Whither structured concurrency?

[John Rose]

On Nov 13, 2019, at 12:58 AM, Andrew Haley <aph at redhat.com> wrote:
> 
> On 11/12/19 7:18 PM, Alan Bateman wrote:
>> On 12/11/2019 18:50, Andrew Haley wrote:
>>> :
>>> OK, so I don't see how I can move forward with scoped locals because
>>> I have nothing to test on.
>>> 
>> I think having scoped locals that are values accessible and shared by 
>> many threads is a topic in itself. I don't think that should stop us 
>> making progress on getting scoped locals working initially with 
>> virtual/lightweight threads, right?
> 
> Sure, that's true. I can still progress with making inheritance work
> correctly with virtual/lightweight threads.

I think you are designing them as values rather than variables,
which means they are easier to share/inherit safely, without
races.  If they were variables, then they would have to be
copied into each fiber if there were any possibility of setting.
By contrast, a whole environment full of shared values can
be inherited with as little overhead as a single pointer.

Note that legacy TLs are designed as variables, with a
TL::set operation.  Their overheads are only excusable
because they are shadowed by the dino-sized appetite
for resources of classic threads.  I’m glad we’re not
repeating this pattern with fibers; it would be a mistake.

— John