DIP60: @nogc attribute

[via Digitalmars-d]

On Thursday, 17 April 2014 at 12:20:06 UTC, Manu via 
Digitalmars-d wrote:
> See, I just don't find managed memory incompatible with 'low 
> level' realtime or embedded code, even on tiny microcontrollers 
> in principle.

RC isn't incompatible with realtime, since the overhead is O(1).

But it is slower than the alternatives where you want maximum 
performance. E.g. raytracing.

And it is slower and less more "safe" than GC for long running 
servers that have uneven loads. E.g. web services.

I think it would be useful to discuss real scenarios when 
discussing performance:

1. Web server request that can be handled instantly (no database 
lookup): small memory requirements and everything is released 
immediately.

Best strategy might be to use a release pool (allocate 
incrementally and free all upon return in one go).

2. Web server, cached content-objects: lots of cycles, shared 
across threads.

Best strategy is global GC.

3. Non-maskable interrupt: can cut into any running code at any 
time. No deallocation must happen and can only touch code that is 
consistent after atomic single instruction CPU operations.

Best strategy is preallocation and single instruction atomic 
communication.

> ARC would be fine in low level code, assuming the language 
> supported it to
> the fullest of it's abilities.

Yes, but that requires whole program optimization, since function 
calls cross compilation unit boundaries frequently.

> No. It misses basically everything that compels the change. 
> Strings, '~',
> closures. D largely depends on it's memory management.

And that is the problem. Strings can usually be owned objects.

What benefits most from GC are the big complex objects that have 
lots of links to other objects, so you get many circular 
references.

You usually have fewer of those.

If you somehow can limit GC to precise collection of those big 
objects, and forbid foreign references to those, then the 
collection cycle could complete quickly and you could use GC for 
soft real time. Which most code application code is.

I don't know how to do it, but global-GC-everything only works 
for batch programming or servers with downtime.

> Take this seriously. I want to see ARC absolutely killed dead 
> rather than dismissed.

Why is that? I can see ARC in D3 with whole program optimization. 
I cannot see how D2 could be extended with ARC given all the 
other challenges.

Ola.