[Gc] RE: about bitmap marking

lijun lijun at ialab.cs.tsukuba.ac.jp
Wed Feb 15 08:51:54 PST 2012


<this is a continuation of the mail sent 2 hours ago>

 

In general, there are three advantages of using mark bit table(bitmap
marking).

+ it is friendly to copy-on-write.

+ it helps to attain a higher locality of reference.

+ 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.

(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.),

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).

 

Best regards

Li Jun

 

 

From: lijun [mailto:lijun at ialab.cs.tsukuba.ac.jp] 
Sent: Thursday, February 16, 2012 12:39 AM
To: 'Boehm, Hans'; 'boehm at acm.org'
Cc: 'gc at linux.hpl.hp.com'
Subject: RE: about bitmap marking

 

Because each page has a separate mark bit table, in most cases, there are
many mark bit tables in the collector.

 

If I use the default configuration, the mark bit tables(gc6.8) help to
attain a higher locality of reference? (in comparison with not using the
mark bit tables)

 

In sweep phase(gc6.8, default configuration), the collector will run faster
because of having mark bit tables?

(scanning/clearing the mark bit tables is faster than scanning/clearing the
mark bits in object headers because of putting the mark bits together?)

 

Best regards

Li Jun

 

From: Boehm, Hans [mailto:hans.boehm at hp.com] 
Sent: Wednesday, February 15, 2012 7:06 AM
To: lijun; boehm at acm.org
Cc: gc at linux.hpl.hp.com
Subject: RE: about bitmap marking

 

That depends on the collector configuration.  It can either use a bit or a
byte per logical mark bit, and use one logical mark bit per allocation
granule or per object.  See MARK_BIT_PER_GRANULE and
USE_MARK_BYTES/USE_MARK_BITS.

 

Hans

 

From: lijun [mailto:lijun at ialab.cs.tsukuba.ac.jp] 
Sent: Monday, February 13, 2012 12:08 AM
To: Boehm, Hans; boehm at acm.org
Cc: gc at linux.hpl.hp.com
Subject: RE: about bitmap marking

 

In each mark bit table, the mark bits are stored contiguously?

 

Best regards.

Li Jun

 

From: Boehm, Hans [mailto:hans.boehm at hp.com] 
Sent: Saturday, February 11, 2012 2:31 AM
To: lijun; boehm at acm.org
Cc: gc at linux.hpl.hp.com
Subject: RE: about bitmap marking

 

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] 
Sent: Friday, February 10, 2012 4:09 PM
To: lijun; boehm at acm.org
Cc: 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] 
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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