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[perl #809] [PATCH] Adding the system stack to the GC

Mike Lambert
July 13, 2002 02:23
[perl #809] [PATCH] Adding the system stack to the GC
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Okay, patch is included below. Here are the relevant times from the gc
bench suite:
gc_alloc_reuse.pbc      12.058001       12.056999
gc_generations.pbc      3.964999        5.769
gc_header_new.pbc       3.924999        3.924999
gc_waves_headers.pbc    0.871997        0.871999
gc_waves_sizeable_data.pbc      7.059   7.140001
gc_waves_sizeable_headers.pbc   5.316999        5.358

I believe gc_generations is slower due to the many DOD calls that are
performed (and each DOD requires a stack walk). In general,
the stack-walk makes the new version a little bit slower, or perhaps the
same, in all cases.

Note that the patch implementation is a bit ugly because of the seperation
of pmc and header pools. I have another GC refactoring patch in the wings
which should remedy this situation.

Also note the problem I mentioned earlier, which may very well crop it's
head up with this patch under 'real-world' conditions. See below sig.

Mike Lambert

> Also, I think I've discovered a situation which might not be
> resolvable. How do we determine id a given buffer is alive? We walk the
> stack and check for potential pointers into buffer memory. If the stack
> contains garbage pointers, we might have bad references into buffer
> memory. I can check for alignment within the buffer pool, and so it should
> be safe to set the BUFFER_live_FLAG on them.
> However, when we perform a collection, that means that we could be taking
> a garbage pointer in the buffer, and attempting to copy the memory it
> pointed to, into new memory.
> This could give us GPFs if we access bad memory, I think. Even if we check
> to ensure the buffer points into valid collectable memory (making it
> slower), we still have the issue of buflen being set to MAX_INT or
> something, and killing the system. :|
> The same caveats apply to pmc headers which happen to have
> PMC_buffer_ptr_FLAG set.
> How should we get around this particular problem, or is it spelling the
> doom of this particular solution?
> Thanks,
> Mike Lambert

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