[Gc] RE: about bitmap marking

lijun lijun at ialab.cs.tsukuba.ac.jp
Sat Feb 18 01:12:43 PST 2012


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.

>>+ 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 1:52 AM
To: 'Boehm, Hans'; 'boehm at acm.org'
Cc: 'gc at linux.hpl.hp.com'
Subject: RE: about bitmap marking



<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

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