Re: Garbage collectors and VM

["Zack Weinberg" <zack@codesourcery.com>]

Jamie Lokier <jamie@shareable.org> writes:

> Zack Weinberg wrote:
>> Thus, a new pseudo-device, with the semantics:
>
> I like it!  I'd use it, too.

Thanks.  Implementing it is way beyond me, but it's good to hear that
other people might find it useful.

>>  * Reading from the descriptor produces a list of user-space pointers
>>    to all the pages that have been reset to read-write since the last
>>    read.
>
> Would it be appropriate to have a limit on the number of pages which
> become writable before a signal is delivered instead of continuing to
> make more pages writable?  Just like the kernel, sometimes its good to
> limit the number of dirty pages in flight in userspace, too.

Maybe.  The GC I wanted to use this with is rather constrained in some
ways - it can't walk the stack, for instance - so it can only collect
when the mutator tells it it's okay.  I suppose it could just copy the
list to its own buffer and continue, which saves the kernel from
having to store a potentially very large list.

>>  * I never decided what to do if the program forks.  The application I
>>    personally care about doesn't do that, but for a general GC like
>>    Boehm it matters.
>
> It should clone the state, obviously, and COW should be invisible to
> the application :)

Well, yeah, that would be the cleanest thing, but it does require two
dirty bits.

> Btw, are these pages swappable?

They certainly ought to be.

> On a different but related topic, it would be most cool if there were
> a way for the kernel to request memory to be released from a userspace
> GC, prior to swapping the GC's memory.  Currently the best strategy is
> for each GC to guess how much of the machine's RAM it can use, however
> this is not a good strategy if you wish to launch multiple programs
> each of which has its own GC, nor is it a particularly good balance
> between GC application pages and other page-cache pages.

Again, this is not particularly useful to me since the collector can't
collect at arbitrary points - but it would be a good thing to have for
a general GC, and maybe I ought to be using a general GC anyway...

zw