4 commands to check thread count per process (threads vs processes) in Linux

In Linux, some processes are divided into pieces called threads. In one liner, threads are essentially just processes with a shared address space on Linux. In this article we will get some brief overview on threads and processes, also some examples to show threads per process, check thread count per process, check number of threads allowed, count threads and some more related topics.

 

Threads vs Processes

  • A thread is very similar to a process, it has an identifier (TID, or thread ID), and the kernel schedules and runs threads just like processes.
  • However, unlike separate processes, which usually do not share system resources such as memory and I/O connections with other processes, all threads inside a single process share their system resources and some memory.
  • A process with one thread is single-threaded, and a process with more than one thread is multithreaded.
  • All processes start out single-threaded. This starting thread is usually called the main thread. The main thread may then start new threads in order for the process to become multithreaded, similar to the way a process can call fork() to start a new process.
  • The primary advantage of a multithreaded process is that when the process has a lot to do, threads can run simultaneously on multiple processors, potentially speeding up computation.
  • Although you can also achieve simultaneous computation with multiple processes, threads start faster than processes, and it is often easier and/or more efficient for threads to intercommunicate using their shared memory than it is for processes to communicate over a channel such as a network connection or a pipe.

 

Show threads per process

There are multiple commands and methods which you can use to show threads per process and count threads in Linux.

 

1. Using PID task

You can count threads with the list of available sub directories inside /proc/<PID>/task/. The count of total available sub-directories inside this part is directly proportional to the thread count per process for the provided PID.

For example to check java thread count, I have a Java process for which you can see I have multiple sub-directories so it means this is a multi threaded process. using ls command under this path you can show threads per process for java

# ls /proc/$(pidof java)/task/
31161  31170  31175  31180  31185  31266  31274  31285  31290  31295  31301  31307
31165  31171  31176  31181  31186  31267  31276  31286  31291  31296  31302  31308
31167  31172  31177  31182  31241  31268  31279  31287  31292  31298  31303  38715
31168  31173  31178  31183  31260  31270  31283  31288  31293  31299  31304  42883
31169  31174  31179  31184  31265  31272  31284  31289  31294  31300  31306  47335

But then again I have another process for which as you can see I have single sub-directory hence we know this is a single thread process

# ls /proc/$(pgrep amsHelper)/task/
6164

 

2. Using ps command

You can also use "ps" command to show threads per process. With "ps" we can list LWP (Light Weight process) which depicts Thread ID of the respective process and NWLP (Number of Threads).

To show threads per process using pscommand you can use below argument

-L	Show threads, possibly with LWP and NLWP columns.
-e	Select all processes
-f	Do full-format listing

For example:

# ps -eLf | less
UID        PID  PPID   LWP  C NLWP STIME TTY          TIME CMD
root         1     0     1  0    1 Nov08 ?        00:01:48 /usr/lib/systemd/systemd --switched-root --system --deserialize 22
root         2     0     2  0    1 Nov08 ?        00:00:00 [kthreadd]
root         4     2     4  0    1 Nov08 ?        00:00:00 [kworker/0:0H]
root         6     2     6  0    1 Nov08 ?        00:00:04 [ksoftirqd/0]
root         7     2     7  0    1 Nov08 ?        00:00:00 [migration/0]
<<Output trimmed>>

 

3. Using pstree command

You can also use pstree to show threads per process. Here as you see java thread count and check number of threads for java process

# pstree -pau -l -G -s 31161
systemd,1 --switched-root --system --deserialize 22
  mqdtomcat-wdg,31160,watchdog /usr/bin/dtomcat-wdg start
      mqjava,31161 -Xms300m -Xmx300m -XX:-UseLargePages -classpath /usr/share/tomcat/bin/bootstrap.jar:/usr/share/tomcat/bin/tomcat-juli.jar:/usr/share/java/commons-daemon.jar:/opt/watchdog/tomcat/lib/onends-tomcat.jar -Dcatalina.base=/opt/watchdog/tomcat -Dcatalina.home=/usr/share/tomcat -Djava.endorsed.dirs= -Djava.io.tmpdir=/opt/watchdog/tomcat/temp -Djava.util.logging.config.file=/opt/watchdog/tomcat/conf/logging.properties -Djava.util.logging.manager=org.apache.juli.ClassLoaderLogManager org.apache.catalina.startup.Bootstrap start
          tq{java},31165
          tq{java},31167
          tq{java},31168
          tq{java},31169
          tq{java},31170
          tq{java},31171
          tq{java},31172
<<Output trimmed>>

 

4. Using top command

In top, by default we will not be able to see thread count per process. But when running top, it is possible to change which fields to display and add this column to print thread count per process can be added manually.

  • Press f
  • This will show a list of fields that top can display. The fields that are displayed in bold are the ones that top will display.
  • Use the down arrow to navigate to "nTH" (Number of Threads).
  • Press to select "nTH"
  • Press 's' to sort on number of threads.
  • Press 'q' to display the data of threads count.

Next you should see a new column at the end of top command with number of thread (nTH) column to show threads per process

top - 15:24:41 up 2 days, 19:31,  2 users,  load average: 0.46, 0.52, 0.52
Tasks: 219 total,   1 running, 218 sleeping,   0 stopped,   0 zombie
%Cpu(s):  0.0 us,  0.0 sy,  0.0 ni, 99.9 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
KiB Mem : 13199273+total, 12801261+free,  1678324 used,  2301800 buff/cache
KiB Swap:  4189180 total,  4189180 free,        0 used. 12937532+avail Mem

  PID USER      PR  NI    VIRT    RES    SHR S  %CPU %MEM     TIME+ COMMAND           nTH
    9 root      20   0       0      0      0 S   0.3  0.0   5:17.22 rcu_sched          1
 6357 ssrun     20   0 5310412 156988  13344 S   0.3  0.1   3:59.69 jsvc              52
51240 root      20   0  168472   2448   1628 R   0.3  0.0   0:00.04 top                1
    1 root      20   0  193420   6408   2652 S   0.0  0.0   1:48.68 systemd            1
    2 root      20   0       0      0      0 S   0.0  0.0   0:00.22 kthreadd           1
    4 root       0 -20       0      0      0 S   0.0  0.0   0:00.00 kworker/0:0H       1

 

 

Check thread count per process

Next you can use the above explained commands to also check thread count per process by customising them a little bit.

 

1. Using PID status

To check thread count per process you can use below command. For example here java thread count is 59 threads in my Linux environment

# cat /proc/$(pgrep java)/status | grep -i Threads
Threads:        59

While amsHelper process has single thread

# cat /proc/$(pgrep amsHelper)/status | grep -i Threads
Threads:        1

 

2. Using ps command

We used ps command to show threads per process and count threads, we can also use "ps" command to get LWP and NLWP details, which when combined with "wc" we can count threads per process.

To check thread count per process for a particular PID for example to check java thread count:

# ps -eL -q 31161 | wc -l
61

 

Check number of threads allowed in Linux system?

Linux doesn't have a separate threads per process limit, just a limit on the total number of processes on the system. This value controls the maximum number of threads that can be created using fork(). During initialization the kernel sets this value such that even if the maximum number of threads is created

To check number of threads which Linux system can allow

# cat /proc/sys/kernel/threads-max
35000

The minimum number of threads that can be written to threads-max is 20.
The maximum value that can be written to threads-max is given by the constant FUTEX_TID_MASK (0x3fffffff).
If a value outside of this range is written to threads-max an error EINVAL occurs.

The default value depends on memory size. You can use threads-max to check number of threads allowed in Linux. You can increase thread count per process limit like this:

#echo 100000 > /proc/sys/kernel/threads-max
IMPORTANT NOTE:

The value written is checked against the available RAM pages. If the thread structures would occupy too much (more than 1/8th) of the available RAM pages threads-max is reduced accordingly.

There is also a limit on the number of processes (an hence threads) that a single user may create, see ulimit for details regarding these limits:

# ulimit -a | grep -i processes
max user processes              (-u) 10000

Here, the system is able to create 35,000 threads/processes in total and a single user can create 10000 number of processes.

The logic is very simple here every CPU can execute 1 process at a time, if there are 8 cores that means 8 to 10 processes at a time can be executed easily without any stress but if number of running or runnable threads per CPU increases drastically then there will be performance issue.

 

What is the maximum processes count allowed in Linux?

From the proc man page

 /proc/sys/kernel/pid_max (since Linux 2.5.34)
              This file specifies the value at which PIDs wrap around (i.e.,
              the value in this file is one greater than the maximum PID).
              PIDs greater than this value are not allocated; thus, the
              value in this file also acts as a system-wide limit on the
              total number of processes and threads.  The default value for
              this file, 32768, results in the same range of PIDs as on ear‐
              lier kernels.

Verify the value for kernel.pid_max

[root@server1 ~]# sysctl -a | grep kernel.pid_max
kernel.pid_max = 35000

Here I can execute 35,000 processes simultaneously in my system that can run in separate memory spaces.

To change the value of kernel.pid_max to 65534:

# echo kernel.pid_max = 65534 >> /etc/sysctl.conf

# sysctl -p

 

Lastly I hope the steps from the article to show threads per process, check thread count per process, check number of threads allowed on Linux was helpful. So, let me know your suggestions and feedback using the comment section.

 

References:
The Linux Programming Interface

 

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