Several high-frequency JVM interview questions

Preface

Hello everyone . I sorted out several high-frequency JVM Interview questions , Guys, come on ！

• JVM Why is memory divided into new generations , Old age

• Why is the new generation divided into Eden and Survivor

• JVM Once complete GC How is the process

• CMS Collectors and G1 The difference between collectors

• JVM tuning

• CPU How to check

1. JVM Why is memory divided into new generations , Old age ？

1.1 JVM Shared memory partition

• Shared memory area = Lasting generation + Pile up （jdk1.8 And above jvm Abandoned the lasting generation ）

• Lasting generation = Method area + other

• Java Pile up = Old age + The new generation

• The new generation = Eden + S0 + S1

1.2 Why is it divided into the old generation and the new generation

• The new generation ： It mainly stores newly created objects , Memory size is usually small , Garbage collection will be more frequent .

• Old age (Tenured Gen)： Main storage JVM Objects with a long life cycle （ After several times of Young GC After recycling, it still exists ）, Or big objects , Garbage collection is also relatively less frequent .

Why divide the old age and the new generation , The main   The object size is different , Object life cycle is different . After division , Provide garbage collection efficiency , Save resources , Improve object utilization   wait .

2. Why is the Cenozoic divided into Eden and Survivor？ Why set two Survivor

• without Survivor,Eden Every time the district goes on Minor GC, The surviving object will be sent to the old age . The old generation is soon filled , Trigger Major GC. The memory space of the old generation is much larger than that of the new generation , Do it once. Full GC The ratio of time spent Minor GC Much longer , So it needs to be divided into Eden and Survivor.

• Survivor The meaning of being , Is to reduce the number of people sent to the old age , And then reduce Full GC Happen ,Survivor Pre screening guarantee , Only experience 16 Time Minor GC Objects that can survive in the new generation , Will be sent to the old age .

• Set the two Survivor The biggest advantage of the zone is that it solves fragmentation , The newly created object is Eden in , Experience once Minor GC,Eden The live object in will be moved to the first piece survivor space S0,Eden Be emptied ; etc. Eden The district is full again , Just trigger it again Minor GC,Eden and S0 The live objects in will be copied to the second block survivor space S1（ This process is very important , Because this replication Algorithm To ensure the S1 From S0 and Eden Two live objects occupy continuous memory space , To avoid fragmentation ）

3. JVM Once complete GC How is the process

• Java The heap is divided into old and new generations

• The new generation Divided into Eden And two Survivor(S0、S1)

• When Eden The area is full of space , Java A virtual opportunity triggers Minor GC, To collect the new generation of garbage , Surviving objects , It will be transferred to Survivor District .

• Big object （ Need a lot of continuous memory space Java object , Like that long string ） Go straight into old age ;

• If the object is Eden born , And after the first time Minor GC Still alive after , And be Survivor If it can be accommodated , Age is set to 1, Every time I get through Minor GC, Age +1, If the age exceeds a certain limit （15）, They are promoted to old age . That is, the long-term survival of the object into the old state .

• The old generation is full and unable to accommodate more objects ,Minor GC After that it usually goes on Full GC,Full GC   Clean up the entire memory heap – Including the young and the old .

• Major GC It happened in the old days GC, Clean up the old age area , Often accompanied at least once Minor GC, Than Minor GC slow 10 More than times .

4. CMS Collectors and G1 The difference between collectors

• CMS Collector is the collector of the old days , Generally with the new generation Serial and ParNew Collector used together ;G1 Collector collection range is old and new generation , It does not need to be used in combination with other collectors ;

• CMS A collector is a way to get   Minimum recovery pause time   Collector for target , G1 The collector   Predictable garbage collection pause time  .

• CMS Collector is used “ Mark - eliminate ” Algorithm Garbage collection , It is easy to generate memory fragmentation ; and G1 The collector uses “ Mark - Arrangement ” Algorithm , Space integration , Reduced memory space fragmentation .

• CMS and G1 The recycling process is different , The garbage collection process is different .CMS yes   Initial marker 、 Concurrent Tags 、 Re label 、 Concurrent cleanup  ;G1 Is the initial tag 、 Concurrent Tags 、 Final marker 、 Screening and recovery .

5. JVM tuning

JVM Tuning is actually through tuning JVM Parameters , That is, the optimization of garbage collector and memory allocation , To achieve higher throughput and performance .JVM Tuning mainly adjusts the following parameters

 

Stack memory dependent

• -Xms Set the size of the initial heap

• -Xmx Set the maximum heap size

• -Xmn Set younger generation size , It is equivalent to configuring at the same time -XX:NewSize and -XX:MaxNewSize For the same value

• -Xss   Stack size per thread

• -XX:NewSize Set younger generation size (for 1.3/1.4)

• -XX:MaxNewSize Young generation maximum (for 1.3/1.4)

• -XX:NewRatio The ratio of the younger generation to the older generation ( Remove the permanent generation )

• -XX:SurvivorRatio Eden District and Survivor The ratio of the region

• -XX:PretenureSizeThreshold When the created object exceeds the specified size , Assign objects directly to the older generation .

• -XX:MaxTenuringThreshold Set the object in Survivor Maximum age threshold for replication , Over the threshold to the old age

Garbage collector related

• -XX:+UseParallelGC： Choose garbage collector as parallel collector .

• -XX:ParallelGCThreads=20： Configure the number of threads for the parallel collector

• -XX:+UseConcMarkSweepGC： Set up concurrent collection of elderly generation .

• -XX:CMSFullGCsBeforeCompaction=5 Because the concurrent collector does not compress the memory space 、 Arrangement , So running for a while will produce “ debris ”, Reduce the efficiency of operation . This value sets to run 5 Time GC Compress the memory space later 、 Arrangement .

• -XX:+UseCMSCompactAtFullCollection： Turn on the compression of the old . May affect performance , But it can eliminate debris

Auxiliary information related

• -XX:+PrintGCDetails Print GC Details

• -XX:+HeapDumpOnOutOfMemoryError Give Way JVM Automatically generate memory snapshot in case of memory overflow , Check the problem with

• -XX:+DisableExplicitGC Disable the system System.gc(), To prevent manual triggering FGC Cause problems .

• -XX:+PrintTLAB see TLAB Use of space

6. CPU How to check

• Input jps, Get the process number .

• top -Hp pid Get the... Of all threads in this process CPU Time consuming performance

• printf %x cpu The highest thread （ Turn into 16 Base number ）

• jstack Command to view the current java The stack state of the process | grep that 16 Base number

• perhaps jstack -l > /tmp/output.txt Type the stack information into a txt file .

If this article helps you , Don't forget to give me a 3 even , give the thumbs-up , forward , Comment on ,

I'll see you next time ！ How to get answers ： Liked Commented Closed ~

Learn more knowledge and skills , Follow up with private bloggers （03）