Shulou(Shulou.com)06/01 Report--

Today's series of articles introduce these four tools:

Jstat (sun.tools.jstat), whose full name is Java Virtual Machine Statistics Monitoring Tool, is a tool used to monitor the status of JVM.

Jstack (sun.tools.jstack), its full name is Java Stack Trace, is used to do java stack tracking tool.

Jmap (sun.tools.jmap), whose full name is Java Memory Map, is the memory management tool of java.

Jhat (com.sun.tools.hat.Main), whose full name is Java Heap Analyse Tool, is java's heap analysis tool.

With these four tools, basically every aspect of JVM during operation can be covered. Below we will explain each tool in detail. This article will first explain the specific use of Jstat.

JStat command

Jstat mainly collects various parameters during the operation of JVM, including viewing the loading of classes, the use of metadata space in jc and so on.

Let's take a look at Jstat's command:

Jstat outputOptions [- t] [- h lines] vmid [interval [count]] outputOptions-choose what to output t-output timestamp in the first column, starting from the start of the target JVM, h-select the name of the output column every n rows. The default is 0, indicating the name of the output column vmid-JVM in the first row only. You can use jps to view the interval-jstat output. The default unit is millisecond count-indicates how much sampled data needs to be displayed. The default is unlimited, indicating that the output will be sampled until JVM stops JStat Output Options.

We talked about the basic format of the JStat command above, and in this section we will explain the output options of JStat in detail.

Using jstat-options, you can see several options options supported by jstat:

Jstat-options-class-compiler-gc-gccapacity-gccause-gcmetacapacity-gcnew-gcnewcapacity-gcold-gcoldcapacity-gcutil-printcompilationclass

To output class loader statistics, let's take an example:

Jstat-class-t 53528 100 5Timestamp Loaded Bytes Unloaded Bytes Time 19822.8 5214 10752.5 0 0 2.91 19823.0 5214 10752.5 0 0 2.91 19823.0 5214 10752.5 0 0.0 2.91 19823.2 5214 10752.5 0 0.02 . 91 19823.2 5214 10752.5 0 0.0 2.91

In the above example, 53528 is the pid,100 of the target JVM, indicating that the sampling interval is 100ms, indicating that only five pieces of data are displayed at the end.

The Timestamp above indicates the time when the JVM was started.

Loaded-how many class are loaded Bytes-loaded class size Unloaded-how many class are reloaded Bytes-reverse loaded class size Time-time spent loading and reloading class compiler

Compiler counts information about the Java HotSpot VM Just-in-Time JIT just-in-time compiler.

The JIT just-in-time compiler is produced to improve the execution speed of the code. JVM for some hot code, such as multiple loops and frequently used methods. For these hot codes, JIT compiles them into machine code to speed up execution.

Let's take the example of JVM just now:

Jstat-compiler 53528 100 5Compiled Failed Invalid Time FailedType FailedMethod 2675 00 5.35 0 2675 00 5.35 0 2675 00 5.35 0 2675 00 5.35 0 2675 00 5.35 0Compiled-number of compilation tasks performed Failed-number of compilation task failures Invalid-number of compilation tasks with invalid settings Time-time spent performing the compilation task FailedType-compilation type FailedMethod of the last failed compilation-method name of the last compilation failure gc

Gc counts gc heap information.

Jstat-gc 53528 5 S0C S1C S0U S1U EC EU OC OU MC MU CCSC CCSU YGC YGCT FGC FGCT CGC CGCT GCT 0.00.0 3978.2 56320.0 33792.0 15360.0 15215.0 21552.0 20680.2688.0 2468.0 4 0.025 0 0.000 2 0.003 0 . 028 0.0 4096.0 0.0 3978.2 56320.0 33792.0 15360.0 15215.0 21552.0 20680.9 2688.0 2468.0 4 0.025 0 0.000 2 0.003 0.028 0.0 4096.0 0.0 3978.2 56320.0 33792.0 15360.0 15215.0 21552.0 20680.9 2688.0 2468.0 4 0.025 0 0.000 2 0.003 0.028 0.0 4096.0 0.0 3978.2 56320.0 33792.0 15360.0 15215.0 21552.0 20680.9 2688.0 2468.0 4 0.025 0 0.000 2 0.003 0.028 0.0 4096.0 0.0 3978.2 56320.0 33792.0 15360.0 15215.0 21552.0 20680.9 2688.0 2468. 0 4 0.025 0 0.000 2 0.003 0.028

There are many output parameters of gc, which we will explain one by one, so that people will have a better understanding of the running kernel of gc:

Capacity of S0C-survivor 0 area, capacity of S1C-survivor 1 area in KB, capacity in KB

Young Gen is divided into 1 Eden Space and 2 Suvivor Space. When the object is first created, it is placed in the Eden space. During garbage collection, Eden Space and a Suvivor Space are scanned. If an object in Eden Space is found to be still valid during garbage collection, it will be copied to another Suvivor Space.

After many times of scanning, it is found that objects that are still valid will be put into Old Gen, indicating that they have a longer life cycle and can reduce garbage collection time.

Usage size of S0U-survivor 0 area, use size of S1U-survivor 1 area in KB, capacity of EC-Eden area in KB, use of EU-Eden area in KB, capacity of OC-old area in KB, use of OU-old area in KB, Committed Size of MC-Metaspace metadata area in KB, size of MU-Metaspace metadata area in KB and KB

Before JDK8, information that rarely changed, such as class definitions, bytecodes, and constants, was placed in the persistent generation Perm Gen. But after JDK8, Perm Gen has been cancelled and is now called Metaspace. Metaspace is not in the java virtual machine, it uses local memory. Metaspace can be controlled through-XX:MaxMetaspaceSize.

Committed Size of CCSC-Compressed class, in KB, CCSU-Compressed class usage size, in KB

CompressedClass Space, it's related to-XX:+UseCompressedOops,-XX:+UseCompressedClassesPointers. It is actually a pointer compression, and you can use 32bits to represent the pointer of the previous 64bits.

Number of GC of YGC-young generation YGCT-GC time of young generation FGC-number of full GC FGCT-time of full GC GCT-total GC time gccapacity

Gccapacity counts statistics such as the creation and size of memory pools.

Jstat-gccapacity 53528 5 NGCMN NGCMX NGC S0C S1C EC OGCMN OGCMX OGC OC MCMN MCMX MC CCSMN CCSMX CCSC YGC FGC CGC 0.0 716800.0 60416.0 4096.0 56320.0 716800.0 15360.0 15360.0 1069056.0 21552.0 0.0 1048576.0 2688 . 04 0 20.0 716800.0 60416.0 0.0 4096.0 56320.0 0.0 716800.0 15360.0 15360.0 0.0 1069056.0 21552.0 0.0 1048576.0 2688.0 40 20.0 716800.0 60416.0 0.0 4096.0 56320.0 0.0 716800.0 15360.0 15360. 00.0 1069056.0 21552.0 0.0 1048576.0 2688.0 40 20.0 716800.0 60416.0 0.0 4096.0 56320.0 0.0 716800.0 15360.0 15360.0 0.0 1069056.0 21552.0 0.0 1048576.0 2688.0 40 20.0 716800.0 60416.0 0.0 4096 .0 56320.0 716800.0 15360.0 15360.0 1069056.0 21552.00.0 1048576.0 2688.040 2NGCMN-smallest Cenozoic size NGCMX-maximum Cenozoic size NGC-capacity of current Cenozoic size S0C-survivor 0 region capacity of S1C-survivor 1 region capacity of EC-Eden region OGCMN-minimum old Generation size OGCMX-maximum old generation size OGC-current old generation size OC-current old space size MCMN-minimum metaspace size MCMX-maximum metaspace size MC-Metaspace Committed SizeCCSMN-Compressed class space minimum capacity CCSMX-Compressed class space maximum capacity CCSC-Compressed class Committed SizeYGC-young generation GC number FGC-full GC number gcnew

Gcnew represents the statistics of the new generation.

Jstat-gcnew 53528 4096.0 5 S0C S1C S0U S1U TT MTT DSS EC EU YGC YGCT 0.00.0 3978.2 15 5120.0 56320.0 33792.0 4 0.025 4096.0 0.03978.2 155120.056320.033792.0 4 0.025 0.04096.0 0.03978.2 155120.056320.033792.0 4 0.025 4096.0 4096.0 3978.2 15 15 5120.0 56320.0 33792.0 4 0.025 4096.0 0.03978.2 155120.056320.033792.04 0.025S0C-survivor 0 area capacity S1C-survivor 1 area capacity S0U-survivor 0 area usage size The usage size of S1U-survivor 1 area in KB and TT-Tenuring threshold in KB. ) MTT-maximum Tenuring thresholdDSS-capacity of required survivor sizeEC-Eden area EU-use of Eden area, in KB, YGC-number of Cenozoic GC YGCT-time gcnewcapacity required for Cenozoic GC

Gcnewcapacity counts the indicators of the new generation, and the results are very similar to those of gccapacity:

Jstat-gcnewcapacity 53528 5 NGCMN NGCMX NGC S0CMX S0C S1CMX S1C ECMX EC YGC FGC CGC 0.0 716800.0 60416.0 716800.0 4096.0 716800.0 56320.0 40 20.0 716800.0 60416.0 0.0 716800.0 4096.0 716800.0 56320.0 40 20.0 716800.0 60416.0 0.0 0.0 716800.0 4096.0 716800.0 56320.0 40 20.0 716800.0 60416.0 0.0 0.0 716800.0 4096.0 716800.0 56320.0 40 20.0 716800.0 60416.0 0.00.0 716800.0 4096.0 716800.0 56320.040 2NGCMN-smallest Cenozoic size NGCMX-maximum Cenozoic size NGC-current Cenozoic size S0CMX-survivor 0 area capacity maximum value S0C-survivor 0 region capacity maximum value S1CMX-survivor 1 region capacity maximum value S1C-survivor 1 region capacity EC-Eden zone The capacity of the domain ECMX-the maximum capacity of the Eden area YGC-the number of GC of young generation FGC-the number of times of full GC gcold

Gcold statistics the information of the Old generation.

Jstat-gcold 53528 5 MC MU CCSC CCSU OC OU YGC FGC FGCT CGC CGCT GCT 21552.0 20680.9 2688.0 2468.0 15360.0 15215.0 4 0 0.000 2 0.003 0.028 21552.0 20680.9 2688.0 2468.0 15360.0 15215.0 400. 000 2 0.003 0.028 21552.0 20680.9 2688.0 2468.0 15360.0 15215.0 4 0 0.000 2 0.003 0.028 21552.0 20680.9 2688.0 2468.0 15360.0 15215.0 4 0 0.000 2 0.003 0.028 21552.0 20680.9 2688.0 2468.0 15360. 0 15215.0 4 0 0.000 2 0.003 0.028

Several indicators of the results have been introduced before, so I will not repeat them here.

Gcoldcapacity

Gcoldcapacity represents the capacity information of the Old generation.

Jstat-gcoldcapacity 53528 100 5 OGCMN OGCMX OGC OC YGC FGC FGCT CGC CGCT GCT 0.0 716800.0 15360.0 15360.0 4 0 0.000 2 0.003 0.028 716800.0 15360.0 15360.0 4 0 0.000 2 0.003 0.028 0.0 716800.0 15360.0 15360.0 4 0 0.000 2 0.003 0.028 0.0 716800.0 15360.0 15360.0 4 0 0.000 2 0.003 0.028 0.0 716800.0 15360.0 15360.0 4 0 0.000 2 0.003 0.028gcmetacapacity

Gcmetacapacity counts the capacity information of metadata areas.

Jstat-gcmetacapacity 53528 100 5 MCMN MCMX MC CCSMN CCSMX CCSC YGC FGC FGCT CGC CGCT GCT 0.0 1069056.0 21552.0 1048576.0 2688.0 4 0 0.000 2 0.003 0.028 0.0 1069056.0 21552.0 0.0 1048576.0 2688.0 4 0 0.000 2 0.003 0.028 0.0 1069056.0 21552.0 0.0 1048576.0 2688.0 4 0 0.000 2 0.003 0.028 0.0 1069056.0 21552.0 0.0 1048576.0 2688.0 4 0 0.000 2 0.003 0.028 0.0 1069056 . 0 21552.0 0.0 1048576.0 2688.0 4 0 0.000 2 0.003 0.028gcutil

Gcutil counts the overall situation of GC.

Jstat-gcutil 53528 100 5 S0 S1 E O M CCS YGC YGCT FGC FGCT CGC CGCT GCT 0.00 97.12 60.00 99.06 95.96 91.82 4 0.025 0 0.000 2 0.003 0.028 0.00 97.12 60.00 99.06 95.96 91.82 4 0.025 0 0.000 2 0.003 0.028 0.00 97.12 60.00 99.06 95.96 91.82 4 0.025 0 0.000 2 0.003 0.028 0.00 97.12 60.00 99.06 95.96 91.82 4 0.025 0 0.000 2 0.003 0.028 0.00 97.12 60.00 99.06 95.96 91.82 4 0.025 00 .000 2 0.003 0.028S0-S0 area usage ratio S1-S1 area usage ratio E-Eden area usage ratio O-Old area usage ratio M-metadata area usage ratio CCS-Compressed class space usage ratio gccause

Gccause and gcutil know each other very well, except for the following two columns:

LGCC-reason for last GC GCC-reason for current GC jstat-gccause 53528 1005 S0 S1 E O M CCS YGC YGCT FGC FGCT CGC CGCT GCT LGCC GCC 0.00 97.12 60.00 99.06 95.96 91.82 4 0.025 0 0.000 2 0.003 0.028 Metadata GC Threshold No GC 0.0097. 12 60.00 99.06 95.96 91.82 4 0.025 0 0.000 2 0.003 0.028 Metadata GC Threshold No GC 0.00 97.12 60.00 99.06 95.96 91.82 4 0.025 0 0.000 2 0.003 0.028 Metadata GC Threshold No GC 0.00 97.12 60.00 99.06 95.96 91.82 4 0.025 0 0.000 2 0.003 0.028 Metadata GC Threshold No GC 0.00 97.12 60.00 99.06 95.96 91.82 4 0.025 0 0.000 2 0.003 0.028 Metadata GC Threshold No GCprintcompilation

Printcompilation is the method statistics of JVM compiler

Jstat-printcompilation 53528 100 5Compiled Size Type Method 2675 23 1 jdk/internal/misc/InnocuousThread eraseThreadLocals 2675 23 1 jdk/internal/misc/InnocuousThread eraseThreadLocals 2675 23 1 jdk/internal/misc/InnocuousThread eraseThreadLocals 2675 23 1 jdk/internal/misc/InnocuousThread eraseThreadLocals 2675 23 1 jdk/internal/misc/InnocuousThread eraseThreadLocalsCompiled-number of recently executed compilation tasks Size-size of the most recent compilation method Type-classes for the latest compilation method Type Method-A summary of the names of the latest compilation methods

This paper introduces the jstat JVM analysis tool that comes with JDK14. I hope you can use it in the actual work.

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