[development of large e-commerce projects] performance pressure test - Performance Monitoring - heap memory and garbage collection -39

One ： Performance monitoring jvm Memory model

1. Program counter Program Counter Register

• It records the address of the virtual machine bytecode instruction being executed ,
• This memory area is the only one in JAVA Nothing is specified in the virtual machine specification OutOfMemoryError Region

2. virtual machine VM Stack

• Describe the JAVA Memory model for method execution , Each method creates a stack frame when it executes , Used to store local variables , The stack of operands , Dynamic links , Method interface, etc
• The local variable table stores various basic data types known at compile time 、 Object reference
• If the stack depth requested by thread is not enough, it will report StackOverflowError abnormal
• If the dynamic expansion capacity of the stack is not enough, it will be reported that OutOfMemoryError abnormal
• The virtual machine stack is thread isolated , That is, each thread has its own independent virtual machine stack

3. Local method Native Stack

• The local method stack is similar to the virtual machine stack , It's just that the local method stack uses local methods

4. Pile up Heap

• Almost all object instances allocate memory on the heap
  

Two ： Pile up

All object instances and arrays are allocated on the heap . The heap is the main area managed by the garbage collector , Also known as “GC Pile up ”; It's also the place that we consider most about optimization . Heaps can be subdivided into ：

1. The new generation ：Eden Space 、From Survivor Space 、To Survivor Space

2. Old age

3. Forever / Meta space ：Java8 The old generation , suffer jvm management ,java8 After the meta space , Use physical memory directly . therefore , By default , The size of the meta space is limited only by local memory .

from Java8 Start ,HotSpot It has been completely replaced forever （Permanent Generation） remove , Instead, a A new area — Meta space （MetaSpace）

• After object creation , Go to the Cenozoic area first , If you can't put it down , Then take GC Garbage collection . Then go to the Cenozoic area , If you still can't put it down , Just go to the elderly area
• The elderly area is a situation that cannot be handled by the Cenozoic area , Will go in . If the elderly area can be put down , Then allocate memory . If you still can't put it down , Then carry out comprehensive GC
• If you still can't put it down , Memory overflow exception will be reported
• Elderly areas generally store objects with long-lasting vitality and large objects
• Eden The park can be put into the survivor area , If you can't put it in, put it in the elderly area

3、 ... and ：jconsole And jvisualvm（ Monitor local and remote applications ）

Jdk Two of my little tools jconsole、jvisualvm（ Upgraded version jconsole）; Start... From the command line , Can monitor local and local Remote applications . Remote applications need to be configured

1.jconsole

1） Get into jconsole
win+r Get into cmd, Input jconsole

Double click the service you want to connect
2） View monitoring information

2.jvisualvm（ Recommended ）

1） start-up
win+r Get into cmd, Input jvisualvm

2）jvisualvm Can do
Monitor memory leaks , Tracking garbage collection , Memory at execution time 、cpu analysis , Thread analysis …

function ： Running
Sleep ：sleep
wait for ：wait
Resident ： Idle threads in the thread pool
monitor ： Blocked threads , Waiting for lock
3） Install plug-ins for easy viewing of garbage collection （gc）

• Tools -> plug-in unit
  
• Available plug-ins -> Check for version updates
  
• If 503 Error resolution ：
  Open the url https://visualvm.github.io/pluginscenters.html
  cmd Check out your jdk edition , Find the corresponding
  
• Copy the links found below . And reset it
  
• download gc plug-in unit
  
• see gc