[gclist] write barrier via VM traps

[Charles Fiterman]

>I agree that "pseudo incremental mode" or abortable collections are cheaper
>than real incremental collections.   And they're often perfectly sufficient.
> But they're not a complete replacement for a write-barrier.  The main problem
>is that not all applications have idle time at predictable and sufficiently
>short intervals, or at all.  This is particularly a problem with multi-threaded
>applications that run a compute-bound thread in the background, while expecting
>interactive performance from a "foreground" thread.

In the problem you describe the answer is to define idle so the background
doesn't inhibit the collector. Since the background is compute-bound this
means it should be lower priority than the foreground and the collector
that supports it. True incremental mode may not help anyway.

Garbage Collection with a large number of threads is a whole new problem we
are just starting to think about. I don't think it can be done with minor
twitches on normal collector algorithms. I'm thinking about a page as object
variation on the Baker Treadmill. This involves a write barrier.

>A less serious issue is that even if there is idle time, you may run into more
>problems than necessary with programs that page, since the computation during
>idle time will most likely replace the mutator's working set, causing it to run
>more slowly when it resumes.

I'm considering a two pass collector first do the pages in ram then the pages
that were missed. This might do a pessimal job of making things worse for
the mutator. I think it needs experiment. The mutator might not need those
pages.
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