Componentizing D's garbage collector

[Benjamin Thaut]

Am 11.01.2014 21:44, schrieb Andrei Alexandrescu:
> On 1/11/14 3:52 AM, Benjamin Thaut wrote:
>> Am 10.01.2014 20:40, schrieb Andrei Alexandrescu:
>>> On 1/10/14 11:34 AM, Benjamin Thaut wrote:
>>>> Am 10.01.2014 09:03, schrieb Andrei Alexandrescu:
>>>>>
>>>>> Destroy.
>>>>>
>>>>> Andrei
>>>>
>>>> Just one question. Did you read "the garbage collection handbook"?
>>>
>>> Yah, the new edition. Unfortunately I didn't find a lot of new stuff or
>>> things that would help in a practical implementation.
>>>
>>> Andrei
>>
>> Very good,
>>
>> In my opinion it should be required for everyone who wants to
>> participate in D's GC to read that book. So everyone has at least a
>> basic understanding of the problem at Hand.
>>
>> But if you read the book I don't understand, why you simply declare the
>> hardest problem, percise pointer discovery, as done. To be able to
>> actually implement and test a GC that is not a reimplementation of what
>> we already have one needs percise pointer discovery of _all_ pointers,
>> write barriers and GC halting points. So please enlighten me why you
>> simply decalre this done?
>
> I'm not considering done as much as separable as a concern. All I'm
> saying is I hope to be able to separate the low-level part of
> discovering roots from the high-level part of marking used memory. BTW
> the way I see this done is "mostly precise", i.e. there will be at least
> for a while some words that will be handled conservatively (stack,
> registers, certain union members).
>
> If there's anything in the GC book that suggest that would be
> impossible, please do let me know!
>
>
> Andrei
>

Well as far as my understanding of GCs goes, you have two options:

1) Impercise pointer discovery => limiting yourself to a mark & sweep
2) Percise pointer disccovery => option to use a semi space GC in 
combination with a mark & sweep and generations, which allows for 
superior handling of short lived allocations (which is the biggest 
problem of D's current GC).

Also without percise pointer discovery you will never be able to move 
objects from one heap into another. This would be especially a problem 
for the immutable heap, because you might want to allocate all strings 
on the thread local heap until you discover that you actually need them 
to be shared between threads. You might also need to move objects into 
antoher heap whenever a casts happens or global variable is assigned.

Kind Regards
Benjamin Thaut