[Gc] Collecting large memory blocks

Boehm, Hans hans.boehm at hp.com
Mon Dec 7 16:37:31 PST 2009

Allocating large blocks is clearly an issue.  A few partial workarounds haven't been mentioned here yet:

1. Build 64-applications.  That greatly reduces the probablility of misidentified pointers.
2. You can use GC_register_displacement instead of GC_all_interior_pointers.  That causes the collector to recognize pointers into objects only if they refer to the right offset within the object. 
3. You can allocate large objects with GC_malloc_ignore_off_page.  This will result in only pointers to near the beginning of the object being treated as valid.  

Actually, I was looking at this, I think we have a bit of a misfeature in this area:

We sometimes do stack marking with GC_push_all and sometimes with GC_push_all_eager, at least in the GC_all_interior_pointers case.  I think the latter applies ignore_off_page to pointers on the stack, but the former doesn't, since I think it ends up using GC_base.  We should be consistent.  The documentation suggests that ignore_off_page is obeyed even for stack pages, though that's a bit risky because of compiler optimizations.  It may require volatile pointers to objects allocated with ignore_off_page.


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