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Linux Processes: Structure, Hangs and Core Dumps

Efficient and effective resolution practices

3.  In a different shell, execute:

# pstree -p

init(1)-+-apmd(1177)

~~~~~~Saving space~~~~~

|-kdeinit(1904)-

~~~~~~Saving space~~~~~

| | -kdeinit(2872)-+-bash(2874)---thread_test (3194)-+-thread_test(3195)
| | | |-thread_test(3196)
| | | |-thread_test(3197)
| | | |-thread_test(3198)
| | | |-thread_test(3199)
| | | |-thread_test(3200)
| | | |-thread_test(3201)
| | | `-thread_test(3202)

~~~~~~Saving space~~~~~

| | `-bash(3204)---pstree(3250)

~~~~~~Saving space~~~~~

4.  We can display more details with the ps command. (Note that the PIDs would have matched if we had run these examples at the same time.)

# ps -eo pid,ppid,state,comm,time,pri,size,wchan | grep test
28807 28275 S thread_test 00:00:12 18 82272 \ schedule_timeout
Display threads with -m.
# ps -emo pid,ppid,state,comm,time,pri,size,wchan | grep test
28807 28275 S thread_test 00:00:00 18 82272 \ schedule_timeout
28808 28807 R thread_test 00:00:03 14 82272 -
28809 28807 R thread_test 00:00:03 14 82272 \ ia64_leave_kernel
28810 28807 R thread_test 00:00:03 14 82272 \ ia64_leave_kernel
28811 28807 R thread_test 00:00:03 14 82272 \ ia64_leave_kernel
28812 28807 R thread_test 00:00:03 14 82272 -
28813 28807 R thread_test 00:00:02 14 82272 \ ia64_leave_kernel
28814 28807 R thread_test 00:00:02 14 82272 \ ia64_leave_kernel
28815 28807 R thread_test 00:00:03 14 82272 -

Even though some UNIX distributions have modified commands such as ps or top to display a process with all its threads by including special options such as -m or -L, HPUX has not. Therefore, the HPUX ps command only shows the HWP process and not the underlying threads that build the process. On the other hand, Solaris can display the LWP of a process by using the -L option with its ps command.

Other vendors have created their own tools for displaying threads of a process. HPUX's glance is a good example. Using the same procedures as earlier, we demonstrate multithreads in HPUX to show the main difference between UNIX threads and Linux's implementation of threads.

HPUX 11.11:
# cc -o thread_test thread_test.c -lpthread

hpux_11.11 #glance
Process Name PID PPID Pri Name ( 700% max) CPU IO Rate RSS \ Cnt
------------------------------------------------------------------------
thread_test 14689 14579 233 root 698/ 588 57.3 0.0/ 0.2 \ 560kb 9

Thus, using HPUX's glance, we can see that the thread count is nine, with one thread representing the main HWP and eight additional threads that were created by the program as shown in the source. Each thread does not have its own PID as with Linux threads. In addition, Linux tools such as top do not show the threads of a process consuming CPU cycles. This can be tested by executing the thread_test program in one tty and the top program in another tty.

Identifying Process Hangs
Now that we have covered the building blocks of processes and threads, it is time to address process hangs and their potential causes. There is little hope that killing an offending process with -9 (sigkill) will lead to discovering the root cause of a process hang. Neither will rebooting the OS unless you are dealing with a stale file handle. Furthermore, these steps will not prevent these anomalies from reoccurring. However, by applying the knowledge of how processes are created and how resources are used, the root cause can be identified.

When a process appears hung, the first step toward a solution is determining certain critical information about the task. Using the ps command, start by determining the task's state, threads, priority, parent, and wait channel. In addition, identify the cumulative CPU time and the initial start time of the task. A holistic approach is needed because although a single ps command is a good start, it will not deliver all the needed information.

Let us first determine whether the process is hung because sometimes a process appears to be blocked when it actually is in the middle of a computation or non-blocking I/O. If cumulative CPU time constantly grows, the task's state will most likely be R. In this state, the process is on the run queue and does not have a wait channel. Monitor its cumulative CPU time. If the process remains in the run queue, it might be performing some calculation that takes a while. Even the fastest computers in the world take a while to calculate an infinite loop! Nevertheless, note that a process in the run state could be the normal operation of that program, an application "feature," or a driver problem.


More Stories By James Kirkland

James Kirkland is the advocate for Red Hat's initiatives and solutions for the Internet of Things(IoT) and is the architect of its three-tier strategy for IoT deployments. For the past five years, James has been focused on IoT solutions for the transportation and energy sectors. A frequent public speaker and writer on a wide range of technical topics, James is also the co-author of Linux Troubleshooting for System Administrators and Power Users (ISBN: 0131855158) published by Prentice Hall PTR. He has been working with UNIX and Linux variants over the course of 20 years in his positions at Red Hat, and in previous roles at Racemi and Hewlett-Packard.

More Stories By David Carmichael

David Carmichael works for Hewlett-Packard as a technical problem manager in Alpharetta, Georgia. He earned a bachelors degree in computer science from West Virginia University in 1987 and has been helping customers resolve their IT problems ever since. David has written articles for HP's IT Resource Center (http://itrc.hp.com) and presented at HP World 2003.

More Stories By Greg Tinker

Greg Tinker began his career while at Bellsouth in Atlanta, Georgia. Greg joined Hewlett-Packard in 1999. Greg's primary role is as a storage business recovery specialist and has participated in HP World, taught several classes in Unix/Linux and Disk Array technology, and obtained various certifications including certifications in Advanced Clusters, SAN, and Linux.

More Stories By Chris Tinker

Chris Tinker began his career in computers while working as a Unix System Administrator for Lockheed Martin in Marietta, Georgia. Chris joined Hewlett-Packard in 1999. Chris's primary role at HP is as a senior software business recovery specialist and has participated in HP World, taught several classes in Unix/Linux and Disk Array technology, and obtained various certifications including certifications in Advanced Clusters, SAN, and Linux.

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Most Recent Comments
Linux News Desk 07/13/06 04:59:25 PM EDT

Troubleshooting a Linux process follows the same general methodology as that used with traditional UNIX systems. In both systems, for process hangs, we identify the system resources being used by the process and attempt to identify the cause for the process to stop responding. With application core dumps, we must identify the signal for which the process terminated and proceed with acquiring a stack trace to identify system calls made by the process at the time it died. There exists neither a 'golden' troubleshooting path nor a set of instructions that can be applied for all cases. Some conditions are much easier to solve than others, but with a good understanding of the fundamentals, a solution is not far from reach.

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