main is org.apache.maven.cli.MavenCli from plexus.core
// 注释掉 // set maven.home default ${user.home}/m2
set maven.home default /Users/yangtao/maven-tutorial/apache-maven-3.1.1
[plexus.core]
optionally ${maven.home}/lib/ext/*.jar
load ${maven.home}/lib/*.jar
load ${maven.home}/conf/logging
What are the benefits of knowing how garbage collection (GC) works in Java? Satisfying the intellectual curiosity as a software engineer would be a valid cause, but also, understanding how GC works can help you write much better Java applications.
This is a very personal and subjective opinion of mine, but I believe that a person well versed in GC tends to be a better Java developer. If you are interested in the GC process, that means you have experience in developing applications of certain size. If you have thought carefully about choosing the right GC algorithm, that means you completely understand the features of the application you have developed. Of course, this may not be common standards for a good developer. However, few would object when I say that understanding GC is a requirement for being a great Java developer.
This is the first of a series of “Become a Java GC Expert” articles. I will cover the GC introduction this time, and in the next article, I will talk about analyzing GC status and GC tuning examples from NHN.
The purpose of this article is to introduce GC to you in an easy way. I hope this article proves to be very helpful. Actually, my colleagues have already published a few great articles on Java Internals which became quite popular on Twitter. You may refer to them as well.
Returning back to Garbage Collection, there is a term that you should know before learning about GC. The term is “stop-the-world.” Stop-the-world will occur no matter which GC algorithm you choose. Stop-the-world means that the JVM is stopping the application from running to execute a GC. When stop-the-world occurs, every thread except for the threads needed for the GC will stop their tasks. The interrupted tasks will resume only after the GC task has completed. GC tuning often means reducing this stop-the-world time.
Generational Garbage Collection
Java does not explicitly specify a memory and remove it in the program code. Some people sets the relevant object to null or use System.gc() method to remove the memory explicitly. Setting it to null is not a big deal, but calling System.gc() method will affect the system performance drastically, and must not be carried out. (Thankfully, I have not yet seen any developer in NHN calling this method.)
In Java, as the developer does not explicitly remove the memory in the program code, the garbage collector finds the unnecessary (garbage) objects and removes them. This garbage collector was created based on the following two hypotheses. (It is more correct to call them suppositions or preconditions, rather than hypotheses.)
Most objects soon become unreachable.
References from old objects to young objects only exist in small numbers.
These hypotheses are called the weak generational hypothesis. So in order to preserve the strengths of this hypothesis, it is physically divided into two - young generation and old generation - in HotSpot VM.
Young generation: Most of the newly created objects are located here. Since most objects soon become unreachable, many objects are created in the young generation, then disappear. When objects disappear from this area, we say a “minor GC” has occurred.
Old generation: The objects that did not become unreachable and survived from the young generation are copied here. It is generally larger than the young generation. As it is bigger in size, the GC occurs less frequently than in the young generation. When objects disappear from the old generation, we say a “major GC” (or a “full GC”) has occurred.
Let’s look at this in a chart.
The permanent generation from the chart above is also called the “method area,” and it stores classes or interned character strings. So, this area is definitely not for objects that survived from the old generation to stay permanently. A GC may occur in this area. The GC that took place here is still counted as a major GC.
Some people may wonder:
What if an object in the old generation need to reference an object in the young generation?
To handle these cases, there is something called the a “card table” in the old generation, which is a 512 byte chunk. Whenever an object in the old generation references an object in the young generation, it is recorded in this table. When a GC is executed for the young generation, only this card table is searched to determine whether or not it is subject for GC, instead of checking the reference of all the objects in the old generation. This card table is managed with write barrier. This write barrier is a device that allows a faster performance for minor GC. Though a bit of overhead occurs because of this, the overall GC time is reduced.
Composition of the Young Generation
In order to understand GC, let’s learn about the young generation, where the objects are created for the first time. The young generation is divided into 3 spaces.
One Eden space
Two Survivor spaces
There are 3 spaces in total, two of which are Survivor spaces. The order of execution process of each space is as below:
The majority of newly created objects are located in the Eden space.
After one GC in the Eden space, the surviving objects are moved to one of the Survivor spaces.
After a GC in the Eden space, the objects are piled up into the Survivor space, where other surviving objects already exist.
Once a Survivor space is full, surviving objects are moved to the other Survivor space. Then, the Survivor space that is full will be changed to a state where there is no data at all.
The objects that survived these steps that have been repeated a number of times are moved to the old generation.
As you can see by checking these steps, one of the Survivor spaces must remain empty. If data exists in both Survivor spaces, or the usage is 0 for both spaces, then take that as a sign that something is wrong with your system.
The process of data piling up into the old generation through minor GCs can be shown as in the below chart:
Note that in HotSpot VM, two techniques are used for faster memory allocations. One is called “bump-the-pointer,” and the other is called “TLABs (Thread-Local Allocation Buffers).”
Bump-the-pointer technique tracks the last object allocated to the Eden space. That object will be located on top of the Eden space. And if there is an object created afterwards, it checks only if the size of the object is suitable for the Eden space. If the said object seems right, it will be placed in the Eden space, and the new object goes on top. So, when new objects are created, only the lastly added object needs to be checked, which allows much faster memory allocations. However, it is a different story if we consider a multithreaded environment. To save objects used by multiple threads in the Eden space for Thread-Safe, an inevitable lock will occur and the performance will drop due to the lock-contention. TLABs is the solution to this problem in HotSpot VM. This allows each thread to have a small portion of its Eden space that corresponds to its own share. As each thread can only access to their own TLAB, even the bump-the-pointer technique will allow memory allocations without a lock.
This has been a quick overview of the GC in the young generation. You do not necessarily have to remember the two techniques that I have just mentioned. You will not go to jail for not knowing them. But please remember that after the objects are first created in the Eden space, and the long-surviving objects are moved to the old generation through the Survivor space.
GC for the Old Generation
The old generation basically performs a GC when the data is full. The execution procedure varies by the GC type, so it would be easier to understand if you know different types of GC.
According to JDK 7, there are 5 GC types.
Serial GC
Parallel GC
Parallel Old GC (Parallel Compacting GC)
Concurrent Mark & Sweep GC (or “CMS”)
Garbage First (G1) GC
Among these, the serial GC must not be used on an operating server. This GC type was created when there was only one CPU core on desktop computers. Using this serial GC will drop the application performance significantly.
Now let’s learn about each GC type.
Serial GC (-XX:+UseSerialGC)
The GC in the young generation uses the type we explained in the previous paragraph. The GC in the old generation uses an algorithm called “mark-sweep-compact.”
The first step of this algorithm is to mark the surviving objects in the old generation.
Then, it checks the heap from the front and leaves only the surviving ones behind (sweep).
In the last step, it fills up the heap from the front with the objects so that the objects are piled up consecutively, and divides the heap into two parts: one with objects and one without objects (compact).
The serial GC is suitable for a small memory and a small number of CPU cores.
Parallel GC (-XX:+UseParallelGC)
From the picture, you can easily see the difference between the serial GC and parallel GC. While the serial GC uses only one thread to process a GC, the parallel GC uses several threads to process a GC, and therefore, faster. This GC is useful when there is enough memory and a large number of cores. It is also called the “throughput GC.”
Parallel Old GC(-XX:+UseParallelOldGC)
Parallel Old GC was supported since JDK 5 update. Compared to the parallel GC, the only difference is the GC algorithm for the old generation. It goes through three steps: mark – summary – compaction. The summary step identifies the surviving objects separately for the areas that the GC have previously performed, and thus different from the sweep step of the mark-sweep-compact algorithm. It goes through a little more complicated steps.
CMS GC (-XX:+UseConcMarkSweepGC)
As you can see from the picture, the Concurrent Mark-Sweep GC is much more complicated than any other GC types that I have explained so far. The early initial mark step is simple. The surviving objects among the objects the closest to the classloader are searched. So, the pausing time is very short. In the concurrent mark step, the objects referenced by the surviving objects that have just been confirmed are tracked and checked. The difference of this step is that it proceeds while other threads are processed at the same time. In the remark step, the objects that were newly added or stopped being referenced in the concurrent mark step are checked. Lastly, in the concurrent sweep step, the garbage collection procedure takes place. The garbage collection is carried out while other threads are still being processed. Since this GC type is performed in this manner, the pausing time for GC is very short. The CMS GC is also called the low latency GC, and is used when the response time from all applications is crucial.
While this GC type has the advantage of short stop-the-world time, it also has the following disadvantages.
It uses more memory and CPU than other GC types.
The compaction step is not provided by default.
You need to carefully review before using this type. Also, if the compaction task needs to be carried out because of the many memory fragments, the stop-the-world time can be longer than any other GC types. You need to check how often and how long the compaction task is carried out.
G1 GC
Finally, let’s learn about the garbage first (G1) GC.
If you want to understand G1 GC, forget everything you know about the young generation and the old generation. As you can see in the picture, one object is allocated to each grid, and then a GC is executed. Then, once one area is full, the objects are allocated to another area, and then a GC is executed. The steps where the data moves from the three spaces of the young generation to the old generation cannot be found in this GC type. This type was created to replace the CMS GC, which has causes a lot of issues and complaints in the long term.
The biggest advantage of the G1 GC is its performance. It is faster than any other GC types that we have discussed so far. But in JDK 6, this is called an early access and can be used only for a test. It is officially included in JDK 7. In my personal opinion, we need to go through a long test period (at least 1 year) before NHN can use JDK7 in actual services, so you probably should wait a while. Also, I heard a few times that a JVM crash occurred after applying the G1 in JDK 6. Please wait until it is more stable.
I will talk about the GC tuning in the next issue, but I would like to ask you one thing in advance. If the size and the type of all objects created in the application are identical, all the GC options for WAS used in our company can be the same. But the size and the lifespan of the objects created by WAS vary depending on the service, and the type of equipment varies as well. In other words, just because a certain service uses the GC option “A,” it does not mean that the same option will bring the best results for a different service. It is necessary to find the best values for the WAS threads, WAS instances for each equipment and each GC option by constant tuning and monitoring. This did not come from my personal experience, but from the discussion of the engineers making Oracle JVM for JavaOne 2010.
In this issue, we have only glanced at the GC for Java. Please look forward to our next issue, where I will talk about how to monitor the Java GC status and tune GC.
I would like to note that I referred to a new book released in December 2011 called “Java Performance” (Amazon, it can also be viewed from safari online, if the company provides an account), as well as “Memory Management in the Java HotSpotTM Virtual Machine,” a white paper provided by the Oracle website. (The book is different from “Java Performance Tuning.”)
By Sangmin Lee, Senior Engineer at Performance Engineering Lab, NHN Corporation.
The content of this article was originally written by Tae Jin Gu on the Cubrid blog.
该文来源于Cubrid blog(不过源地址已没有相关内容，本文翻译系转载)
When there is an obstacle, or when a Java based Web application is running much slower than expected, we need to use thread dumps. If thread dumps feel like very complicated to you, this article may help you very much. Here I will explain what threads are in Java, their types, how they are created, how to manage them, how you can dump threads from a running application, and finally how you can analyze them and determine the bottleneck or blocking threads. This article is a result of long experience in Java application debugging.
A web server uses tens to hundreds of threads to process a large number of concurrent users. If two or more threads utilize the same resources, a contention between the threads is inevitable, and sometimes deadlock occurs.
Thread contention is a status in which one thread is waiting for a lock, held by another thread, to be lifted. Different threads frequently access shared resources on a web application. For example, to record a log, the thread trying to record the log must obtain a lock and access the shared resources.
Deadlock is a special type of thread contention, in which two or more threads are waiting for the other threads to complete their tasks in order to complete their own tasks.
死锁是线程竞争的一个特殊形式.多个线程都在等待其它线程先于自己完成任务,于是陷入无尽的等待.
Different issues can arise from thread contention. To analyze such issues, you need to use the thread dump. A thread dump will give you the information on the exact status of each thread.
A thread can be processed with other threads at the same time. In order to ensure compatibility when multiple threads are trying to use shared resources, one thread at a time should be allowed to access the shared resources by using thread synchronization.
Thread synchronization on Java can be done using monitor. Every Java object has a single monitor. The monitor can be owned by only one thread. For a thread to own a monitor that is owned by a different thread, it needs to wait in the wait queue until the other thread releases its monitor.
NEW: The thread is created but has not been processed yet.
RUNNABLE: The thread is occupying the CPU and processing a task. (It may be in WAITING status due to the OS’s resource distribution.)
BLOCKED: The thread is waiting for a different thread to release its lock in order to get the monitor lock.
WAITING: The thread is waiting by using a wait, join or park method.
TIMED_WAITING: The thread is waiting by using a sleep, wait, join or park method. (The difference from WAITING is that the maximum waiting time is specified by the method parameter, andWAITING can be relieved by time as well as external changes.)
Daemon threads stop working when there are no other non-daemon threads. Even if you do not create any threads, the Java application will create several threads by default. Most of them are daemon threads, mainly for processing tasks such as garbage collection or JMX.
A thread running the ‘static void main(String[] args)’ method is created as a non-daemon thread, and when this thread stops working, all other daemon threads will stop as well. (The thread running this main method is called the VM thread in HotSpot VM.)
We will introduce the three most commonly used methods. Note that there are many other ways to get a thread dump. A thread dump can only show the thread status at the time of measurement, so in order to see the change in thread status, it is recommended to extract them from 5 to 10 times with 5-second intervals.
Use the extracted PID as the parameter of jstack to obtain a thread dump.
把pid作为参数传给jstack,以获取该pid对应java进程的线程转储.
1
[user@linux ~]$ jstack -f 5824
A Thread Dump Using jVisualVM
使用jVisualVM
Generate a thread dump by using a program such as jVisualVM.
jVisualVM是JDK提供的可视化工具.
Figure 2: A Thread Dump Using visualvm.
The task on the left indicates the list of currently running processes. Click on the process for which you want the information, and select the thread tab to check the thread information in real time. Click the Thread Dump button on the top right corner to get the thread dump file.
Obtain the process pid by using ps -ef command to check the pid of the currently running Java process.
1234
[user@linux ~]$ ps - ef | grep java
user 2477 1 0 Dec23 ? 00:10:45 ...
user 25780 25361 0 15:02 pts/3 00:00:02 ./jstatd -J -Djava.security.policy=jstatd.all.policy -p 2999
user 26335 25361 0 15:49 pts/3 00:00:00 grep java
Use the extracted pid as the parameter of kill –SIGQUIT(3) to obtain a thread dump.
Thread Information from the Thread Dump File
线程转储文件包含的线程信息
1234567891011121314151617
"pool-1-thread-13" prio=6 tid=0x000000000729a000 nid=0x2fb4 runnable [0x0000000007f0f000] java.lang.Thread.State: RUNNABLE
at java.net.SocketInputStream.socketRead0(Native Method)
at java.net.SocketInputStream.read(SocketInputStream.java:129)
at sun.nio.cs.StreamDecoder.readBytes(StreamDecoder.java:264)
at sun.nio.cs.StreamDecoder.implRead(StreamDecoder.java:306)
at sun.nio.cs.StreamDecoder.read(StreamDecoder.java:158)
- locked <0x0000000780b7e688> (a java.io.InputStreamReader)
at java.io.InputStreamReader.read(InputStreamReader.java:167)
at java.io.BufferedReader.fill(BufferedReader.java:136)
at java.io.BufferedReader.readLine(BufferedReader.java:299)
- locked <0x0000000780b7e688> (a java.io.InputStreamReader)
at java.io.BufferedReader.readLine(BufferedReader.java:362)
)
Thread name: When using Java.lang.Thread class to generate a thread, the thread will be named Thread-(Number), whereas when using java.util.concurrent.ThreadFactory class, it will be named pool-(number)-thread-(number).
Priority: Represents the priority of the threads.
Thread ID: Represents the unique ID for the threads. (Some useful information, including the CPU usage or memory usage of the thread, can be obtained by using thread ID.)
Thread status: Represents the status of the threads.
Thread callstack: Represents the call stack information of the threads.
Thread name 线程名: 使用 Java.lang.Thread类去创建线程时,线程的命名方式为Thread-(序号);当使用java.util.concurrent.ThreadFactory类去创建线程时,线程的命名方式为pool-(序号)-thread-(序号).
Priority 优先级: 表示线程的优先级.
Thread ID 线程ID: 线程的ID是唯一的(一些有用的信息,比如线程的CPU使用率或者内存使用率,可以通过线程ID找到).
Thread status 线程状态: 表示线程的状态.
Thread callstack 线程调用栈: 表示线程的调用栈信息.
Thread Dump Patterns by Type
线程转储的类型布局
When Unable to Obtain a Lock (BLOCKED)
无法获得锁的时候
This is when the overall performance of the application slows down because a thread is occupying the lock and prevents other threads from obtaining it. In the following example, BLOCKED_TEST pool-1-thread-1 thread is running with <0x0000000780a000b0> lock, while BLOCKED_TEST pool-1-thread-2 and BLOCKED_TEST pool-1-thread-3 threads are waiting to obtain <0x0000000780a000b0> lock.
"BLOCKED_TEST pool-1-thread-1" prio=6 tid=0x0000000006904800 nid=0x28f4 runnable [0x000000000785f000]
java.lang.Thread.State: RUNNABLE
at java.io.FileOutputStream.writeBytes(Native Method)
at java.io.FileOutputStream.write(FileOutputStream.java:282)
at java.io.BufferedOutputStream.flushBuffer(BufferedOutputStream.java:65)
at java.io.BufferedOutputStream.flush(BufferedOutputStream.java:123)
- locked <0x0000000780a31778> (a java.io.BufferedOutputStream)
at java.io.PrintStream.write(PrintStream.java:432)
- locked <0x0000000780a04118> (a java.io.PrintStream)
at sun.nio.cs.StreamEncoder.writeBytes(StreamEncoder.java:202)
at sun.nio.cs.StreamEncoder.implFlushBuffer(StreamEncoder.java:272)
at sun.nio.cs.StreamEncoder.flushBuffer(StreamEncoder.java:85)
- locked <0x0000000780a040c0> (a java.io.OutputStreamWriter)
at java.io.OutputStreamWriter.flushBuffer(OutputStreamWriter.java:168)
at java.io.PrintStream.newLine(PrintStream.java:496)
- locked <0x0000000780a04118> (a java.io.PrintStream)
at java.io.PrintStream.println(PrintStream.java:687)
- locked <0x0000000780a04118> (a java.io.PrintStream)
at com.nbp.theplatform.threaddump.ThreadBlockedState.monitorLock(ThreadBlockedState.java:44)
- locked <0x0000000780a000b0> (a com.nbp.theplatform.threaddump.ThreadBlockedState)
at com.nbp.theplatform.threaddump.ThreadBlockedState$1.run(ThreadBlockedState.java:7)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
at java.lang.Thread.run(Thread.java:662)
Locked ownable synchronizers:
- <0x0000000780a31758> (a java.util.concurrent.locks.ReentrantLock$NonfairSync)
"BLOCKED_TEST pool-1-thread-2" prio=6 tid=0x0000000007673800 nid=0x260c waiting for monitor entry [0x0000000008abf000]
java.lang.Thread.State: BLOCKED (on object monitor)
at com.nbp.theplatform.threaddump.ThreadBlockedState.monitorLock(ThreadBlockedState.java:43)
- waiting to lock <0x0000000780a000b0> (a com.nbp.theplatform.threaddump.ThreadBlockedState)
at com.nbp.theplatform.threaddump.ThreadBlockedState$2.run(ThreadBlockedState.java:26)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
at java.lang.Thread.run(Thread.java:662)
Locked ownable synchronizers:
- <0x0000000780b0c6a0> (a java.util.concurrent.locks.ReentrantLock$NonfairSync)
"BLOCKED_TEST pool-1-thread-3" prio=6 tid=0x00000000074f5800 nid=0x1994 waiting for monitor entry [0x0000000008bbf000]
java.lang.Thread.State: BLOCKED (on object monitor)
at com.nbp.theplatform.threaddump.ThreadBlockedState.monitorLock(ThreadBlockedState.java:42)
- waiting to lock <0x0000000780a000b0> (a com.nbp.theplatform.threaddump.ThreadBlockedState)
at com.nbp.theplatform.threaddump.ThreadBlockedState$3.run(ThreadBlockedState.java:34)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
at java.lang.Thread.run(Thread.java:662)
Locked ownable synchronizers:
- <0x0000000780b0e1b8> (a java.util.concurrent.locks.ReentrantLock$NonfairSync)
When in Deadlock Status
死锁的时候
This is when thread A needs to obtain thread B’s lock to continue its task, while thread B needs to obtain thread A’s lock to continue its task. In the thread dump, you can see that DEADLOCK_TEST-1 thread has 0x00000007d58f5e48 lock, and is trying to obtain 0x00000007d58f5e60 lock. You can also see that DEADLOCK_TEST-2 thread has 0x00000007d58f5e60 lock, and is trying to obtain 0x00000007d58f5e78 lock. Also, DEADLOCK_TEST-3 thread has 0x00000007d58f5e78 lock, and is trying to obtain 0x00000007d58f5e48 lock. As you can see, each thread is waiting to obtain another thread’s lock, and this status will not change until one thread discards its lock.
"DEADLOCK_TEST-1" daemon prio=6 tid=0x000000000690f800 nid=0x1820 waiting for monitor entry [0x000000000805f000]
java.lang.Thread.State: BLOCKED (on object monitor)
at com.nbp.theplatform.threaddump.ThreadDeadLockState$DeadlockThread.goMonitorDeadlock(ThreadDeadLockState.java:197)
- waiting to lock <0x00000007d58f5e60> (a com.nbp.theplatform.threaddump.ThreadDeadLockState$Monitor)
at com.nbp.theplatform.threaddump.ThreadDeadLockState$DeadlockThread.monitorOurLock(ThreadDeadLockState.java:182)
- locked <0x00000007d58f5e48> (a com.nbp.theplatform.threaddump.ThreadDeadLockState$Monitor)
at com.nbp.theplatform.threaddump.ThreadDeadLockState$DeadlockThread.run(ThreadDeadLockState.java:135)
Locked ownable synchronizers:
- None
"DEADLOCK_TEST-2" daemon prio=6 tid=0x0000000006858800 nid=0x17b8 waiting for monitor entry [0x000000000815f000]
java.lang.Thread.State: BLOCKED (on object monitor)
at com.nbp.theplatform.threaddump.ThreadDeadLockState$DeadlockThread.goMonitorDeadlock(ThreadDeadLockState.java:197)
- waiting to lock <0x00000007d58f5e78> (a com.nbp.theplatform.threaddump.ThreadDeadLockState$Monitor)
at com.nbp.theplatform.threaddump.ThreadDeadLockState$DeadlockThread.monitorOurLock(ThreadDeadLockState.java:182)
- locked <0x00000007d58f5e60> (a com.nbp.theplatform.threaddump.ThreadDeadLockState$Monitor)
at com.nbp.theplatform.threaddump.ThreadDeadLockState$DeadlockThread.run(ThreadDeadLockState.java:135)
Locked ownable synchronizers:
- None
"DEADLOCK_TEST-3" daemon prio=6 tid=0x0000000006859000 nid=0x25dc waiting for monitor entry [0x000000000825f000]
java.lang.Thread.State: BLOCKED (on object monitor)
at com.nbp.theplatform.threaddump.ThreadDeadLockState$DeadlockThread.goMonitorDeadlock(ThreadDeadLockState.java:197)
- waiting to lock <0x00000007d58f5e48> (a com.nbp.theplatform.threaddump.ThreadDeadLockState$Monitor)
at com.nbp.theplatform.threaddump.ThreadDeadLockState$DeadlockThread.monitorOurLock(ThreadDeadLockState.java:182)
- locked <0x00000007d58f5e78> (a com.nbp.theplatform.threaddump.ThreadDeadLockState$Monitor)
at com.nbp.theplatform.threaddump.ThreadDeadLockState$DeadlockThread.run(ThreadDeadLockState.java:135)
Locked ownable synchronizers:
- None
When Continuously Waiting to Receive Messages from a Remote Server
持续等待接收远程服务器数据的时候
The thread appears to be normal, since its state keeps showing as RUNNABLE. However, when you align the thread dumps chronologically, you can see that socketReadThread thread is waiting infinitely to read the socket.
"socketReadThread" prio=6 tid=0x0000000006a0d800 nid=0x1b40 runnable [0x00000000089ef000]
java.lang.Thread.State: RUNNABLE
at java.net.SocketInputStream.socketRead0(Native Method)
at java.net.SocketInputStream.read(SocketInputStream.java:129)
at sun.nio.cs.StreamDecoder.readBytes(StreamDecoder.java:264)
at sun.nio.cs.StreamDecoder.implRead(StreamDecoder.java:306)
at sun.nio.cs.StreamDecoder.read(StreamDecoder.java:158)
- locked <0x00000007d78a2230> (a java.io.InputStreamReader)
at sun.nio.cs.StreamDecoder.read0(StreamDecoder.java:107)
- locked <0x00000007d78a2230> (a java.io.InputStreamReader)
at sun.nio.cs.StreamDecoder.read(StreamDecoder.java:93)
at java.io.InputStreamReader.read(InputStreamReader.java:151)
at com.nbp.theplatform.threaddump.ThreadSocketReadState$1.run(ThreadSocketReadState.java:27)
at java.lang.Thread.run(Thread.java:662)
When Waiting
等待的时候
The thread is maintaining WAIT status. In the thread dump, IoWaitThread thread keeps waiting to receive a message from LinkedBlockingQueue. If there continues to be no message for LinkedBlockingQueue, then the thread status will not change.
"IoWaitThread" prio=6 tid=0x0000000007334800 nid=0x2b3c waiting on condition [0x000000000893f000]
java.lang.Thread.State: WAITING (parking)
at sun.misc.Unsafe.park(Native Method)
- parking to wait for <0x00000007d5c45850> (a java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject)
at java.util.concurrent.locks.LockSupport.park(LockSupport.java:156)
at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.await(AbstractQueuedSynchronizer.java:1987)
at java.util.concurrent.LinkedBlockingDeque.takeFirst(LinkedBlockingDeque.java:440)
at java.util.concurrent.LinkedBlockingDeque.take(LinkedBlockingDeque.java:629)
at com.nbp.theplatform.threaddump.ThreadIoWaitState$IoWaitHandler2.run(ThreadIoWaitState.java:89)
at java.lang.Thread.run(Thread.java:662)
When Thread Resources Cannot be Organized Normally
线程资源管理失控的时候
Unnecessary threads will pile up when thread resources cannot be organized normally. If this occurs, it is recommended to monitor the thread organization process or check the conditions for thread termination.
From the application, find out which thread is using the CPU the most.
Acquire the Light Weight Process (LWP) that uses the CPU the most and convert its unique number (10039) into a hexadecimal number (0x2737).
找出应用里最占用CPU的那个线程,并获得它的轻量进程 Light Weight Process (LWP). 把这个唯一的LWP数字 (10039) 转成十六进制 (0x2737).
After acquiring the thread dump, check the thread’s action.
得到线程转储文件之后, 检查线程执行的动作.
Extract the thread dump of an application with a PID of 10029, then find the thread with an nid of 0x2737.
把PID为10029的应用进行线程转储, 然后找到nid为 0x2737 的线程.
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"NioProcessor-2" prio=10 tid=0x0a8d2800 nid=0x2737 runnable [0x49aa5000] java.lang.Thread.State: RUNNABLE
at sun.nio.ch.EPollArrayWrapper.epollWait(Native Method)
at sun.nio.ch.EPollArrayWrapper.poll(EPollArrayWrapper.java:210)
at sun.nio.ch.EPollSelectorImpl.doSelect(EPollSelectorImpl.java:65)
at sun.nio.ch.SelectorImpl.lockAndDoSelect(SelectorImpl.java:69)
- locked <0x74c52678> (a sun.nio.ch.Util$1)
- locked <0x74c52668> (a java.util.Collections$UnmodifiableSet)
- locked <0x74c501b0> (a sun.nio.ch.EPollSelectorImpl)
at sun.nio.ch.SelectorImpl.select(SelectorImpl.java:80)
at external.org.apache.mina.transport.socket.nio.NioProcessor.select(NioProcessor.java:65)
at external.org.apache.mina.common.AbstractPollingIoProcessor$Worker.run(AbstractPollingIoProcessor.java:708)
at external.org.apache.mina.util.NamePreservingRunnable.run(NamePreservingRunnable.java:51)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
at java.lang.Thread.run(Thread.java:662)
Extract thread dumps several times every hour, and check the status change of the threads to determine the problem.
按一小时的时间间隔获取多个线程转储文件, 查看线程状态的变化来定位问题.
Example 2: When the Processing Performance is Abnormally Slow
例 2: 应用的效率过低
After acquiring thread dumps several times, find the list of threads with BLOCKED status.
获取多个线程转储文件之后, 找到状态为 BLOCKED 的所有线程.
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"DB-Processor-13" daemon prio=5 tid=0x003edf98 nid=0xca waiting for monitor entry [0x000000000825f000]
java.lang.Thread.State: BLOCKED (on object monitor)
at beans.ConnectionPool.getConnection(ConnectionPool.java:102)
- waiting to lock <0xe0375410> (a beans.ConnectionPool)
at beans.cus.ServiceCnt.getTodayCount(ServiceCnt.java:111)
at beans.cus.ServiceCnt.insertCount(ServiceCnt.java:43)
"DB-Processor-14" daemon prio=5 tid=0x003edf98 nid=0xca waiting for monitor entry [0x000000000825f020]
java.lang.Thread.State: BLOCKED (on object monitor)
at beans.ConnectionPool.getConnection(ConnectionPool.java:102)
- waiting to lock <0xe0375410> (a beans.ConnectionPool)
at beans.cus.ServiceCnt.getTodayCount(ServiceCnt.java:111)
at beans.cus.ServiceCnt.insertCount(ServiceCnt.java:43)
"" "DB-Processor-3"" daemon prio=5 tid=0x00928248 nid=0x8b waiting for monitor entry [0x000000000825d080]
java.lang.Thread.State: RUNNABLE
at oracle.jdbc.driver.OracleConnection.isClosed(OracleConnection.java:570)
- waiting to lock <0xe03ba2e0> (a oracle.jdbc.driver.OracleConnection)
at beans.ConnectionPool.getConnection(ConnectionPool.java:112)
- locked <0xe0386580> (a java.util.Vector)
- locked <0xe0375410> (a beans.ConnectionPool)
at beans.cus.Cue_1700c.GetNationList(Cue_1700c.java:66)
at org.apache.jsp.cue_1700c_jsp._jspService(cue_1700c_jsp.java:12)
Acquire the list of threads with BLOCKED status after getting the thread dumps several times.
If the threads are BLOCKED, extract the threads related to the lock that the threads are trying to obtain.
Through the thread dump, you can confirm that the thread status stays BLOCKED because <0xe0375410> lock could not be obtained. This problem can be solved by analyzing stack trace from the thread currently holding the lock.
There are two reasons why the above pattern frequently appears in applications using DBMS. The first reason is inadequate configurations. Despite the fact that the threads are still working, they cannot show their best performance because the configurations for DBCP and the like are not adequate. If you extract thread dumps multiple times and compare them, you will often see that some of the threads that were BLOCKED previously are in a different state.
The second reason is the abnormal connection. When the connection with DBMS stays abnormal, the threads wait until the time is out. In this case, even after extracting the thread dumps several times and comparing them, you will see that the threads related to DBMS are still in a BLOCKED state. By adequately changing the values, such as the timeout value, you can shorten the time in which the problem occurs.
When a thread is created using java.lang.Thread object, the thread will be named Thread-(Number). When a thread is created using java.util.concurrent.DefaultThreadFactory object, the thread will be named pool-(Number)-thread-(Number). When analyzing tens to thousands of threads for an application, if all the threads still have their default names, analyzing them becomes very difficult, because it is difficult to distinguish the threads to be analyzed.
Therefore, you are recommended to develop the habit of naming the threads whenever a new thread is created.
所以,要养成给线程命名的好习惯.
When you create a thread using java.lang.Thread, you can give the thread a custom name by using the creator parameter.
使用 java.lang.Thread时, 通过构造函数给线程一个定制的名字.
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public Thread(Runnable target, String name);
public Thread(ThreadGroup group, String name);
public Thread(ThreadGroup group, Runnable target, String name);
public Thread(ThreadGroup group, Runnable target, String name, long stackSize);
When you create a thread using java.util.concurrent.ThreadFactory, you can name it by generating your own ThreadFactory. If you do not need special functionalities, then you can use MyThreadFactory as described below:
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ThreadFactory;
import java.util.concurrent.atomic.AtomicInteger;
public class MyThreadFactory implements ThreadFactory {
private static final ConcurrentHashMap<String, AtomicInteger> POOL_NUMBER =
new ConcurrentHashMap<String, AtomicInteger>();
private final ThreadGroup group;
private final AtomicInteger threadNumber = new AtomicInteger(1);
private final String namePrefix;
public MyThreadFactory(String threadPoolName) {
if (threadPoolName == null) {
throw new NullPointerException("threadPoolName");
}
POOL_NUMBER.putIfAbsent(threadPoolName, new AtomicInteger());
SecurityManager securityManager = System.getSecurityManager();
group = (securityManager != null) ? securityManager.getThreadGroup() :
Thread.currentThread().getThreadGroup();
AtomicInteger poolCount = POOL_NUMBER.get(threadPoolName);
if (poolCount == null) {
namePrefix = threadPoolName + " pool-00-thread-";
} else {
namePrefix = threadPoolName + " pool-" + poolCount.getAndIncrement() + "-thread-";
}
}
public Thread newThread(Runnable runnable) {
Thread thread = new Thread(group, runnable, namePrefix + threadNumber.getAndIncrement(), 0);
if (thread.isDaemon()) {
thread.setDaemon(false);
}
if (thread.getPriority() != Thread.NORM_PRIORITY) {
thread.setPriority(Thread.NORM_PRIORITY);
}
return thread;
}
}
Obtaining More Detailed Information by Using MBean
通过MBean获得更详细的信息
You can obtain ThreadInfo objects using MBean. You can also obtain more information that would be difficult to acquire via thread dumps, by using ThreadInfo.
You can acquire the amount of time that the threads WAITed or were BLOCKED by using the method in ThreadInfo, and by using this you can also obtain the list of threads that have been inactive for an abnormally long period of time.
In this article I was concerned that for developers with a lot of experience in multi-thread programming, this material may be common knowledge, whereas for less experienced developers, I felt that I was skipping straight to thread dumps, without providing enough background information about the thread activities. This was because of my lack of knowledge, as I was not able to explain the thread activities in a clear yet concise manner. I sincerely hope that this article will prove helpful for many developers.
gem sources --remove http://rubygems.org/gem sources -a http://ruby.taobao.org/gem sources -l
进入Git Bash cmd，执行如下命令安装bundler
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gem install bundler
安装Octopress 下载Octopress源代码
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git clone git://github.com/imathis/octopress.git octopress
cd octopress # If you use RVM, You'll be asked if you trust the .rvmrc file (say yes).
ruby --version # Should report Ruby 1.9.2
安装依赖模块
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cd octopress
vi GemFile
将行 ： source "http://rubygems.org/"改为 ： source "http://ruby.taobao.org/"$ bundle install
