MutVar# and GC

chessai . chessai1996 at
Fri Jan 18 04:33:29 UTC 2019


That makes perfect sense, thanks. Is that documented explicitly anywhere?
If not, I'd like to add the documentation to any place relevant.


On Thu, Jan 17, 2019, 8:19 PM Ryan Yates <fryguybob at 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> 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
>> _______________________________________________
>> ghc-devs mailing list
>> ghc-devs at
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <>

More information about the ghc-devs mailing list