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Linux Processes: Structure, Hangs and Core Dumps

Efficient and effective resolution practices

Troubleshooting the hung process involves these steps:

  1. Identify all the tasks (threads) for the program.
  2. Assess the hanging process. Is it easily reproducible?
  3. Assess the other things going on. What else is the machine doing? Check load and other applications' response time.
The following scenario demonstrates a way of troubleshooting a process that periodically hangs and then continues.

Scenario 2: Troubleshooting a Hanging Web Browser
A user complains that her machine is working great except for her Web browsing application. When accessing Web sites, the browser hangs for a few minutes every so often. The user has installed several different versions and tried other browsers to no avail.

You ask her the following question: Has it ever worked? The reply is "Yes . . . several days ago it was fine."

For the sake of simplicity, we attempt to find the problem with a light Web browser with little overhead.

# strace -f -F -r -T -o /tmp/lynx.strace_2 lynx http://www.hp.com

Using the vi editor and greping for network calls, such as poll(), we can identify what seems to be a problem right away:

:g/poll/p
3660 0.000085 poll([{fd=0, events=POLLIN}], 1, 0) = 0 <0.000020>
3660 0.000186 poll([{fd=0, events=POLLIN}], 1, 0) = 0 <0.000008>
3660 0.000049 poll([{fd=3, events=POLLIN}], 1, 5000) = 0 <5.005154>
3660 0.000043 poll([{fd=3, events=POLLIN}], 1, 5000) = 0 <5.009763>
3660 0.000042 poll([{fd=3, events=POLLIN}], 1, 5000) = 0 <5.008875>
3660 0.000043 poll([{fd=3, events=POLLIN}], 1, 5000) = 0 <5.009264>
3660 0.000042 poll([{fd=3, events=POLLIN}], 1, 5000) = 0 <5.009216>
3660 0.000043 poll([{fd=3, events=POLLIN, revents=POLLIN}], 1, \ 5000) = 1 <0.001146>
3660 0.000081 poll([{fd=0, events=POLLIN}], 1, 0) = 0 <0.000017>
3660 0.000088 poll([{fd=0, events=POLLIN}], 1, 0) = 0 <0.000008>
3660 0.000088 poll([{fd=0, events=POLLIN}], 1, 0) = 0 <0.000022>

We see that some poll() calls took over five seconds each. That would explain the Web browser hanging and taking a long time to browse sites.

Focusing on the trace, we see the following:

3660 0.000254 socket(PF_INET, SOCK_DGRAM, IPPROTO_IP) = 3 <0.000044>
3660 0.000095 connect(3, {sa_family=AF_INET, sin_port=htons(53), \
sin_addr=inet_addr("15.50.74.40")}, 28) = 0 <0.000017>
3660 0.000108 send(3, "\245`\1\0\0\1\0\0\0\0\0\0\3www\2hp\3com\0\0\
34\0\1", 28, 0) = 28 <0.000404>
3660 0.000476 gettimeofday({1097369839, 928119}, NULL) = 0 <0.000005>
3660 0.000049 poll([{fd=3, events=POLLIN}], 1, 5000) = 0 <5.005154>
3660 5.005262 send(3, "\245`\1\0\0\1\0\0\0\0\0\0\3www\2hp\3com\0\0\
34\0\1", 28, 0) = 28 <0.000426>

Checking the man page on poll, we see that it is waiting for an event to take place on a file descriptor.

# ls -al /proc/3660/fd/3 -> socket:[52013]

This confirms a network issue. After reviewing the /etc/resolv.conf file, we see that 15.50.64.40 is the nameserver.

The user contacted her IT department and found that the nameserver configuration for her network had changed. Switching to a different nameserver in the resolv.conf file alleviated the five-second poll() call and resolved the problem.
Commands commonly used to troubleshoot a hung process include the following:

  • ps-Concentrate on pid, ppid, state, comm, time, pri, size, and wchan flags.
  • lsof-Determine the open files on the system.
  • pstree-Focus on processes and how they relate to each other.
  • strace-Flags most commonly used: -f -F -r -T -o <outfile>.
  • man pages-Believe it or not, the manual helps.
  • source code-Use it to determine what the application is doing.
  • /proc filesystem-It offers a wealth of information.
In Scenario 3, we show a process that appears to be hung but is not.

Scenario 3: Troubleshooting an Apparent Process Hang
In this scenario, a user's goal is to create a file that takes data, automatically compresses it, and sends it to a new file. To perform this task, the user creates a named pipe and issues gzip, redirecting input from the pipe to a new file. The odd part is that the gzip process seems to hang, and the user cannot find the gzip process when searching ps -ef. So you devise an action plan: Re-create the event and trace the process involved.

1.  Create a named pipe.

$ mknod /tmp/named_pipe p
$ ll /tmp/named_pipe
prw-r--r-- 1 chris chris 0 Oct 9 16:53 /tmp/named_pipe|


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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