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This article introduces the knowledge of "what are the common Java performance commands". In the operation of practical cases, many people will encounter such a dilemma, so let the editor lead you to learn how to deal with these situations. I hope you can read it carefully and be able to achieve something!

Performance optimization has always been the focus of back-end service optimization, but online performance problems do not often occur, or are limited by business products, so there is no way to have performance problems, including not many performance problems encountered by the author. Therefore, in order to store knowledge in advance, you will not be in a hurry when problems occur.

Since it is a positioning problem, it must be with the help of tools, let's first find out which tools are needed to help locate the problem.

Top command

The top command is one of the most commonly used Linux commands, which can display the CPU utilization, memory utilization and other system information of the currently executing process in real time. Top-Hp pid can view the system resource usage of threads.

Vmstat command

Vmstat is a virtual memory detection tool with specified cycles and collection times, which can count the usage of memory and CPU,swap. It also has an important common function to observe the context switching of the process. The fields are described as follows:

R: the number of processes in the run queue (when the number is greater than the number of CPU cores indicates blocked threads)

B: number of processes waiting for IO

Swpd: using virtual memory siz

Free: free physical memory size

Buff: the amount of memory used as a buffer (memory and hard disk buffers)

Cache: the amount of memory used as the cache (buffer between CPU and memory)

Si: the amount of memory written from the swap area to memory per second, transferred from disk to memory

So: the amount of memory written to the swap area per second, transferred from memory to disk

Bi: number of blocks read per second

Bo: number of blocks written per second

In: interrupts per second, including clock interrupts.

Cs: context switches per second.

Us: percentage of user process execution time (user time)

Sy: percentage of kernel system process execution time (system time)

Wa: percentage of IO wait time

Id: percentage of idle time

Pidstat command

Pidstat is a component of Sysstat and a powerful performance monitoring tool. Both top and vmstat commands monitor the memory, CPU and Icano usage of the process, while the pidstat command can detect thread-level. The pidstat command thread switch field is described as follows:

UID: the real user ID of the monitored task.

TGID: thread group ID.

TID: thread ID.

Cswch/s: the number of times to actively switch contexts. Here, threads are switched due to resource blocking, such as lock waiting.

Nvcswch/s: the number of times to passively switch contexts. In this case, CPU scheduling switches threads.

Jstack command

Jstack is a JDK tool command, it is a thread stack analysis tool, the most commonly used function is to use the jstack pid command to view the stack information of threads, and is often used to eliminate deadlocks.

Jstat command

It can detect the real-time running of Java programs, including heap memory information and garbage collection information, which we often use to check the program garbage collection. The common command is jstat-gc pid. The information field is described as follows:

S0C: the capacity of To Survivor in the younger generation (in KB)

S1C: the capacity of From Survivor in the younger generation (in KB)

S0U: To Survivor of the younger generation currently uses space (in KB)

S1U: From Survivor in the younger generation currently uses space (in KB)

EC: the capacity of Eden in the younger generation (in KB)

EU: Eden of the younger generation currently uses space (in KB)

OC: capacity of the old age (in KB)

OU: space is now used in the old years (unit KB)

MC: the capacity of metaspace (in KB)

MU: Metaspace is currently used (in KB)

YGC: the number of gc in the younger generation from application startup to sampling

YGCT: the time taken by gc in the younger generation from application startup to sampling (s)

FGC: number of Full Gc gc from application startup to sampling

FGCT: time from application startup to sampling time of the Full Gc gc (s)

GCT: the total time (s) that the gc took from application startup to sampling.

Jmap command

Jmap is also a JDK tool command, which can view heap memory initialization information and heap memory usage, and can also generate dump files for detailed analysis. The command jmap-heap pid to view heap memory.

Mat memory tool

MAT (Memory Analyzer Tool) tool is a plug-in of eclipse (MAT can also be used alone). When analyzing dump files with large memory, you can directly see the memory occupied by each object in the heap space, the number of class instances, the object reference relationship, the use of OQL object query, and it is very convenient to find out the relevant information of the object GC Roots.

There is also such a plug-in in idea, which is JProfiler.

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