What To Do About Shared?

[Andrei Alexandrescu]

On 3/23/11 10:22 AM, Don wrote:
> dsimcha wrote:
>> == Quote from dsimcha (dsimcha at yahoo.com)'s article
>>> On 3/23/2011 9:09 AM, Jason House wrote:
>>>> dsimcha Wrote:
>>>>
>>>>> Some discussions about std.parallelism have prompted an examination of
>>>>> how far D's guarantees against low level data races should extend and
>>>>> how safety and practicality should be balanced.
>>>> I didn't follow the review of std.parallelism, can you give some
>>>> specific
>> examples?
>>>> Users of languages look to standard libraries as a model for how to
>>>> write
>> their own apps. I don't like your proposal and think std.parallelism
>> should use
>> shared properly. I'd like to understand better what your issues with
>> shared were.
>> I've done a descent amount of shared-correct code, so I'm pretty sure
>> it's usable.
>> In fact, the only really nasty bug I had could have been caught if
>> std.thread had
>> been shared-correct...
>>> I have already decided that, unless shared is drastically improved in
>>> ways I don't foresee (I'm not even sure exactly how, this would need to
>>> be discussed), I will not be making std.parallelism shared correct.
>>
>> One small clarification/relaxation of this position: I will seriously
>> consider
>> making individual artifacts of std.parallelism shared correct if this can
>> demonstrably be done without affecting efficiency, flexibility or ease
>> of use.
>> For example, certain uses of future/promise parallelism via task() are
>> shared-correct, marked as @safe/@trusted and documented as such.
>>
>> However, I will not make shared correctness a higher priority than
>> efficiency,
>> ease of use or flexibility and I will not cripple or remove artifacts
>> that cannot
>> reasonably be made shared correct.
>
> Seems to me, that you're making use of some primitive that I'll call a
> 'DivisableArray' -- an array that can be sliced up (into other
> DivisibleArrays), and different DivisableArrays can be sent to different
> threads. You can extract a normal array slice from a DivisibleArray, but
> cannot send that slice to other threads: only DivisibleArrays can do that.
> In debug mode, a DivisibleArray could keep track of how it has been
> sliced, and disallow overlapping slices. The DivisibleArray could even
> ensure that all slices lie on word/paragraph boundaries, thus dealing
> with word tearing.

That's a great idea!

I agree with the sentiment that std.parallelism is not necessarily 
supposed to work with the shared qualifier, which is intended for 
general inter-thread communication. std.parallelism is using implicit 
sharing: as Don described, most often data in std.parallelism is 
factually not shared (different threads work on different portions of 
the data).


Andrei