beforeGarbageCollection

[Michel Fortin]

On 2010-06-17 01:52:57 -0400, Andrei Alexandrescu 
<SeeWebsiteForEmail at erdani.org> said:

> I'm thinking of a feature that would improve the memory footprint of D 
> programs. Say druntime provided a function:
> 
> void beforeGarbageCollection(void delegate() callMe);
> 
> The runtime would guarantee that callMe gets called before any 
> collection. That would allow modules to clean up caches (e.g. free 
> lists) to improve their memory profile.
> 
> Any thoughts, please share.

Hum, cleaning up caches will reduce the memory used, but not using a 
cache will too. If you use a cache, it's probably because you need one 
to improve performance, so it may have an adverse effect on performance 
if you clear caches at every collection cycle. Are collections only 
done when you're low on memory? I doubt it. It'd be more useful I think 
to have a callback for when you're actually low on physical RAM 
(because swapping caches to disk through virtual memory is quite 
unproductive).

On a side note, that looks like how things work on the iPhone: your 
application receives a memory warning when memory is near-exhausted, 
and it should release the unessential memory it holds. For instance, 
what Mobile Safari does when it receives a memory warning is to flush 
the cached representation of the non-frontmost tabs, except for the URL 
and a small previous picture; when you switch back to another tab, it 
has to reload the content.

I'm also wondering if the idea of using a *callback* for this is really 
a good idea. If you're in the process of updating a cache, you call 
'new', it trigger the garbage collector which reentrantly asks your 
cache to clear itself, won't this easily become trouble? Apple use an 
event for this on the iPhone, which is handled by the event loop of the 
application; perhaps a low memory *message* could be sent to all the 
threads that are listening to the message passing system, this would 
avoid the reentrancy issue and would work better with multithreading.

-- 
Michel Fortin
michel.fortin at michelf.com
http://michelf.com/