GC via low-priority thread

[Sean Kelly]

Luis Marques wrote:
> Hello,
> 
> I've been wondering, I've read on this newsgroup that one could use a
> low-priority thread to do GC, perhaps setting watermarks of memory usage after
> which that thread's priority would raise.
> 
> What I question is the following. Here http://digitalmars.com/d/memory.html says
> that "All threads other than the collector thread must be halted while the
> collection is in progress". So, wouldn't you still experience the halting that
> makes GC problematic?

The mark/sweep method of garbage collection must "stop the world" for at 
least a portion of its processing to make sure that it hasn't missed 
anything (Boehm does have a "mostly parallel mark/sweep GC 
implementation that reduces the time that the world is stopped, 
however).  When people usually mention a low-priority thread performing 
GC they're thinking of incremental GC where a "stop the world" phase 
typically isn't required at all.  Instead, garbage collection is 
happening perpetually in the background at a relatively low priority.

> Or the ideia supposed to be that more frequent GC cycles
> would make each cycle faster and imperceptible? But in that case I can't really
> see how the thread priority would matter much.

For mark/sweep, it shouldn't.  Once the world is stopped, the GC thread 
should perform at normal priority by default since all other threads in 
the program are blocked.


Sean