Android /proc file systemß

Aug 15, 2015


I attended AnDevCon Boston 2015 weeks ago. Compared with Google IO, it's much more like a training camp combined with some company advertisements. Fortunately I met Jonathan Levin who is the author of "Android Internals". The content of this blog is mainly from his speech in the conference.

The /proc file system in Android is in a standard format. That is to say, compared with /sys, its content are equal across different Android devices, so it's safe to have dependencies on /proc in you Android app. Lots of the files under this directory are useful for helping identifying Android app performance issues.

We all know that process information is inside /proc/PID. In fact, Linux kernel doesn't have any concept of process. Instead, it cares about thread-group, which is normally named "TGID". When a new thread group (process) is created/forked, kernel assign a new TGID, and the first thread (UI thread in Android) has the same number of TID as TGID. Information about a thread of a given process can be found at /proc/PID/task/TID. If you get into this dir you will find that it has the exact same structure as /proc/PID. The interesting thing is that each thread, which is regarded as "task", is sensible by Linux kernel, so you can directly reach them from /proc.
# cd /proc/TID
// succed, that means /proc/FOO isn't necessarily a process.
But if you try:
# ls /proc | grep <TID>
// no match

memory related

With /proc/PID/status you can get lots of detail about the process. I need to point out that 'Pid' and 'Tgid' in this file are quit comfusing. In fact the field 'Tgid' is what we think as PID (process ID); while 'Pid' is actually the thread/task ID. Other useful fields in this file for the app developer:
  • State: S(sleeping), R(running), T(stopped), D(uninterruptable sleep)
  • TracerPid: any ptrace attached process, like strace or gdb
  • VmHWN: RSS peak usage (hight-water mark)
  • VmRSS: current RSS usage
  • VmPeak: virtual memory hight-water mark
  • VmSize: current virtual memory
  • VmData: size of data segment(heap)
  • VmStk: size of thread stacks
  • VmExe: size of executable
  • VmLib: shared library usage
  • VmPTE: page table entries
  • Threads: number of threads
  • Cpus_allowed: bitmask of CPUS allowed
  • voluntary_ctxt_switches: voluntray (system call induced) context switches
  • novoluntary_ctxt_switches: nonvoluntary(preemption induced) context switches
The same values are also stored in machine-readable format in /proc/PID/stat, if you want to obtain these values.

/proc/PID/maps also stores the memory space occupied by different sections (libraries, stacks, heap, ...). More information can be found in /proc/PID/smaps, like RSS, PSS, clean/dirty memory usage for each section.

oom_score

The kernel scores of the memory usage of each process, and will kill some processes if the system is in low memory. The score for each process is stored in /proc/PID/oom_score. The higher the value it is, the more possible your process will be killed. The score can be adjusted by /proc/PID/oom_adj or oom_score_adj, with different value range (-1000 to 1000 for oom_score_adj). I'm not sure the exact difference between them, but it seems that oom_score_adj which was introduced in Linux kernel 2.6.36, is newer than oom_adj. If the system specify a nagative value in any of these files, the related process is not likely to be terminated, so you may see very large negative value (-1000 etc.) specified for many system services. From an app developer's perspective, by comparing the values in oom_score for all the processes, we can forecast whether our app are more likely to be recycled in the near future.

fd

All the file descriptors opened by current process can be found in /proc/PID/fd. So as the app developer, if you'd like to investigate which files are currently opened, take a look into this directory. For each file named N in 'fd', you can find the offset of the file descriptor in the file as well as the flag from /proc/PID/fdinfo/N.

As Jonathan mentioned, getting familiar with these file systems like /proc and /sys "just opens a door to the broad Linux world" for our app developers. With this knowledge we are not limited to the SDK framework. Hopefully I will use any of the knowledge in my Hurdle project --- by plugin it into your Android application, you can easily detect many performance issues. (shameless advertisement!)