# Are you Sure your Linux PID is the Process ID?

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## IDs for Processes and Threads in Linux

1. In Linux, all processes and threads get a unique identifier (ID) and are all listed as directories of the same level under the /proc pseudo file system.
2. These processes and threads are represented as subdirectories in the form of /proc/[pid] , where the pid is the unique numerical ID for each.
3. So, technically, the “pid” is not just a process ID but could be an ID for either a thread or a process.
4. htop shows both processes and threads, and doesn’t distinguish them by default.
• Consequently, its “PID” column contains IDs for both processes and threads.
• Linux originally didn’t have the notion of threads.
• It only had separate processes, each of which has a unique pid, potentially sharing some resources like virtual memory and file descriptors.
• In 2001, Linux 2.4 introduced “Thread groups”, which gave rise to threads within a process. From the clone(2) man page:

Thread groups were a feature added in Linux 2.4 to support the POSIX threads notion of a set of threads that share a single PID. Internally, this shared PID is the so-called thread group identifier (TGID) for the thread group. Since Linux 2.4, calls to getpid(2) return the TGID of the caller.

• So, the PID of the parent process is overloaded with another meaning, the TGID :)
• That’s said, the kernel still doesn’t have a separate implementation for processes and threads.
6. In summary, threads within a process will get the same PID , while each of which has a unique thread ID (TID).
• The values returned by the getpid() are the same for all of them.
• But the TID from the gettid() are always unique.

## How to Distinguish between a Thread and a real “Process”?

1. As mentioned above, threads and process are similar in nature (kernel implementation), and both are “schedulable” entities to the kernel.
2. The main difference between them is the scope of their namespaces (address space, resources, etc.).
3. To distinguish between threads and processes, we need to look into the /proc/[pid]/task/[tid] subdirectories where tid is the kernel thread ID.
4. Kernel threads are the ones registered in and scheduled by the kernel.
• There are user-level threads (e.g., Java threads) invisible to the kernel and managed by the application layer.
• See here for more.
5. The threads within parent process are under the same thread group (TG) umbrella, having the same TGID as the main thread but different tids.
• The main thread is the “process” that spawned all the children threads and has the same TGID as its PID.
6. /proc/[pid]/task/[tid] shares the same content as /proc/[pid]/ if pid==tid , i.e., it contains the same information describing the same process/thread.
7. Therefore, when you look into the /proc/[pid] directory of a multithreaded process:
• The task/[tid] subdirectories are all the threads within the same thread group.
• They have the same TGID being the pid.
• The so-called process is the main thread that spawned all other threads.
• The main thread has the same TGID as its PID (usually the smallest one under the task/ directory).

## Last Note: Multiprocessing vs. Multithreading

1. To avoid confusion, apart from the thread group ID (TGID), there is also a process group ID (PGID).
2. The former is for multithreading, and the later is used in multiprocessing.
3. A process is spawned by invoking the fork() syscall, a thread is created by e.g., pthread_create() in C. (Under the hood, they all use the syscall clone() but with different parameters)
• The process that spawns other subprocesses is the parent.
• When spawning a new process, the parent can either create a new process group (and puts itself and its children into it) or inherits that of an existing process (usually it’s the grandparent process).
• If a new process group is created, the PGID will be the PID of the process that creates it.
• If it’s inherited, the PGID is the same as that of the grandparent.
• The process (the main thread) and all the threads it creates are siblings.
• They share the same TGID.
• They also have the same PGID as that of the main thread.
• NB: The threads created by a process are NOT visible to its parent process.

## Example

1. We easily can launch a multithreaded program with stress-ng on Linux.
2. A “stressor/hog” is a process.
3. We can run the stress test for memory accesses with the following command, which will spawn a stressor that runs with 5 threads reading and writing to two different mappings of the same underlying physical page.

 hy@node-0:~$stress-ng --mcontend 1 -t 10h stress-ng: info: [56472] dispatching hogs: 1 mcontend  4. With htop, we can see the process and threads therein in hierarchy (the PGRP is the GPID, and the PID is the ID for threads/processes). • We can see that all the threads and processes have a unique ID, PID (process/thread ID). • 56472: The single-threaded parent process spawned from the bash command. • 56473: The multithreaded child process (and the main thread) spawned from the parent 56472. • 56473-56477: The 5 sibling threads created by the main thread 56473. 5. Then, by using pidof, we get the pid of the main thread.  hy@node-0:~$ pidof stress-ng-mcontend
56473

6. Since the bash command 56472 is the parent process that spawned the child process 56473, we can examine this relationship by checking:

 hy@node-0:~$cat /proc/56472/task/56472/children 56473  7. Then, navigate to the /proc directory and check the /proc/[pid]/task/ subdirectories. We get the 5 threads within this process:  hy@node-0:~$ ll /proc/56473/task/
total 0
dr-xr-xr-x 7 hy hy 0 Dec 31 22:21 ./
dr-xr-xr-x 9 hy hy 0 Dec 31 22:21 ../
dr-xr-xr-x 7 hy hy 0 Dec 31 22:21 56473/
dr-xr-xr-x 7 hy hy 0 Dec 31 22:21 56474/
dr-xr-xr-x 7 hy hy 0 Dec 31 22:21 56475/
dr-xr-xr-x 7 hy hy 0 Dec 31 22:21 56476/
dr-xr-xr-x 7 hy hy 0 Dec 31 22:21 56477/

8. To examine directory of a child thread, we get the same output as above since they are siblings.

 hy@node-0:~$ll /proc/56476/task/ total 0 dr-xr-xr-x 7 hy hy 0 Dec 31 22:21 ./ dr-xr-xr-x 9 hy hy 0 Dec 31 22:21 ../ dr-xr-xr-x 7 hy hy 0 Dec 31 22:21 56473/ dr-xr-xr-x 7 hy hy 0 Dec 31 22:21 56474/ dr-xr-xr-x 7 hy hy 0 Dec 31 22:21 56475/ dr-xr-xr-x 7 hy hy 0 Dec 31 22:21 56476/ dr-xr-xr-x 7 hy hy 0 Dec 31 22:21 56477/  9. Note that the parent process 56472 is a single-threaded process, so its /task directory contains only itself.  hy@node-0:~$ ll /proc/56472/task/
total 0
dr-xr-xr-x 7 hy hy 0 Dec 31 22:21 ./
dr-xr-xr-x 9 hy hy 0 Dec 31 22:21 ../
dr-xr-xr-x 7 hy hy 0 Dec 31 22:21 56472/


### Resource Accounting

1. The resource usage of a process/thread can be obtained in various ways, and the information is not consistent.
2. From above, htop aggregate all the resource usages in the main thread 56473 by default.
3. We can obtain per-process information from ps as well:
• Like htop, It aggregates usage of all threads to the main threads by default.

  hy@node-0:~$ps -p 56473 -o %cpu,%mem,cmd %CPU %MEM CMD 473 0.0 stress-ng-mcontend  • It only works on the main thread but not with the siblings:  hy@node-0:~$ ps -p 56476 -o %cpu,%mem,cmd
%CPU %MEM CMD

• To see detailed thread-level information, we can use the -L flag on the main thread:

  hy@node-0:~$ps -L 56473 -o %cpu,%mem,cmd %CPU %MEM CMD 97.3 0.0 stress-ng-mcontend 94.0 0.0 stress-ng-mcontend 94.0 0.0 stress-ng-mcontend 94.0 0.0 stress-ng-mcontend 94.0 0.0 stress-ng-mcontend  • With -F option, we can obtain the full glory:  hy@node-0:~$ ps -L 56473 -F
UID          PID    PPID     LWP  C NLWP    SZ   RSS PSR STIME TTY      STAT   TIME CMD
hy         56473   56472   56473 97    5 22792  2604  13 08:10 pts/2    RLl+ 302:30 stress-ng-mcontend
hy         56473   56472   56474 94    5 22792  2604   7 08:10 pts/2    RLl+ 292:03 stress-ng-mcontend
hy         56473   56472   56475 94    5 22792  2604  31 08:10 pts/2    RLl+ 292:00 stress-ng-mcontend
hy         56473   56472   56476 94    5 22792  2604  15 08:10 pts/2    RLl+ 291:59 stress-ng-mcontend
hy         56473   56472   56477 94    5 22792  2604   0 08:10 pts/2    RLl+ 292:05 stress-ng-mcontend

• Note that ALL threads share the same PID but each of them has a unique TID (LWP).

4. We can monitor those threads using top with -H:

 hy@node-0:/proc$top -H -p 56476 .... PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND 56473 hy 20 0 91168 2708 2272 R 97.3 0.0 127:24.91 stress-ng-mcont 56474 hy 20 0 91168 2708 2272 R 94.0 0.0 122:55.16 stress-ng-mcont 56475 hy 20 0 91168 2708 2272 R 94.0 0.0 122:54.44 stress-ng-mcont 56476 hy 20 0 91168 2708 2272 R 93.7 0.0 122:55.33 stress-ng-mcont 56477 hy 20 0 91168 2708 2272 R 92.3 0.0 122:56.57 stress-ng-mcont  • Without the -H flag, however, it aggregates all usages to any of the sibling threads:  hy@node-0:~$ top -p 56476
....
PID USER      PR  NI    VIRT    RES    SHR S  %CPU  %MEM     TIME+ COMMAND
56473 hy        20   0   91168   2708   2272 R 476.3   0.0 621:39.36 stress-ng-mcont

• Also, we can get a single number out:

  # Get the CPU utilization of thread 56476.
# * Aggregated.
hy@node-0:~$top -b -n 2 -d 0.2 -p 56476 | tail -1 | awk '{print$9}'
465.0
hy@node-0:~$top -b -H -n 2 -d 0.2 -p 56476 | tail -1 | awk '{print$9}'
75.0

5. How to get per-thread resource usages? We need to again look into the /proc sysfs.
• The file /proc/pid/stat of a thread (whose TID==pid) contains the information aggregated from all threads.
• The file /proc/pid/task/pid/stat contains per-thread information:

  # * Get total cpu time (user and kernel) of all threads belonging to the same TG as that of 56476.
hy@node-0:~$cat /proc/56476/stat | awk '{print$14, $15}' 9460932 12361 # * Get the cpu time for only thread 56476. hy@node-0:~$ cat /proc/56476/task/56476/stat | awk '{print $14,$15}'
1879429 3032

6. Alternatively, we can use the mighty python with psutil.

 >>> import psutil

# The great grandparent process.
>>> tmux_session = psutil.Process(54711)
# * It's spawned from the mother of all processes of PID=1 -- the init(old distros)/systemd(new distros).
>>> tmux_session.ppid()
1
>>> [(child.name(), child.pid) for child in tmux_session.children(recursive=True)]
[('bash', 54712), ('bash', 56236), ('python', 56613), ('stress-ng', 56472), ('stress-ng-mcontend', 56473)]

# The parent process.
>>> parent = psutil.Process(56472)
# * The parent was spawned from one of the above bash sessions (grandparent)
>>> parent.ppid()
54712
# * The sibling threads art NOT children, and invisable to the parent process.
>>> parent.children(recursive=True)
[psutil.Process(pid=56473, name='stress-ng-mcontend', status='running', started='11:21:57')]
# * The parent is single-threaded.
1

# The child process spawned from the parent process.
>>> child = psutil.Process(56473)
# * The child created 5 threads (including itself).
5
[56473, 56474, 56475, 56476, 56477]

# One of the sibling threads.
>>> sibling = psutil.Process(56476)
# * The sibling thread inherits the parent process of the main thread.
>>> child.ppid()
56472
>>> sibling.ppid()
56472

# Accounting resouces.
# * The parent process is in sleep state (S), so it doesn't take any CPU time.
>>> parent.cpu_percent(interval=1)
0.0
# ! psutil **aggregates** all sibling resources to **any** of the siblings.
>>> sibling.cpu_percent(interval=1)
471.5
>>> child.cpu_percent(interval=1)
472.4
# ! Also, its cpu time accounting for children processes is broken somehow ...
>>> tmux_session.cpu_times()
pcputimes(user=7.46, system=3.19, children_user=102.18, children_system=153.15, iowait=0.0)
>>> parent.cpu_times()
pcputimes(user=0.0, system=0.0, children_user=0.0, children_system=0.0, iowait=0.0)
>>> child.cpu_times()
pcputimes(user=45250.11, system=57.79, children_user=0.0, children_system=0.0, iowait=0.0)
>>> sibling.cpu_times()
pcputimes(user=45255.42, system=57.79, children_user=0.0, children_system=0.0, iowait=0.0)

# * Memory usages are accounted the same as it does for CPU.
>>> parent.memory_full_info()
pfullmem(rss=6475776, vms=59777024, shared=6078464, text=1728512, lib=0, data=32018432, dirty=0, uss=3051520, pss=3749888, swap=0)
>>> child.memory_full_info()
pfullmem(rss=2772992, vms=93356032, shared=2326528, text=1728512, lib=0, data=65581056, dirty=0, uss=126976, pss=735232, swap=0)
>>> sibling.memory_full_info()
pfullmem(rss=2772992, vms=93356032, shared=2326528, text=1728512, lib=0, data=65581056, dirty=0, uss=126976, pss=735232, swap=0)
>>> tmux_session.memory_percent()
0.007239506814671662
>>> parent.memory_percent()
0.009602063988251591
>>> child.memory_percent()
0.004111699759675096
>>> sibling.memory_percent()
0.004111699759675096

7. In summary, there are various ways of accounting resources for processes and threads on Linux.
• However, there are nuances that must be taken into account.
• psutil is a convenient tool for sys admins when scripting in python.
• However, the resource usage of any threads is the aggregated values of all threads!
• It doesn’t explicitly distinguish between PIDs and TIDs either.
• And it’s time to end our running example:

  # ! "Note this will return True also if the process is a zombie (p.status() == psutil.STATUS_ZOMBIE)"
>>> parent.is_running() == child.is_running() == sibling.is_running() == True
True

>>> import signal
>>> sibling.send_signal(signal.SIGINT)

>>> parent.is_running() == child.is_running() == sibling.is_running() == False
True


(It seems that interrupting one thread has a bottom-up cascading effect in stress-ng 🥴 )

Happy New Year 🎆 ~

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