MutVar# and GC

[Ryan Yates]

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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