Mercurial > hg > truffle

comparison src/share/vm/oops/markOop.hpp @ 1546:a00b51b2dda4

Find changesets by keywords (author, files, the commit message), revision number or hash, or revset expression.
6948539: CMS+UseCompressedOops: placement of cms_free bit interferes with promoted object link Summary: When using compressed oops, use compressed promoted pointers in b63:b31 of the mark word, so as not to interfere with the CMS "freeness bit" at b7. Updated mark-word layout documentation. Reviewed-by: minqi, poonam, jmasa, coleenp
author ysr
date Mon, 17 May 2010 00:47:28 -0700
parents 1ee8caae33af
children c18cbe5936b8
comparison
equal deleted inserted replaced
1545:cc387008223e 1546:a00b51b2dda4
25 // The markOop describes the header of an object. 25 // The markOop describes the header of an object.
26 // 26 //
27 // Note that the mark is not a real oop but just a word. 27 // Note that the mark is not a real oop but just a word.
28 // It is placed in the oop hierarchy for historical reasons. 28 // It is placed in the oop hierarchy for historical reasons.
29 // 29 //
30 // Bit-format of an object header (most significant first): 30 // Bit-format of an object header (most significant first, big endian layout below):
31 // 31 //
32 // 32 bits: unused:0 hash:25 age:4 biased_lock:1 lock:2 32 // 32 bits:
33 // 64 bits: unused:24 hash:31 cms:2 age:4 biased_lock:1 lock:2 33 // --------
34 // unused:20 size:35 cms:2 age:4 biased_lock:1 lock:2 (if cms 34 // hash:25 ------------>| age:4 biased_lock:1 lock:2 (normal object)
35 // free chunk) 35 // JavaThread*:23 epoch:2 age:4 biased_lock:1 lock:2 (biased object)
36 // size:32 ------------------------------------------>| (CMS free block)
37 // PromotedObject*:29 ---------->| promo_bits:3 ----->| (CMS promoted object)
38 //
39 // 64 bits:
40 // --------
41 // unused:25 hash:31 -->| unused:1 age:4 biased_lock:1 lock:2 (normal object)
42 // JavaThread*:54 epoch:2 unused:1 age:4 biased_lock:1 lock:2 (biased object)
43 // PromotedObject*:61 --------------------->| promo_bits:3 ----->| (CMS promoted object)
44 // size:64 ----------------------------------------------------->| (CMS free block)
45 //
46 // unused:25 hash:31 -->| cms_free:1 age:4 biased_lock:1 lock:2 (COOPs && normal object)
47 // JavaThread*:54 epoch:2 cms_free:1 age:4 biased_lock:1 lock:2 (COOPs && biased object)
48 // narrowOop:32 unused:24 cms_free:1 unused:4 promo_bits:3 ----->| (COOPs && CMS promoted object)
49 // unused:21 size:35 -->| cms_free:1 unused:7 ------------------>| (COOPs && CMS free block)
36 // 50 //
37 // - hash contains the identity hash value: largest value is 51 // - hash contains the identity hash value: largest value is
38 // 31 bits, see os::random(). Also, 64-bit vm's require 52 // 31 bits, see os::random(). Also, 64-bit vm's require
39 // a hash value no bigger than 32 bits because they will not 53 // a hash value no bigger than 32 bits because they will not
40 // properly generate a mask larger than that: see library_call.cpp 54 // properly generate a mask larger than that: see library_call.cpp
59 // times were seen when these bits were absent and an arbitrary age 73 // times were seen when these bits were absent and an arbitrary age
60 // assigned to all biased objects, because they tended to consume a 74 // assigned to all biased objects, because they tended to consume a
61 // significant fraction of the eden semispaces and were not 75 // significant fraction of the eden semispaces and were not
62 // promoted promptly, causing an increase in the amount of copying 76 // promoted promptly, causing an increase in the amount of copying
63 // performed. The runtime system aligns all JavaThread* pointers to 77 // performed. The runtime system aligns all JavaThread* pointers to
64 // a very large value (currently 128 bytes) to make room for the 78 // a very large value (currently 128 bytes (32bVM) or 256 bytes (64bVM))
65 // age bits when biased locking is enabled. 79 // to make room for the age bits & the epoch bits (used in support of
80 // biased locking), and for the CMS "freeness" bit in the 64bVM (+COOPs).
66 // 81 //
67 // [JavaThread* | epoch | age | 1 | 01] lock is biased toward given thread 82 // [JavaThread* | epoch | age | 1 | 01] lock is biased toward given thread
68 // [0 | epoch | age | 1 | 01] lock is anonymously biased 83 // [0 | epoch | age | 1 | 01] lock is anonymously biased
69 // 84 //
70 // - the two lock bits are used to describe three states: locked/unlocked and monitor. 85 // - the two lock bits are used to describe three states: locked/unlocked and monitor.