[dmd-concurrency] Vot de hekk is shared good for, anyway?

[Graham St Jack]

This all sounds excellent to me.

A question:
How will the message-passing API avoid exposing "shared"? Surely the 
message channel itself is a shared mutable object, even though the 
messages passing through it are immutable. If it isn't shared, aren't we 
cheating by hiding the fact that multiple threads are accessing it?


Walter Bright wrote:
>
> There's another aspect here. Consider all the problems we have getting 
> across the idea of an immutable type. What hope is there for shared? I 
> see mass confusion everywhere. Frankly, I see little hope of any but a 
> handful of programmers ever being able to grok shared and use it 
> correctly for concurrent programs. The notion that one can just slap 
> 'shared' on a data type and have it work correctly across threads 
> without further thought is a pipe dream.
>
> So what to do?
>
> I want to pin the mainstream concurrency on message passing. The 
> message passing user never sees shared, never has to deal with locks, 
> never has to deal with memory barriers. It just works. Message passing 
> should be a robust, scalable solution for most users. I believe the 
> Erlang experience validates this. Go and Scala also rely entirely on 
> message passing (but they don't have immutable data, so their models 
> are unsafe and I predict many rude surprises).
>
> So why bother with shared at all?
>
> Because message passing does not cover all the bases, and D is 
> supposed to be a systems programming language. So we need a paradigm 
> for synchronization and shared data structures. What shared provides is:
>
> 1. A way to identify shared data. This is incredibly important. A lot 
> of sharing bugs come about because of inadvertant unrecognized sharing 
> of data. This should be pretty much impossible in D. Furthermore, if 
> you do have a sharing bug in your code, you look at the 1% of the data 
> tagged as shared, rather than every freakin' line of code and every 
> piece of data. Half the battle in debugging code is figuring out where 
> to look for the problem. Shared pares that problem down to a 
> reasonable size.
>
> 2. Shared comes with a collection of static typing rules and 
> guarantees that will head off a number of concurrency bugs, such as 
> sequential consistency.
>
> I view shared as sort of like the latest electric arc welders which 
> automatically adjust the current and wire feed for you. They 
> dramatically shorten (but don't eliminate) the learning curve for 
> people trying to master the art of welding. D is the only language to 
> even attempt this. C++ leaves you completely on your own, Java offers 
> no help, Erlang, Scala and Go throw in the towel and won't allow 
> anything but message passing.
>
> As for a shared gc vs thread local gc, I just see an awful lot of 
> strange irreproducible bugs when someone passes data from one to the 
> other. I doubt it's worth it, unless it can be done with compiler 
> guarantees, which seem doubtful.
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