MutVar# and GC

[Shao, Cheng]

Hi,

I believe it's mentioned here:

https://ghc.haskell.org/trac/ghc/wiki/Commentary/Rts/Storage/GC/RememberedSets

Regards,
Shao Cheng

On Fri, Jan 18, 2019, 12:34 PM chessai . <chessai1996 at gmail.com> wrote:

> Ryan,
>
> That makes perfect sense, thanks. Is that documented explicitly anywhere?
> If not, I'd like to add the documentation to any place relevant.
>
> Thanks
>
> On Thu, Jan 17, 2019, 8:19 PM Ryan Yates <fryguybob at gmail.com wrote:
>
>> Hi,
>>
>> Because GHC's GC is generational it needs a way to handle heap objects
>> from older generations that point into younger generations.  This only
>> happens when an older object is mutated to point to a younger object.  GHC
>> solves this by invoking the GC write barrier (not to be confused with write
>> barriers for memory synchronization) `dirty_MUT_VAR`.  This will add that
>> mutable object to a mutable list that will be traversed in minor GCs along
>> with young generation roots.  Additionally the write barrier will mark the
>> heap object as "dirty" to avoid adding it to the list more than once.
>>
>> Ryan
>>
>> On Thu, Jan 17, 2019 at 4:29 PM chessai . <chessai1996 at gmail.com> wrote:
>>
>>> Devs,
>>>
>>> I've heard from a few friends that MutVars, TVars, etc. are more
>>> challenging for the garbage collector. I'm writing to ask if someone can
>>> answer: 1. Is this true, and 2: Why? I can't seem to find anything like a
>>> writeup or documentation that mentions this. The HeapObjects trac page also
>>> mentions nothing about these supposed difficulties that GC faces with
>>> mutable heap objects.
>>>
>>> Thanks
>>> _______________________________________________
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>>> ghc-devs at haskell.org
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>>>
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