6장 Thread Synchronization

Java Performance
artdb@ex-em.com | performeister.tistory.com | twitter @novathinker

Thread Synchronization

Java Performance
2


1) Java Thread
2) Thread Synchronization
3) Cooperation
4) Mutual execution
5) Light-Weight Lock
Hotspot Synchronization
1) Light-Weight Lock
2) Biased Locking
IBM JVM Synchronization
1) Thin Lock
2) Lock Reservation

Java Performance

Thread
Synchnonization

Java Performance
4
Java Performance Fundamental | twitter @novathinker
artdb@ex-em.com | performeister.tistory.com

• Java Thread
– Green Thread
• JVM Created, scheduling Thread
• 1.1 JVM Thread model
• OS Thread n:1
• OS Thread
• CPU
– Native Thread
• OS Kernel Thread
• 1.3 Default

Java Performance
5

• Thread Synchronization
– Monitor : Java Synchronization Mechanism
– Monitor
• Cooperation
– Class object wait(),notify()
– Thread
• Mutual Exclusion
– Class class
–

Java Performance
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• Cooperation
– Cooperation
•

wait()
Reader
Thread
Buffer empty
Listen notify()
Thread

Buffer full

Java Performance
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• Cooperation
– Monitor
Monitor

Critical Section

Monitor suspend

Java Performance
8

• Cooperation
– Monitor

Java Performance
9

– Shared Data
• Runtime Data Area Data Thread

• Heap  Instance Variables
• Method Area  Class Variables
• Object Class Monitor lock
• Mutex object lock
• Object Lock
– Thread Object

Java Performance
10

– Object Lock
• Class ﬁle java.lang.class instance

• Object lock object(instance) locking
• Thread object locking
• object lock counter
– Lock count
– Lock count
– Count = 0 thread lock
• Thread Critical Section lock

Java Performance
11

– Critical Section
• Critical Section
– Synchronized statement
– Synchronized method
• Critical section object reference
• Critical section instruction thread
object lock
• Critical section lock release
• Object Lock critical
section JVM

Java Performance
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• Light-Weight Lock
– light-weight lock
• JVM Light-Weight Lock
• Object OS Mutex heavy-
weight lock
• locking

No Atomic Operation
Enter, Exit monitor

Contended

Heavy-Weight OS Lock
Yes

Java Performance

Hotspot
Synchronization

Java Performance
14

– Object Header
• Object Header word Mark Work

Mark Word Hash Code (0) Age Biased Tag
Thread ID, epoch(1)

23 bit 6 bit 1 bit 2 bit

Hash Code (0) Age Biased Tag Biased bit Tag
Thread ID, epoch(1)
0 01 Unlocked
(000) Lock Record Address 0 00 Light-weight locked

(010) Monitor Address 0 10 Heavy-weight locked
0 11 Marked for GC
(011) Forwarding Address 1 01 Biased / Biasable

Java Performance
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– Light-Weight Locking
• Object Header word Mark Work
• Atomic compare-and-swap(CAS) instruction
lock
• CAS thread lock
• CAS heavy weight lock
• Object lock thread stack frame mark
word move (displaced mark )
• Lock Mark Word

Java Performance
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Execution Execution
Stack Stack
Locking
Method Method
Activation Activation
Mark word 01 Stack pointer

Lock Displaced hdr Mark word 01

Record Object Object
owner owner

Java Performance
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Unlock
Hash Code Age 0 01

Recursive lock

Light weight lock Pointer to lock record 00 Unlock

heavy weight lock Pointer to heavyweight 10

Java Performance
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• Atomic Operation multiprocessor

• lock Thread
locking
• Thread Lock

• Thread lock

–

Java Performance
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• Biased Locking
– Java 6 default
• Java 5 update 6
– Light Weight lock
• Object lock thread lock

• CAS
– Biased lock
• lock CAS
• Thread (biased) lock
CAS

Java Performance
20

• Biased Locking
Object
Allocate
Bias enabled Bias disabled
Unlock Unlock

0 epoch Age 1 01 Hash Code Age 0 01

Recursive lock
Initial
lock Rebias Revoke
bias Pointer to lock record 00
locked
Thread
ID
epoch Age 1 01
Pointer to heavyweight
monitor 10
Lock / Unlock

Java Performance
21

• Biased Locking
– Biased lock
• Mark word tag 1
• 101  biased or biasable
(thread id 0 unlocked)
– Bias revocation
• Lock Thread Thread lock
light weight lock
• Owner thread bias
• Object mark word
– Thread suspend  expensive

Java Performance
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• Biased Locking
– Bulk Revocation
• Class revocation
• Class instance bias invalid
– Bulk Rebiasing
• Thread
lock class
• class instance bias invalid

Java Performance
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• Biased Locking
– Option
• -XX:+UseBiasedLocking (Java5 update
6~)
: biased lock
java 6 default
• -XX:BiasedLockingStartupDelay=0
(Java6~)
: JVM Delay second
• -XX:+TraceBiasedLocking
(Java6~)
: Biased Lock Debugging

Java Performance

IBM JVM
Synchronization

Java Performance
25
IBM JVM Synchronization

• Thin Lock
– Thin lock Fat lock
• Fat Lock : heavy-weight locking system
• Thin Lock : Light-weight locking system
– Object lock
– Recursive lock
• Lock default thin
• Lock
• IBM JDK 1.1.2

Java Performance
IBM JVM Synchronization artdb@ex-em.com | performeister.tistory.com 26

• Thin Lock
– Object Layout for Thin Lock
• Lock Word
Garbage Collection
(2bits) bit (2bits)
Pinned
Object Size G P
(double word) Array bit (1bit)
Class pointer or array size
Monitor Shape bit Thread index count
(1bit) S H T A
Monitor index Type
Hash code state (2bits)
Object Data

Hash code

Java Performance
IBM JVM Synchronization artdb@ex-em.com | performeister.tistory.com27

• Thin Lock
– Object Layout for Thin Lock
• Object Lock Word 24 bit
• Lock Word bit Lock
– Monitor Shape Lock
– 0  Thin lock
– 1  Fat lock
• Thin Lock •Recursive (nested) lock count
•Maximum 255 nested lock

Count
0 Thread ID (15 bits)
(8bits)

Java Performance
IBM JVM Synchronization artdb@ex-em.com | performeister.tistory.com 28

• Thin Lock
Spin
– Thin Locking lock
loop

Fail Own No Get
CAS
Lock? Lock? No
Success YES YES

0 Nested Lock Lock
24bit monitor shape, Count
count 0 Lock Release Fat lock
Thread ID Monitor index monitor
Count
Lock Release index

Lock word
CAS Monitor shape 1

Java Performance
IBM JVM Synchronization artdb@ex-em.com | performeister.tistory.com
29

• Lock Reservation
– Java 5
– Atomic Operation
• 75% Lock thread

• Performance
– Idea
• Object Lock (reserve) cost

Java Performance
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– Object Layout
• Object Header Lock Word
• LRV bit : Lock reserved
–1 Reserved
–0 locking System LRV bit

Thread ID Recursion Cnt* 1

(deﬁned by base locking algorithm) 0

Java Performance
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– Reserved mode
• Reserve Thread ID
• Recursion Count Lock Acquisition

Thread ID 0 1 Reserved, Released

Thread ID >0 1 Reserved ,

0 0 1 Reserved

Java Performance
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– Locking
• Reserved mode atomic operation
Recursion count
• Thread Reservation cancel
unreserve
0 0 1 Thread ID 0 1 aaaaaa 0
acquire release
acquire unreserve
Thread ID 1 1 bbbbbb 0
acquire release
unreserve
Thread ID 2 1 ccccccc 0

Reserved Mode Base Mode

Java Performance
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6장 Thread Synchronization

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