaintChan(ref c) = *self;
c.send(Msg::CollectReports(reports_chan)).is_ok() // pass a
channel over a channel!
}
}
In multi-process Servo it would make sense to have one memory profiler
per process, so I'm not sure if this is a showstopper or not.
Nick
On Thu, Jun 18, 2015 at 4:40 PM, Patrick Walton <pwal...@mozilla.com> wrote:
> Hi everyone,
>
> I'm rebasing my multiprocess patch and found that many of the new features
> that have landed were implemented in a way that was incompatible with
> multiprocess Servo. In particular:
>
> * WebDriver passes channels over channels between script and the chrome
> process.
>
> * Canvas relies on passing channels over channels between code that can
> touch the GPU and script.
>
> * The new messages for pipeline/iframe control rely on passing channels
> over channels between the chrome process and the script task.
>
> To be sure, it's my fault that I wasn't keeping the multiprocess branch
> current. I didn't anticipate so much breakage, though, and in order to fix
> these issues I will have to make severe modifications to all of these
> features. (I'm two days into rebasing and large crates like compositing and
> script aren't even building yet, and I've had to disable lots of features
> such as canvas.)
>
> In an effort to keep this sort of thing from happening again, I'd like to
> suggest that all new code that spawns threads and passes channels or boxed
> objects over channels not be allowed to pass review until audited for
> multiprocess safety up until the multiprocess branch lands. Obviously, all
> other browser engines are cautionary tales for the amount of technical debt
> that can accrue if this is allowed to continue to happen unchecked. Even
> though we've been good about keeping things in separate threads, the
> passing-channels-over-channels pattern does not work among separate
> processes, and the more we rely on it the harder it will be to harden Servo.
>
> Thoughts?
> Patrick
> _______________________________________________
> dev-servo mailing list
> dev-servo@lists.mozilla.org
> https://lists.mozilla.org/listinfo/dev-servo
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