Java 8 고급 (4/6)

Java 나흘째
TmaxSoft R&D Center
1
113년 10월 15일 화

자바 교육 계획
1일 : Language, String,
ClassLoader, Proxy
2일 : GC, Collections
3일 : Thread, Java Memory
Model
4일 : AQS, ForkJoin,
Concurrent Utils
5일 : IO, Generics, Annotation,
RMI
6일 : Unsafe, Lambda (Java 8)
2
213년 10월 15일 화

오늘 내용
Hotspot JVM Locking Optimization Review
java.util.concurrent
Atomic CAS
AbstractQueueSynchronizer, Lock & Condition
Concurrent Collections
Fork-Join framework
3
313년 10월 15일 화

지난 시간 리뷰 :
Hotspot JVM Lock Optimization
volatile (JMM)
park/unpark
4
413년 10월 15일 화

Locking
Lightweight locking
Atomic CAS
Heavyweight locking
mutex
Using the assumption that most locking is
uncontended
try lightweight ﬁrst and if failed then use heavyweight
5
513년 10월 15일 화

Mark word (Object Header)
2bits are used for lock state
01 : unlocked
00 : light-weight locked
10 : heavy-weight locked
11 : marked for GC
1 more bit is used for biased locking (obtained via CAS)
X01 : unlocked
X is 0 for not biased, X is 1 for biased or biasable
2bit epoch is used for indicate the bias-holding thread’s lock/unlock
6
Hashcode Age 0 01
lock record address 00
monitor address 10
forward address, etc.. 11
thread id Age 1 01epoch
613년 10월 15일 화

Biased Locking
(quickly reacquirable lock)
make it cheap for a single thread to re-acquire the same
lock
when biased bit is 1
when the biased thread locks/unlocks
word에서 thread id 만을 검사하고 epoch bit 변경
when other thread locks
revoke (expensive) : 먼저 light-weight lock 상태로 되돌린
다
7
713년 10월 15일 화

volatile
단순한 메모리 변수가 아니라 barrier를 필요로 함
thread가 접근한 변수들의 visibility와 ordering 모두 만족해야
x86 등에서 실제 구현 형태
보통 volatile ﬁeld에 set 후에 StoreLoad(x86에서는 mfence
등에 해당)를 호출하는 것으로 구현
CPU마다 다르긴 하나 x86, SPARC에서는 다른 instrunction
은 모두 no-op
8
813년 10월 15일 화

park/unpark 모델 (JDK 5)
9
Park Unpark
Parked
Unparked
None
Impossible None
None Unparked
Parked Unparked
현재 상태
Operation
[표] 현재 상태 + Operation => 다음 상태
park/unpark는 biased를 사용하지 않으므로 thread간 lock 경
쟁이 심할 경우 synchronized에 비해 더 효율적
synchronized는 single thread이거나 contention이 약하면 더
효율적
913년 10월 15일 화

java.util.concurrent (JDK 5)
10
1013년 10월 15일 화

Atomic CAS
11
package java.util.concurrent.atomic
e.g, AtomicInteger
volatile value
compareAndSet : cpu dependent instruction
other methods : set, getAndIncrement, ...
while loop (spin lock) and CAS
1113년 10월 15일 화

AbstractQueuedSynchronizer (1)
package java.util.concurrent.locks
a small common framework of most provided synchronizers
acquire : blocks the calling thread unless/until the synchronization
state allows it to proceed
release : changes synchronization state in a way that may allow
one or more blocked threads to unblock
tryAcquire/tryRelease : non-blocking version (child class should
implement if necessary)
acquireShared/releaseShared : shared mode
12
1213년 10월 15일 화

acquire operation
while (synchronization state does not allow acquire) {
enqueue current thread if not already queued;
possibly block current thread;
}
dequeue current thread if it was queued;
release operation
update synchronization state;
if (state may permit a blocked thread to acquire)
unblock one or more queued threads;
13
1313년 10월 15일 화

int synchronization state (32bit)
blocking
LockSupport.park
LockSupport.unpark(Thread)
FIFO wait queue of blocked threads (CLH queue)
Condition queue of blocked threads
class AbstractQueuedSynchronizer.ConditionObject
14
1413년 10월 15일 화

class Mutex {
class Sync extends AbstractQueuedSynchronizer {
public boolean tryAcquire(int ignore) {
return compareAndSetState(0, 1); // locked 이면 state == 1
}
public boolean tryRelease(int ignore) {
setState(0); return true; // unlocked 이면 state == 0
}
}
private ﬁnal Sync sync = new Sync();
public void lock() { sync.acquire(0); }
public void unlock() { sync.release(0); }
}
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1513년 10월 15일 화

ReentrantLock (and Condition) : state is recursive lock count
ReentrantReadWriteLock : state 16bit writer count, 16bit reader count
Semaphore : state is current count, acquire calls acquireShared to
decrement or block (if <= 0), release calls releaseShared to increment
and unblock (if > 0)
CountDownLatch : state is count, countDown calls releaseShared
(decrements 1), await calls acquireShared (if state is 0, return
immediately)
FutureTask : state is state machine (initial|running|cancelled|done). set/
cancel calls release which eventually unblock the awaiting threads which
are trying to acquire to get the value
16
1613년 10월 15일 화

Lock overhead
Measured in JDK 5
(So, builtin is not
meaningful)
17
1713년 10월 15일 화

ThreadPoolExecutor
Core pool size, Max pool size, blocking queue size
java.util.concurrent.Executors helper class
newFixedThreadPool (Core pool size == max pool size)
newCachedThreadPool
Core pool size = 0, Max pool size = Integer.MAX_VALUE,
SynchronousQueue
newSingleThreadExecutor (Unbounded queue)
newScheduledThreadPool (Can execute by schedule)
18
1813년 10월 15일 화

Fork/Join (JDK 7)
a parallel version of divide-conquer algorithm
Result solve(Problem problem) {
if (problem is small)
directly solve problem
else {
split problem into independent parts
fork new subtasks to solve each part
join all subtasks
compose result from subresults
}
}
19
1913년 10월 15일 화

Fork/Join
work-stealing
deque : array, top, base
push/pop/take(steal)
owner thread : stack(LIFO) push/pop
idle thread : take and if fail then yield or set inactive
20
base top
deque
push
pop
steal (thread 2)
(thread 1)
2013년 10월 15일 화

Fork/Join
ForkJoinTask methods
child classes should override exec(), setRawResult(), getRawResult()
public static void invokeAll(ForkJoinTask<?> t1, ForkJoinTask<?> t2) {
t2.fork();
t1.invoke();
t2.join();
}
ForkJoinPool : can specify parallelism (thread pool size)
RecursiveAction (result가 없는 ForkJoinTask)
RecursiveTask (result가 있는 ForkJoinTask)
21
2113년 10월 15일 화

Concurrent Collections
CopyOnWriteArrayList : read-mostly, high contention
LinkedBlockingQueue : 2-lock queue algorithm
separation of takeLock and putLock
ConcurrentHashMap
lock striping
no lock when get (multiple access to volatile data)
ConcurrentSkipListMap
concurrent version of skip-list
a concurrent replacement for TreeMap
22
2213년 10월 15일 화

다음 시간 예정
some more concurrent collections
Generics
Annotation
I/O
Object Serialization
RMI
23
2313년 10월 15일 화

Java 8 고급 (4/6)

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