[Gc] RE: about bitmap marking

lijun lijun at ialab.cs.tsukuba.ac.jp
Mon Feb 20 04:14:10 PST 2012


Thank you for your reply.

You emphasized that the main goal of keeping mark bits in a separate region(mark bit table) is to avoid touching pages containing pointer-free objects.
If I only use GC_malloc(not GC_malloc_atomic) to allocate memories, does the collector(boehm gc6.8) access pages containing pointer-free objects?

Is the following conclusion correct?
・In essence, Not GC_malloc_atomic but mark bit table(keeping mark bits in a separate region, even if I use the default configuration) avoids touching pages containing pointer-free objects during a GC, 
but if I use GC_malloc_atomic to allocate pointer-free or untraced memory, I can obtain better performance.

Best regards.
Li jun

-----Original Message-----
From: Hans Boehm [mailto:Hans.Boehm at hp.com] 
Sent: Sunday, February 19, 2012 4:26 AM
To: lijun
Cc: 'Boehm, Hans'; boehm at acm.org; gc at linux.hpl.hp.com
Subject: RE: about bitmap marking



On Sat, 18 Feb 2012, lijun wrote:

> 
> I sent 2 emails 2 days ago, but I didn’t get an answer.
> 
>  
> 
> Can you tell me whether the implememtation of bitmap marking in boehm
> gc(gc6.8) has the advantages listed in my previous email(February 16, 2012
> 1:52 AM) if I use the default configuration(boehm gc 6.8).
> 
>  
> 
> >>In general, there are three advantages of using mark bit table(bitmap
> marking).
> 
> >>+ it is friendly to copy-on-write.
Probably.  The writes during marking are certainly better isolated.
> 
> >>+ it helps to attain a higher locality of reference.
In some senses.  As I pointed out earlier, the main goal is to avoid
touching pages containing pointer-free objects, which is a special kind
of locality.
> 
> >>+ in sweep phase, scanning/clearing the mark bit table is faster than
> scanning/clearing the mark bits in object headers because of putting the
> mark bits together.
Right.  Kind of.  It makes it much faster to tell whether a page is 
entirely empty.  I'm not sure it helps when building a free list, since
we write free-list links in any case.  The trade-off there may be
complicated.

> 
> >>(the collector will run faster because of having a mark bit table)
> 
> >>
> 
> >>There are several differences between the usual bitmap marking and that
> of boehm gc
> 
> >>(For example, because each page has a separate mark bit table in boehm
> gc, in most cases, there are many mark bit tables in the collector.),
I'm not sure that matters much for the issues you brought up.  The mark 
bit tables tend to be in adjacent sections of memory.

> 
> >>So I want to know whether the implememtation of bitmap marking in boehm
> gc(gc6.8) has the advantages listed above if I use the default
> configuration(boehm gc 6.8).
Generally yes.  But the fact that you're not touching pointerfree objects
is probably still the most important one.

Hans
> 
>  
> 
> Best regards
> 
> Li Jun




More information about the Gc mailing list