Ordinary Object Pointer in JVM

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Go Tanaka 1)14DBMB+BWB4DSJQU+BWB 8 : 1.5 : 0.5 : 0
【運営スタッフ】関西Javaエンジニアの会 Scala関西 Summit Freelance Engineer @UBO@HP

ࠂ஌ ,"/+"7"1"35: ೔࣌೥݄೔ ౔ ৔ॴ೔ຊϚΠΫϩιϑτؔ੢ࢧࣾ

ࠂ஌ 4DBMBؔ੢4VNNJU ೔࣌೥݄೔ ౔ ৔ॴఱຬݚमηϯλʔʢ+3େࡕͷྡͷӺͷ͙ۙ͘͢ʣ

0QFO+%,ͷιʔείʔυ΍+0-ɺ)4%#ͳͲͰͰ͖ΔݶΓ֬ೝ͍ͯ͠·͕͢ 001·ΘΓͷ࿩͸+BWBݴޠ࢓༷΍+BWBԾ૝Ϛγϯ࢓༷ʹ͸هࡌ͞Ε͍ͯ·ͤΜɻ ΑͬͯɺҰ෦࣮ࡍͷಈ࡞ͱҟͳΔ͜ͱ͕͋Δ͔΋͠Ε·ͤΜɻ ঘɺ͜ͷηογϣϯͰ͸+7.͸)PU4QPU7.ͷ͜ͱΛࢦ͠·͢ɻ ஫ҙ

໨࣍

໨࣍ 0SEJOBSZ0CKFDU1PJOUFS ɹPPQ%FTD ɹ$PNQSFTTFE001 ,MBTT ɹ,MBTTϙΠϯλͷѹॖ .BSL ɹ.BSLͷঢ়ଶ ɹ#JBTFE-PDLJOH +BWBΦϒδΣΫτͷϝϞϦߏ଄
001ʹ͍ͭͯͷ֓ཁ

001ϔομͷઆ໌

ͲΕ͘Β͍ϝϞϦ࢖͏͔ͱ͔

ώʔϓ্ͷ͢΂ͯͷ+BWBΦϒδΣΫτ͸001ͱͯ͠දݱ͞ΕΔ 001͸$$ ͷϙΠϯλͰώʔϓ಺ͷҐஔΛࢦ͢ 001ͷϔομ͸.BSLͱ,MBTTͷ̎ϚγϯޠͰߏ੒͞ΕΔ z.BSLz͸($ɺಉظԽͷͨΊͷ৘ใΛ΋͍ͬͯΔ z,MBTTz͸Ϋϥεͷϝλσʔλ΁ͷϙΠϯλ 0SEJOBSZ0CKFDU1PJOUFS

0SEJOBSZ0CKFDU1PJOUFS _mark _klass Fields 32-bit cpu 64-bit cpu 4 byte
8 byte 4 byte 8 byte -XX:ObjectAlignmentInBytes=8 ͢΂ͯͷΦϒδΣΫτ͸όΠτڥքʹ഑ஔ͞ΕΔ

0SEJOBSZ0CKFDU1PJOUFS java.lang.Object object internals: OFFSET SIZE TYPE DESCRIPTION VALUE 0
16 (object header) N/A Instance size: 16 bytes Space losses: 0 bytes internal + 0 bytes external = 0 bytes total +0-Ͱग़ྗͯ͠Έͨ݁Ռ new Object() すると、64-bit cpu では 16 byte 消費することになる

PPQ%FTD IPUTQPUTSDTIBSFWNPPQTPPQIQQ +%, PPQ%FTDJTUIFUPQCBTFDMBTTGPSPCKFDUTDMBTTFT 5IF\OBNF^%FTDDMBTTFTEFTDSJCF UIFGPSNBUPG+BWBPCKFDUTTPUIFpFMETDBOCFBDDFTTFEGSPN$ PPQ%FTDJTBCTUSBDU TFFPPQ)JFSBSDIZGPSDPNQMFUFPPQDMBTTIJFSBSDIZ
PPQ%FTDΫϥε͸($ର৅ͱͳΔΦϒδΣΫτͷந৅తͳجఈΫϥε PPQ%FTDΫϥεΛܧঝͨ͠ΫϥεͷΠϯελϯε͕($ର৅ͷΦϒδΣΫτʹͳΔ

PPQ%FTD private: volatile markOop _mark; union _metadata { wideKlassOop _klass;
narrowOop _compressed_klass; } _metadata; TSDTIBSFWNPPQTPPQIQQ +%,

PPQ%FTD TSDTIBSFWNPPQTPPQIQQ +%, private: volatile markOop _mark; union _metadata {
Klass* _klass; narrowKlass _compressed_klass; } _metadata;

PPQ%FTD IPUTQPUTSDTIBSFWNPPQTPPQT)JFSBSDIZIQQ +%, typedef class oopDesc* oop; typedef class instanceOopDesc*
instanceOop; typedef class methodOopDesc* methodOop; typedef class constMethodOopDesc* constMethodOop; typedef class methodDataOopDesc* methodDataOop; typedef class arrayOopDesc* arrayOop; typedef class objArrayOopDesc* objArrayOop; typedef class typeArrayOopDesc* typeArrayOop; typedef class constantPoolOopDesc* constantPoolOop; typedef class constantPoolCacheOopDesc* constantPoolCacheOop; typedef class klassOopDesc* klassOop; typedef class markOopDesc* markOop; typedef class compiledICHolderOopDesc* compiledICHolderOop; PPQ%FTD͔Β೿ੜͨ͠Ϋϥε ˞Ұ෦ͷΫϥε͕+%,Ͱ.FUBTQBDFʹҠಈ

PPQ%FTD IPUTQPUTSDTIBSFWNPPQTPPQT)JFSBSDIZIQQ +%, typedef class oopDesc* oop; typedef class instanceOopDesc*
instanceOop; typedef class arrayOopDesc* arrayOop; typedef class objArrayOopDesc* objArrayOop; typedef class typeArrayOopDesc* typeArrayOop; // class MetaspaceObj class ConstMethod; class ConstantPoolCache; class MethodData; // class Metadata class Method; class ConstantPool; // class CHeapObj class CompiledICHolder; PPQ%FTD͔Β೿ੜͨ͠Ϋϥε ˞+%,Ͱ.FUBTQBDFʹҠಈ

PPQ%FTD JOTUBODF0PQ%FTDʜ+BWBΦϒδΣΫτͷ001 BSSBZ0PQ%FTDʜ഑ྻΛද͢001ͷ਌ PCK"SSBZ0PQ%FTDʜ഑ྻΦϒδΣΫτͷ001 UZQF"SSBZ0PQ%FTDʜϓϦϛςΟϒͷ഑ྻͷ001

$PNQSFTTFE001 CJUϚγϯͰ΋ΦϒδΣΫτͷΞυϨεΛCJUʹ཈͑Δ CZUF +BWBώʔϓͷ࢖༻ྔΛઅ໿ ώʔϓͷϕʔεΞυϨε͔ΒͷΦϑηοτΛར༻ ѹॖ001͸+BWBV͔ΒσϑΥϧτͰΦϯ 996TF$PNQSFTTFE0PQTͰ੾Δ͜ͱ͕Ͱ͖Δ http://www.oracle.com/technetwork/jp/articles/java/compressedoops-427542-ja.html ৄ͘͠͸

͜͜·Ͱͷ·ͱΊ ώʔϓ্ͷ͢΂ͯͷ+BWBΦϒδΣΫτ͸001ͱͯ͠දݱ͞ΕΔ 001ͷϔομ͸.BSLͱ,MBTTͰߏ੒͞ΕΔ $16͕CJU͔CJU͔Ͱ001ͷαΠζ͕มΘΔ ѹॖͰ͖Δ +BWBͰ.FUBTQBDF͕ಋೖ͞Εͨ͜ͱʹΑΓ001ͷߏ଄͕มΘͬͨ

,MBTT 001ͷϔομͷҰ෦ ,MBTT͸Ϋϥεͷ.FUBTQBDF಺ͷ؅ཧϙΠϯλΛද͢ CJUϚγϯͰCZUFɺCJUϚγϯͰCZUF࢖༻͢ΔʢѹॖͰ͖Δʣ http://otndnld.oracle.co.jp/ondemand/javaday2014/pdf/C2-JavaDay-304328.pdf

,MBTTϙΠϯλͷѹॖ $PNQSFTTFE001ͱಉ͡Α͏ʹ+BWB)FBQΛઅ໿ CZUF $PNQSFTTFE$MBTT1PJOUFS4QBDF $$14 ͱ͍͏ϝϞϦΤϦΞΛ༻ҙ *OTUBODF,MBTT "SSBZ,MBTT W5BCMFͳͲύϑΥʔϚϯεʹӨڹ͢ΔΫϥε
ͷΈ֨ೲ͠ɺͦΕҎ֎͸.FUBTQBDFʹ֨ೲʢϝιουɺίϯελϯτϓʔϧͳͲʣ

,MBTT ,MBTT͸͞·͟·ͳܕ৘ใͷந৅తͳجఈΫϥε

*OTUBODF,MBTTʜܕΫϥεͷΠϯελϯε *OTUBODF.JSSPS,MBTTʜKBWBMBOH$MBTTΛද͢ *OTUBODF$MBTT-PBEFS,MBTTʜ*OTUBODF,MBTTͷΫϥεϩʔμ༻ *OTUBODF3FG,MBTTʜKBWBMBOHSFG3FGFSFODFͷαϒΫϥε ɹɹɹɹɹɹɹɹɹɹ TPGUXFBLpOBMQIBOUPN ,MBTT

"SSBZ,MBTTʜ഑ྻΛද͢਌Ϋϥε 0CK"SSBZ,MBTTʜΦϒδΣΫτͷ഑ྻͷΫϥε 5ZQF"SSBZ,MBTTʜϓϦϛςΟϒͷ഑ྻͷΫϥε ,MBTT

001ͱ,MBTTͷؔ܎ InstanceOop _klass InstanceKlass InstanceKlass _super *OTUBODF0PQTUS ,MBTT4USJOH ,MBTT0CKFDU Metaspace
InstanceOop PPQ$MBTT4USJOH _java_mirror Java Heap String str = “hoge”; InstanceMirrorKlass _klass

͜͜·Ͱͷ·ͱΊ ,MBTT͸001ͷϔομͷҰ෦ ௨ৗͷ ഑ྻ͡Όͳ͍ +BWBΠϯελϯε͸*OTUBODF0PQͰදݱ͞ΕΔ Πϯελϯεͷܕ৘ใ͸*OTUBODF,MBTTʹ֨ೲ͞Ε͍ͯΔ *OTUBODF,MBTTͷKBWB@NJSSPSʹKBWBMBOH$MBTTͷ001΁ͷࢀর͕͋Δ

.BSL ($ɺಉظԽͷͨΊͷ৘ใΛ΋͍ͬͯΔ CJUϚγϯͰ͸όΠτɺCJUϚγϯͰ͸όΠτͷ௕͞ʹͳΔ .BSL͸ϙΠϯλͰ͸ͳ͘σʔλߏ଄

.BSL hash age CJU CJU unused:25 hash:31 -->| unused:1 age:4
biased_lock:1 lock:2 (normal object) hash:25 ------------>| age:4 biased_lock:1 lock:2 (normal object) src/share/vm/oops/markOop.hpp ʢྫʣ6OMPDLFE ޙड़ ͷ.BSLͷϝϞϦߏ଄ hash age

.BSL .BSLͷঢ়ଶ͸ओʹҎԼͷ̑ͭ ɹ#JBTFE ɹ6OMPDLFE લड़ͷOPSNBMPCKFDU ɹ-JHIUXFJHIU-PDLFE ɹ)FBWZXFJHIU-PDLFE ɹ.BSLFEʢ($ͷΈʣ https://wiki.openjdk.java.net/display/HotSpot/Synchronization
Mark には5種類のデータ構造がある

.BSLͷঢ়ଶ#JBTFE ΦϒδΣΫτׂ͕Γ౰ͯΒΕΔ #JBTFE-PDLJOH͸+BWB͔ΒσϑΥϧτͳͷͰCJBTBCMFPCKFDUʹͳΔ ϩοΫ͢ΔͱUISFBE*%͕ઃఆ͞ΕΔ CJBTFEPCKFDU
SFCJBT͢ΔͱCJBTBCMFPCKFDUʹ໭Δ ҰఆपظͰSFCJBT͢ΔΒ͍͠ ଞͷεϨουͰར༻͞ΕΔͱجຊϩοΫʹͳΔ #JBTFE-PDLJOH͕ղআ͞ΕΔ 0 epoch age 1 01 thread ID epoch age 1 01 JOJUJBMMPDL SFCJBT CJBTBCMFPCKFDU CJBTFEPCKFDU

#JBTFE-PDLJOH جຊϩοΫͰ͸ճͷϩοΫͰ࠷௿ճͷ$"4໋ྩΛ࢖͏ #JBTFE-PDLJOHͷίϯηϓτ ɹɾ͋ΔεϨου͔ΒϩοΫ͞ΕͨΦϒδΣΫτ͸ ɹɹಉ͡εϨου͔ΒϩοΫ͞ΕΔ܏޲͕͋Δ ɹɾΦϒδΣΫτͷதʹεϨου৘ใΛຒΊࠐΜͰಛఆͷεϨουʹ ภ͍ͬͯΔ CJBTFE ͜ͱΛࣔ͢ɻͦΕʹΑΓͦͷεϨου͔Βͷ ϩοΫ͸$"4໋ྩͳ͠Ͱߦ͏͜ͱ͕Ͱ͖Δ

.BSLͷঢ়ଶ hash code age 0 01 pointer to lock record
00 #JBTFE-PDLJOH͕ར༻Ͱ͖ͳ͍৔߹͸جຊϩοΫͷϓϩηεʹͳΔ جຊϩοΫͰϩοΫ͢Δͱ5IJO-PDL͕͔͔Δ εϨου͕͢ͰʹͦͷΦϒδΣΫτͷϩοΫΛ͍࣋ͬͯΔ৔߹͸3FDVSTJWF-PDLʹͳΔ ฒߦॲཧͳͲͰෳ਺ͷҟͳΔεϨου͔Βಉ͡ΦϒδΣΫτʹରͯ͠ ಉ࣌ʹಉظॲཧ͕ߦΘΕΔͱ*OqBUF-PDLʹͳΔ pointer to heavyweight monitor 10 6OMPDLFE -JHIUXFJHIU-PDLFE )FBWZXFJHIU-PDLFE 5IJO-PDL *OqBUF-PDL ʜVOMPDLFE ʜMPDLFE ʜNPOJUPS

͜͜·Ͱͷ·ͱΊ .BSL͸001ϔομͷҰ෦ .BSL͸ϙΠϯλͰ͸ͳ͘ঢ়ଶΛද͢σʔλߏ଄ ओʹಉظॲཧͷͨΊʹར༻͞ΕΔ ಉظॲཧ͸ύϑΥʔϚϯεʹӨڹ͢ΔͷͰঢ়گʹԠͨ͡ϩοΫॲཧ͕͋Δ ϩοΫॲཧʹԠͯ͡σʔλߏ଄͕มΘΔ

+BWBΫϥεͷϝϞϦߏ଄

͜͜·Ͱ͸001ϔομͷ࿩

_mark _klass ͜͜·ͰͰCZUF෼ͷ࿩͔ͯ͠͠ͳ͍ ˞CJUϚγϯͷ৔߹ɻ001͓Αͼ,MBTTϙΠϯλະѹॖ CZUF CZUF

+BWBΦϒδΣΫτͷϝϞϦߏ଄ -XX:ObjectAlignmentInBytes=8 ͢΂ͯͷΦϒδΣΫτ͸όΠτڥքʹ഑ஔ͞ΕΔ _mark _klass Fields

java.lang.Object object internals: OFFSET SIZE TYPE DESCRIPTION VALUE 0 16
(object header) N/A Instance size: 16 bytes Space losses: 0 bytes internal + 0 bytes external = 0 bytes total ѹॖ001ͳ͠ java.lang.Object object internals: OFFSET SIZE TYPE DESCRIPTION VALUE 0 12 (object header) N/A 12 4 (loss due to the next object alignment) Instance size: 16 bytes Space losses: 0 bytes internal + 4 bytes external = 4 bytes total ѹॖ001͋Γ +BWBΦϒδΣΫτͷϝϞϦߏ଄

001ϔομͷޙʹΫϥεϑΟʔϧυ͕ଓ͘ ϝϞϦઅ໿ͷͨΊʹ+7.͸ఆٛ͞Εͨॱ൪Ͱ͸ͳ͘ฒͼସ͑Λߦ͏ ϑΟʔϧυ͸جຊతʹ࣍ͷ༏ઌॱҐͰϝϞϦʹߏ੒͢Δ 1 double, long 8 2 int, ﬂoat
4 3 short, char 2 4 boolean, byte 1 5 ΦϒδΣΫτࢀর 8 (64bit) +BWBΦϒδΣΫτͷϝϞϦߏ଄

class A { byte a; int c; short d; long
e; Object f; } ᶃ ᶄ ᶆ ᶅ ᶇ jol.samples.A object internals: OFFSET SIZE TYPE DESCRIPTION VALUE 0 16 (object header) N/A 16 8 long D.e N/A 24 4 int D.c N/A 28 2 short D.d N/A 30 1 byte D.a N/A 31 1 (alignment/padding gap) 32 8 java.lang.Object D.f N/A Instance size: 40 bytes Space losses: 1 bytes internal + 0 bytes external = 1 bytes total ᶃ ᶄ ᶅ ᶆ ᶇ ѹॖ001ͳ͠ +BWBΦϒδΣΫτͷϝϞϦߏ଄

class A { byte a; int c; short d; long
e; Object f; } ᶃ ᶄ ᶆ ᶅ ᶇ jol.samples.A object internals: OFFSET SIZE TYPE DESCRIPTION VALUE 0 12 (object header) N/A 12 4 int D.c N/A 16 8 long D.e N/A 24 2 short D.d N/A 26 1 byte D.a N/A 27 1 (alignment/padding gap) 28 4 java.lang.Object D.f N/A Instance size: 32 bytes Space losses: 1 bytes internal + 0 bytes external = 1 bytes total ᶃ ᶄ ᶅ ᶆ ᶇ ѹॖ001͋Γ +BWBΦϒδΣΫτͷϝϞϦߏ଄

αϒΫϥεͷϝϞϦϨΠΞ΢τ ɹ֊૚͕ҟͳΔΫϥεͷϑΟʔϧυ͸ࠞ߹͠ͳ͍ ɹ਌ΫϥεͷϑΟʔϧυ͕ઌʹ͖ͯɺαϒΫϥεͷϑΟʔϧυ͕ͦͷޙʹ͘Δ +BWBΦϒδΣΫτͷϝϞϦߏ଄

class A { byte a; } class B extends A
{ long b; short c; byte d; } jol.samples.B object internals: OFFSET SIZE TYPE DESCRIPTION VALUE 0 12 (object header) N/A 12 1 byte A.a N/A 13 3 (alignment/padding gap) 16 8 long B.b N/A 24 2 short B.c N/A 26 1 byte B.d N/A 27 5 (loss due to the next object alignment) Instance size: 32 bytes Space losses: 3 bytes internal + 5 bytes external = 8 bytes total +BWBΦϒδΣΫτͷϝϞϦߏ଄

String str = “abcdefghij”; ͜ͷίʔυ͸ԿCZUFͷϝϞϦΛফඅ͢Δ͔ +BWBΦϒδΣΫτͷϝϞϦߏ଄

+0-Ͱ$MBTT-BZPVUΛΈͯΈΔ String str = “abcdefghij"; System.out.println(ClassLayout.parseInstance(str).toPrintable()); +BWBΦϒδΣΫτͷϝϞϦߏ଄

+0-Ͱ$MBTT-BZPVUΛΈͯΈΔ java.lang.String object internals: OFFSET SIZE TYPE DESCRIPTION VALUE 0
4 (object header) 01 00 00 00 4 4 (object header) 00 00 00 00 8 4 (object header) da 02 00 f8 12 4 char[] String.value [a, b, c, d, e, f, g, h, i, j] 16 4 int String.hash 0 20 4 (loss due to the next object alignment) Instance size: 24 bytes Space losses: 0 bytes internal + 4 bytes external = 4 bytes total +BWBΦϒδΣΫτͷϝϞϦߏ଄

+BWBΦϒδΣΫτͷϝϞϦߏ଄ String str = “abcdefghij"; System.out.println(GraphLayout.parseInstance(str).toPrintable()); +0-ͰΠϯελϯεͷ(SBQI-BZPVUΛΈͯΈΔ

+0-ͰΠϯελϯεͷ(SBQI-BZPVUΛΈͯΈΔ java.lang.String@532760d8d object externals: ADDRESS SIZE TYPE PATH VALUE 76b98fb58
24 java.lang.String (object) 76b98fb70 40 [C .value [a, b, c, d, e, f, g, h, i, j] DIBSͷ഑ྻͰCZUFফඅ͍ͯ͠Δ +BWBΦϒδΣΫτͷϝϞϦߏ଄

[C object internals: OFFSET SIZE TYPE DESCRIPTION VALUE 0 4
(object header) 01 00 00 00 4 4 (object header) 00 00 00 00 8 4 (object header) 41 00 00 f8 12 4 (object header) 0a 00 00 00 16 20 char [C.<elements> N/A 36 4 (loss due to the next object alignment) Instance size: 40 bytes Space losses: 0 bytes internal + 4 bytes external = 4 bytes total ͜ͷDIBSͷ഑ྻͷ$MBTT-BZPVUΛΈͯΈΔ <B C D E F G H I J K>͸ཁૉ਺YB ഑ྻͷ௕͕͞ϔομʹઃఆ͞ΕΔ +BWBΦϒδΣΫτͷϝϞϦߏ଄

String str = “abcdefghij”; ͜ͷίʔυ͸ԿCZUFͷϝϞϦΛফඅ͢Δ͔ 4USJOHCZUF DIBS<>CZUF ߹ܭɿCZUF +BWBΦϒδΣΫτͷϝϞϦߏ଄

·ͱΊͱײ૝ ώʔϓ্ͷ͢΂ͯͷ+BWBΦϒδΣΫτ͸001ͱͯ͠දݱ͞ΕΔ 001ͷϔομ͸.BSLͱ,MBTTͰߏ੒͞ΕΔ .BSL͸͓΋ʹಉظԽɺ,MBTT͸Ϋϥεͷ৘ใΛ΋͍ͬͯΔ 001ϔομͷ͋ͱʹ͸ϑΟʔϧυ৘ใ͕͸͍Δ ΦϯϥΠϯʹ͋Δ৘ใͷ΄ͱΜͲ͕+BWBҎલͳͷͰ஫ҙ

͋Γ͕ͱ͏͍͟͝·ͨ͠ʂ

4ZODISPOJ[BUJPOBOE0CKFDU-PDLJOH )PU4QPUVOEFSUIF)PPE -PDLJOHBOE4ZODISPOJ[BUJPO +BWBΦϒδΣΫτͷϝϞϦߏ଄ &WFSZUIJOH*&WFS-FBSOFE"CPVU+7.1FSGPSNBODF5VOJOH!5XJUUFS +%,+7.*NQSPWFNFOUT ࢀߟࢿྉ https://www.infoscoop.org/blogjp/2014/06/16/java-objects-memory-structure/ https://www.slideshare.net/aszegedi/everything-i-ever-learned-about-jvm-performance-tuning-twitter https://www.slideshare.net/nminoru_jp/jvm-readingsynchronization
https://www.infoq.com/presentations/hotspot-memory-data-structures https://wiki.openjdk.java.net/display/HotSpot/Synchronization http://otndnld.oracle.co.jp/ondemand/javaday2014/pdf/C2-JavaDay-304328.pdf

Ordinary Object Pointer in JVM

Ordinary Object Pointer in JVM

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