[Gc] RE: about bitmap marking

Boehm, Hans hans.boehm at hp.com
Wed Feb 8 07:56:30 PST 2012

[Copying response to gc mailing list.]

6.8 is old at this point, but this hasn't changed much in 7.2alpha6.

The collector always stores mark bits in a map off to the side.  When I last looked at this ages ago, that seemed to be a clear win.  We need to look up a page descriptor for the object being marked in any case.  Thus the overhead is smaller than it would probably be for a non-conservative collector.  Having the mark bit in the object, or even on the same page, would mean accessing pages containing pointer-free objects during a GC.  In my experience, in well-tuned applications, a large fraction of the heap is pointer-free.  The GC could often touch twice as many pages during a GC with the mark bits in the objects.

An ancient version of the GC stored mark bits at the beginning of each page.  That potentially  causes other cache-related problems.  All mark bits contend for a small fraction of the cache.

The GC allocates memory from a free-list, but free-lists are built only a page at a time, when needed.  Building the free-list will force memory into the cache, but the hope is that it will still be there when the object is allocated by the client.


From: lijun [mailto:lijun at ialab.cs.tsukuba.ac.jp]
Sent: Tuesday, February 07, 2012 7:26 PM
To: Boehm, Hans; boehm at acm.org
Subject: about bitmap marking

Dear Boehm:

I'm sorry about my poor English.

I would like to hear about boehm gc.

Boehm gc(gc6.8) uses Bitmap marking as default?

In Boehm gc, bitmap marking rules out storing mark-bits in object headers?(move mark-bits to other area)

Does Boehm gc get memory area from a freelist when allocate memory?

I'm waiting for your reply eagerly.

Best regards.

Li Jun
Univ. of Tsukuba
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