[Gc] RE: about bitmap marking

Boehm, Hans hans.boehm at hp.com
Fri Feb 10 09:30:33 PST 2012


Each page has a separate mark bit table, since the heap is not presumed to be contiguous.  The mark bit table is allocated separately from the page itself, but also contains other information about the page.  Much of this is outlined in http://www.hpl.hp.com/personal/Hans_Boehm/gc/gcdescr.html .

Hans

From: lijun [mailto:lijun at ialab.cs.tsukuba.ac.jp]
Sent: Friday, February 10, 2012 3:32 AM
To: Boehm, Hans; boehm at acm.org
Cc: gc at linux.hpl.hp.com
Subject: RE: about bitmap marking

Thank you for your information.

The mark bits in a separate region of memory are stored continuously in a table?
(In sweep phase, the collector scans mark bits in the table)
There is a bitmap table in each page?

Best regards.

Li Jun

From: Boehm, Hans [mailto:hans.boehm at hp.com]<mailto:[mailto:hans.boehm at hp.com]>
Sent: Friday, February 10, 2012 4:09 PM
To: lijun; boehm at acm.org<mailto:boehm at acm.org>
Cc: gc at linux.hpl.hp.com<mailto:gc at linux.hpl.hp.com>
Subject: RE: about bitmap marking

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]<mailto:[mailto:lijun at ialab.cs.tsukuba.ac.jp]>
Sent: Thursday, February 09, 2012 6:06 AM
To: Boehm, Hans; boehm at acm.org<mailto:boehm at acm.org>
Cc: gc at linux.hpl.hp.com<mailto: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]<mailto:[mailto:hans.boehm at hp.com]>
Sent: Thursday, February 09, 2012 12:57 AM
To: lijun; boehm at acm.org<mailto:boehm at acm.org>
Cc: gc at linux.hpl.hp.com<mailto: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]<mailto:[mailto:lijun at ialab.cs.tsukuba.ac.jp]>
Sent: Tuesday, February 07, 2012 7:26 PM
To: Boehm, Hans; boehm at acm.org<mailto: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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