Memory Leaks


About this document
When a system hangs

About this document

This document addresses memory leaks and what to do when paging space runs out. When a process first touches a page it is allocated in memory (RAM), an adjacent page in designated paging space is allocated. As a process continues grabbing memory, paging space continues being "reserved" or allocated. When this process finishes, it is responsible for deallocating the paging space it used. A memory leak occurs when a process fails to deallocate its paging space. This may result in paging space becoming entirely consumed, which in time will hang any system.

This document applies to AIX Versions 4.x.

When a system hangs

Processes requesting additional memory are killed once the system runs low on paging space. The system appears hung as new processes and telnet connections are terminated. Error messages such as Not enough memory or Fork function failed are generated. There are three ways to resolve this situation.

  1. Add additional paging space. To know how much paging space is "enough", use the lsps -s command often to get a feel for the %Used of the paging space. Based on this percentage, a system at its maximum workload should have no more than 80% of paging space used.

    Example output of the command lsps -s looks like the following:

     Total Paging Space   Percent Used 
          200MB              51% 
  2. Systems often have plenty of paging space (sometimes 3-4 times RAM) and can still run out. This could be due to a memory leak. The question then is which process is causing the memory leak.

    Discussed below are ways to find out what process is causing the memory leak and the tools used to accomplish this task.

    1. The command ps vg provides useful information. In this case the data in the column labeled SIZE is needed. The SIZE column reports virtual memory (paging space) usage on a per-process basis, in 1KB units.

      Sample output from ps vg | pg looks like the following:

         0     - A   87:42    6   20    8    xx    0    0  0.1  0.0 swapper 
         1     - A  191:58   94  240  240    xx   25   28  0.3  0.0 /etc/init 
       516     - A 70228:47   0   16   20    xx    0    0 97.0  0.0 kproc 
       774     - A    5:53    1   24   28    xx    0    0  0.0  0.0 kproc 
      1032     - A   28:40    0   56   56    xx    0    0  0.0  0.0 kproc 
      1866     - A    0:00    0   24   20    xx    0    0  0.0  0.0 kproc 
      2174 pts/1 A    2:55   31  420  544 32768  260  164  0.0  1.0 aixterm 
      2454     - A    1:32   62  272  224    xx   96   60  0.0  0.0 /usr/dt/b 

      Collect ps vg output at different instances throughout the period of time that %Used from lsps -s grows to 99%. The output can then be examined for large numerical increases from the SIZE column. This process would exhibit extraordinarily large increases in the amount of paging space it uses between the two ps vg readings.

    2. Another tool that can be used to track a memory leak is svmon.

      NOTE: PAIDE/6000 must be installed in order use svmon (and others, such as tprof, netpmon, and filemon). To check if this is installed, enter: lslpp -1

      If you are at AIX Version 4.3.0 or higher, this file can be found on the AIX Base Operating System media. Otherwise, to order PAIDE/6000, contact your AIX support center.

      As root, enter the following command:

          svmon -Pu | more 

      This will list the top memory consumers in decreasing order, the first process being the largest consumer. The rest of the report shows memory and paging space usage for each segment of each process.

      Sample output looks like the following:

      Pid            Command        Inuse        Pin      Pgspace 
      13794             dtwm         1603          1          449 
      Pid:  13794 
      Command:  dtwm 
      Segid  Type  Description         Inuse Pin Pgspace Address Range 
      b23    pers  /dev/hd2:24849          2   0       0 0..1 
      14a5   pers  /dev/hd2:24842          0   0       0 0..2 
      6179   work  lib data              131   0      98 0..891 
      280a   work  shared library text  1101   0      10 0..65535 
      181    work  private               287   1     341 0..310:65277..65535 
      57d5   pers  code,/dev/hd2:61722    82   0       0 0..135 

      In each process report, find items in the Type column identified as work and in the Description column identified as private, and check how many 4KB (4096-byte) pages are used under the Pgspace column. This is the minimum number of working pages this segment is using in all of virtual memory. A Pgspace number that grows but never decreases may indicate a memory leak.

  3. The system may be reaching its Maximum number of PROCESSES allowed per user, or maxuproc. Depending on what maxuproc is set to (default is 40), if a user has already forked a number of processes equal to maxuproc, the system will not allow that user to fork any more processes.

    The maxuproc parameter can be increased via SMIT. Enter SMIT and proceed in sequence through the panels System Environments and then Change / Show Characteristics of the Operating System. The first line on this last screen is maxuproc. Increasing this number by a conservative increment (50-100 at a time) allows users to fork more processes, thus avoiding any Out of memory or Cannot fork messages.

[ Doc Ref: 90605210114766     Publish Date: Jan. 19, 2001     4FAX Ref: 8173 ]