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
6

• Cooperation
– Cooperation
•

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

Buffer full

Java Performance
7

• 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
12

• 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
15

– 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
16


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
17


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
18

• Atomic Operation multiprocessor

• lock Thread
locking
• Thread Lock

• Thread lock

–

Java Performance
19

• 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
22

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

Java Performance
23

• 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
30

– 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
31

– 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
32

– 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
33

6장 Thread Synchronization

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