# JVM 菜鸟进阶高手之路九（解惑）

## 关于MAT工具相关知识解惑

MAT 不是一个万能工具，它并不能处理所有类型的堆存储文件。但是比较主流的厂家和格式，例如 Sun, HP, SAP 所采用的 HPROF 二进制堆存储文件，以及 IBM 的 PHD 堆存储文件等都能被很好的解析，MAT下载地址，以及相关文档

jmap -dump:format=b,file=heap.bin pid


## 关于第八篇Xmn问题

time ls


Real, User and Sys process time statistics
One of these things is not like the other. Real refers to actual elapsed time; User and Sys refer to CPU time used only by the process.
Real is wall clock time - time from start to finish of the call. This is all elapsed time including time slices used by other processes and time the process spends blocked (for example if it is waiting for I/O to complete).

User is the amount of CPU time spent in user-mode code (outside the kernel) within the process. This is only actual CPU time used in executing the process. Other processes and time the process spends blocked do not count towards this figure.

Sys is the amount of CPU time spent in the kernel within the process. This means executing CPU time spent in system calls within the kernel, as opposed to library code, which is still running in user-space. Like 'user', this is only CPU time used by the process. See below for a brief description of kernel mode (also known as 'supervisor' mode) and the system call mechanism.

User+Sys
will tell you how much actual CPU time your process used. Note that this is across all CPUs, so if the process has multiple threads (and this process is running on a computer with more than one processor) it could potentially exceed the wall clock time reported by Real
(which usually occurs). Note that in the output these figures include the User
and Sys
time of all child processes (and their descendants) as well when they could have been collected, e.g. by wait(2)
or waitpid(2)
, although the underlying system calls return the statistics for the process and its children separately.
Origins of the statistics reported by time (1)

The statistics reported by time
are gathered from various system calls. 'User' and 'Sys' come from wait (2)
or times (2)
, depending on the particular system. 'Real' is calculated from a start and end time gathered from the gettimeofday (2)
call. Depending on the version of the system, various other statistics such as the number of context switches may also be gathered by time
.
On a multi-processor machine, a multi-threaded process or a process forking children could have an elapsed time smaller than the total CPU time - as different threads or processes may run in parallel. Also, the time statistics reported come from different origins, so times recorded for very short running tasks may be subject to rounding errors, as the example given by the original poster shows.
A brief primer on Kernel vs. User mode
On Unix, or any protected-memory operating system, 'Kernel' or 'Supervisor' mode refers to a privileged mode that the CPU can operate in. Certain privileged actions that could affect security or stability can only be done when the CPU is operating in this mode; these actions are not available to application code. An example of such an action might be manipulation of the MMU to gain access to the address space of another process. Normally, user-mode code cannot do this (with good reason), although it can request shared memory from the kernel, which could be read or written by more than one process. In this case, the shared memory is explicitly requested from the kernel through a secure mechanism and both processes have to explicitly attach to it in order to use it.
The privileged mode is usually referred to as 'kernel' mode because the kernel is executed by the CPU running in this mode. In order to switch to kernel mode you have to issue a specific instruction (often called a trap) that switches the CPU to running in kernel mode and runs code from a specific location held in a jump table. For security reasons, you cannot switch to kernel mode and execute arbitrary code - the traps are managed through a table of addresses that cannot be written to unless the CPU is running in supervisor mode. You trap with an explicit trap number and the address is looked up in the jump table; the kernel has a finite number of controlled entry points.
The 'system' calls in the C library (particularly those described in Section 2 of the man pages) have a user-mode component, which is what you actually call from your C program. Behind the scenes, they may issue one or more system calls to the kernel to do specific services such as I/O, but they still also have code running in user-mode. It is also quite possible to directly issue a trap to kernel mode from any user space code if desired, although you may need to write a snippet of assembly language to set up the registers correctly for the call. A page describing the system calls provided by the Linux kernel and the conventions for setting up registers can be found here.
There are things that your code cannot do from user mode - things like allocating memory or accessing hardware (HDD, network, etc.). These are under the supervision of the kernel, and it alone can do them. Some operations that you do (like malloc
/fwrite
) will invoke these Kernel functions and that then will count as 'sys' time. Unfortunately it's not as simple as "every call to malloc will be counted in 'sys' time". The call to malloc
will do some processing of its own (still counted in 'user' time) and then somewhere along the way it may call the function in kernel (counted in 'sys' time). After returning from the kernel call, there will be some more time in 'user' and then malloc
will return to your code. As for when the switch happens, and how much of it is spent in kernel mode... you cannot say. It depends on the implementation of the library. Also, other seemingly innocent functions might also use malloc
and the like in the background, which will again have some time in 'sys' then.

yum install sysstat


-d option in the sar command is used to display the block device statistics report. Using option -p (pretty-print) along with -d make the dev column more readable, example is shown below :

sar -d -p 1


## 选择UseParallelOldGC垃圾回收器相关坑

• 新生代对象每经历依次minor gc，年龄会加一，当达到年龄阀值会直接进入老年代。阀值大小一般为15。
• Survivor空间中年龄所有对象大小的总和大于survivor空间的一半，年龄大于或等于该年龄的对象就可以直接进入老年代，而无需等到年龄阀值。
• 大对象直接进入老年代。
• 新生代复制算法需要一个survivor区进行轮换备份，如果出现大量对象在minor gc后仍然存活的情况时，就需要老年代进行分配担保，让survivor无法容纳的对象直接进入老年代。

jstat -gc pid 3s


jinfo -flag UseAdaptiveSizePolicy pid


ps然后开启UseAdaptiveSizePolicy之后，晋升年龄阈值最后减到1了，是因为ps在开启这个之后，会判断young gc和full gc的上消耗的时间差。如果young gc>full gc1.1，threshold就会一直减少。反之，如果young1.1<full,threshold就会一直增加。 而我的应用确实young gc比较频繁，所以threshold一直减到了1，最小是1。

posted @ 2017-09-24 14:58  匠心零度  阅读(643)  评论(0编辑  收藏