A different kind of Walter? :-)

[Frits van Bommel]

Alexander Panek wrote:
> On Thu, 19 Apr 2007 01:07:22 -0700
> Walter Bright <newshound1 at digitalmars.com> wrote:
> 
>> David B. Held wrote:
>>> That being said, I think it would be *particularly* clever of you
>>> to use your start-from-scratch kernel to explore new ideas in
>>> multi-processing and massive multithreading, but this would imply
>>> having decent multi-CPU hardware to play with.
>> I'd also like to see better operating system support for garbage 
>> collection - either making gc an operating system service, or
>> providing hooks to the virtual memory subsystem so the gc can tell
>> when pages are dirty.
> 
> Well, something along the lines of that is what I was thinking of. I'm
> not so sure how to make the GC an operating system service, though..?

http://www-cs.canisius.edu/~hertzm/bc.html has a nice idea on support 
the OS could give to garbage collection: they modified Linux to notify 
the process when it was about to get some of its memory swapped out. 
Then the process could perform a quick GC (focusing on freeing up pages 
of memory) and tell the OS which pages to unmap or swap out. The GC 
freeing pages means the GC can tell the OS memory manager that certain 
pages can be thrown out without saving them to disk first (no disk 
writes is faster than disk writes :) ).
If it can't avoid swapping pages out, the GC checks the page about to be 
swapped out for pointers and marks the pointed-to objects as referenced 
by a swapped-out page, then tells the OS to put that page back at the 
head of the swap-out queue (since it has just been touched by the GC, it 
was removed from that queue). In this way, their GC also avoids touching 
swapped-out pages except in rare cases, so most GC cycles shouldn't 
cause a single page fault.
Pretty cool stuff.

About making it a system service: I'm not sure how much help that would 
be, though I guess it could cut down on privilege level switches (caused 
by warnings about swapping and/or system calls to nominate pages for 
swapping) if the kernel could perform something like the above GC itself.