进程文件系统被挂载到/proc目录，其本质上并不是一个真正的文件系统因此并不会占用磁盘空间，最早出现在UNIX 8th Edition版本里，在最开始的设计上的根本目的实际上是为了方便debug也就是尝试取代ptrace而让进程信息透明化，其规定了进程文件命名格式以及相关数据的布局，实际本质上都感觉没什么可说的，直接贴别人的回答吧：
In the beginning (way back in Unix), the way that programs found out about the running processes on the system was via directly reading process structures from the kernel memory (opening /dev/mem, and interpreting the raw data directly). This is how the very first 'ps' commands worked. Over time, some information was made available via system calls.
However, it is bad form to expose system data directly to user-space via /dev/mem, and obnoxious to be constantly creating new system calls every time you wanted to export some new piece of process data, and so a newer method was created to access structured data for user-space applications to find out about process attributes. This was the /proc filesystem. With /proc, the interfaces and structures (directories and files) could be kept the same, even as the underlying data structures in the kernel changed. This was much less fragile than the earlier system, and it scaled better.
The /proc filesystem was originally designed to publish process information and a few key system attributes, required by 'ps', 'top', 'free' and a few other system utilities. However, because it was easy to use (both from the kernel side and the user-space side), it became a dumping ground for a whole range of system information. Also, it started to gain read/write files, to be used to adjust settings and control the operation of the kernel or its various subsystems. However, the methodology of implementing control interfaces was ad-hoc, and /proc soon grew into a tangled mess.
The sysfs (or /sys filesystem) was designed to add structure to this mess and provide a uniform way to expose system information and control points (settable system and driver attributes) to user-space from the kernel. Now, the driver framework in the kernel automatically creates directories under /sys when drivers are registered, based on the driver type and the values in their data structures. This means that drivers of a particular type will all have the same elements exposed via sysfs.
Many of the legacy system information and control points are still accessible in /proc, but all new busses and drivers should expose their info and control points via sysfs.