[Gc] RE: about bitmap marking

Boehm, Hans hans.boehm at hp.com
Thu Feb 9 23:09:10 PST 2012


gc6.8 and all relevant versions always keep mark bits in a separate region of memory.  There is no way to configure the collector differently.

I do not understand your second question.  The collector, including 6.8, always uses free-lists for small objects.  There is no way to configure that differently.

Hans

From: lijun [mailto:lijun at ialab.cs.tsukuba.ac.jp]
Sent: Thursday, February 09, 2012 6:06 AM
To: Boehm, Hans; boehm at acm.org
Cc: gc at linux.hpl.hp.com
Subject: RE: about bitmap marking

I am sorry that i did not ask more clearly.
I just want to know about gc6.8.

I don't know whether gc6.8 uses bitmap marking or not if I build the collector without any options or if I don't set any parameters in my C program?
(For example, if I build gc6.8 with -DGC_LINUX_THREADS -DPARALLEL_MARK -DTHREAD_LOCAL_ALLOC, the collector will run the mark phase in parallel.
If I call GC_enable_incremental and set GC_time_limit in my program, the collector will perform an incremental gc. etc)

If yes, I want to know whether mark bits are out of the objects or not in gc6.8.

About freelist, I want to know whether the following is right or wrong in gc6.8?
C program <- (memory block) <- freelist(list of free memory blocks)

I'm waiting for your reply eagerly.
Best Regards.

Li Jun


From: Boehm, Hans [mailto:hans.boehm at hp.com]
Sent: Thursday, February 09, 2012 12:57 AM
To: lijun; boehm at acm.org
Cc: gc at linux.hpl.hp.com
Subject: RE: about bitmap marking

[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.

Hans

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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