The /proc file system, doesn't contain 'real' files. Most of the 'files' within /proc have a file size of 0.

/proc simply acts as an interface to internal data structures in the kernel. It can be used to obtain information about the system (such as memory, disks mounted, hardware configuration, etc.) and to change certain kernel parameters at runtime (sysctl).

It not only allows access to process data but also allows you to request the kernel status by reading files in the hierarchy.

The /proc directory contains (among other things) one sub-directory for each process running on the system, which is named after the process ID (PID).

Each process sub-directory has the following entries.

Similar to the process entries, the kernel data files give information about the running kernel. The files used to obtain this information are contained in /proc and are listed here. Not all of these will be present in your system. It depends on the kernel configuration and the loaded modules, which files are there, and which are missing.

/proc/<pid>/oom_adj and /proc/<pid>/oom_score_adj adjust the oom-killer score.

These file can be used to adjust the badness heuristic used to select which process gets killed in out of memory conditions.

The badness heuristic assigns a value to each candidate task ranging from 0 (never kill) to 1000 (always kill) to determine which process is targeted. The units are roughly a proportion along that range of allowed memory the process may allocate from based on an estimation of its current memory and swap use. For example, if a task is using all allowed memory, its badness score will be 1000. If it is using half of its allowed memory, its score will be 500.

There is an additional factor included in the badness score: root processes are given 3% extra memory over other tasks.

The amount of “allowed” memory depends on the context in which the oom killer was called. If it is due to the memory assigned to the allocating task's cpuset being exhausted, the allowed memory represents the set of mems assigned to that cpuset. If it is due to a mempolicy's node(s) being exhausted, the allowed memory represents the set of mempolicy nodes. If it is due to a memory limit (or swap limit) being reached, the allowed memory is that configured limit. Finally, if it is due to the entire system being out of memory, the allowed memory represents all allocatable resources.

The value of /proc/<pid>/oom_score_adj is added to the badness score before it is used to determine which task to kill. Acceptable values range from -1000 (OOM_SCORE_ADJ_MIN) to +1000 (OOM_SCORE_ADJ_MAX). This allows userspace to polarize the preference for oom killing either by always preferring a certain task or completely disabling it. The lowest possible value, -1000, is equivalent to disabling oom killing entirely for that task since it will always report a badness score of 0.

Consequently, it is very simple for userspace to define the amount of memory to consider for each task. Setting a /proc/<pid>/oom_score_adj value of +500, for example, is roughly equivalent to allowing the remainder of tasks sharing the same system, cpuset, mempolicy, or memory controller resources to use at least 50% more memory. A value of -500, on the other hand, would be roughly equivalent to discounting 50% of the task's allowed memory from being considered as scoring against the task.

For backwards compatibility with previous kernels, /proc/<pid>/oom_adj may also be used to tune the badness score. Its acceptable values range from -16 (OOM_ADJUST_MIN) to +15 (OOM_ADJUST_MAX) and a special value of -17 (OOM_DISABLE) to disable oom killing entirely for that task. Its value is scaled linearly with /proc/<pid>/oom_score_adj.

Writing to /proc/<pid>/oom_score_adj or /proc/<pid>/oom_adj will change the other with its scaled value.

Caveat: When a parent task is selected, the oom killer will sacrifice any first generation children with separate address spaces instead, if possible. This avoids servers and important system daemons from being killed and loses the minimal amount of work.

/proc/<pid>/oom_score displays the current oom-killer score. This file can be used to check the current score used by the oom-killer is for any given <pid>. Use it together with /proc/<pid>/oom_adj to tune which process should be killed in an out-of-memory situation.

/proc/<pid>/io display the IO accounting fields, which are IO statistics for each running process.

dd if=/dev/zero of=/tmp/test.dat &
[1] 3828
 
 
cat /proc/3828/io
 
rchar: 323934931
wchar: 323929600
syscr: 632687
syscw: 632675
read_bytes: 0
write_bytes: 323932160
cancelled_write_bytes: 0

where:

• rchar is count of chars read. The number of bytes which this task has caused to be read from storage. This is simply the sum of bytes which this process passed to read() and pread(). It includes things like tty IO and it is unaffected by whether or not actual physical disk IO was required (the read might have been satisfied from pagecache)
• wchar is count of chars written. The number of bytes which this task has caused, or shall cause to be written to disk. Similar caveats apply here as with rchar.
• syscr is a count of read syscalls. Attempt to count the number of read I/O operations, i.e. syscalls like read() and pread().
• syscw is a count of writen syscalls. Attempt to count the number of write I/O operations, i.e. syscalls like write() and pwrite().
• read_bytes is count of bytes read. Attempt to count the number of bytes which this process really did cause to be fetched from the storage layer. Done at the submit_bio() level, so it is accurate for block-backed filesystems. TODO: Add status regarding NFS and CIFS at a later time.
• write_bytes is count of bytes written. Attempt to count the number of bytes which this process caused to be sent to the storage layer. This is done at page-dirtying time.
• cancelled_write_bytes The big inaccuracy here is truncate. If a process writes 1MB to a file and then deletes the file, it will in fact perform no writeout. But it will have been accounted as having caused 1MB of write. In other words: The number of bytes which this process caused to not happen, by truncating pagecache. A task can cause “negative” IO too. If this task truncates some dirty pagecache, some IO which another task has been accounted for (in its write_bytes) will not be happening. We _could_ just subtract that from the truncating task's write_bytes, but there is information loss in doing that.

http://tldp.org/LDP/Linux-Filesystem-Hierarchy/html/proc.html

https://www.cyberciti.biz/files/linux-kernel/Documentation/filesystems/proc.txt