JavaEE Profiling concepts
This Blog article contains some JavaEE profiling/performance concepts.
CPU time Vs Wall time
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CPU time (or process time) is the amount of time for which a central processing unit (CPU) was used for processing / executing method code.
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Wall time (or wall-clock time) is the human perception of the passage of time from the start to the completion of a task.
Wall time can also mean the actual time taken by a computer to complete a task. It is the sum of three terms: CPU time, I/O time, and the communication channel delay (e.g. if data are scattered on multiple machines).
CPU, Core and Processors
如下命令分别用来在 Linux 操作系统下统计物理CPU个数（socket 个数)，每个物理CPU 的逻辑核数，以及系统整个cpu线程数。
- 物理CPU个数：
[root@localhost ~]# cat /proc/cpuinfo | grep "physical id" | sort | uniq
physical id : 0
physical id : 1
可以看到物理CPU个数为2，物理CPU也就是机器外面就能看到的一个个CPU，每个物理CPU还带有单独的风扇。
- 每个物理CPU的逻辑核数：
[root@localhost ~]# cat /proc/cpuinfo | grep "cores" | uniq
cpu cores : 6
- 系统整个cpu线程数：
[root@localhost ~]# cat /proc/cpuinfo | grep "processor" | wc -l
24
假设，物理CPU个数（socket 个数) 为 socket, 每个物理CPU的逻辑核数为 cores, 系统整个cpu线程数(硬件线程数)为 total_hardware_threads, 则我们有如下公式：
total_hardware_threads = socket * cores * 2
每个逻辑核跑了2个cpu线程，上面例子中服务器共有两个物理CPU，即 socket = 2，每个物理CPU的逻辑核数为6，即 cores = 6，所以，上面的cpu线程数 24 = 2 * 6 * 2
Intel Hardware List
4 cores, 8 processors
2 cores, 4 processors
14 cores, 28 processors
12 cores, 24 processors
12 cores, 24 processors
10 cores, 20 processors
8 cores, 16 processors
6 cores, 12 processors
4 cores, 8 processors