Monitoring Shared Memory

You can monitor use of memory that your UDR allocates (explicitly or implicitly) with the following options of the onstat utility:

• The -g ses option outputs session information.

  You can specify a particular session identifier after the ses keyword. If you do not include a session identifier, onstat -g ses outputs a one-line summary for each active session. Look for any session whose memory allocation or usage steadily increases.

• The -g mem option outputs statistics for a memory pool.

  Internally, the database server organizes memory allocations into memory pools by duration. You can specify a particular pool name after the mem keyword. If you do not include a pool name, onstat -g mem outputs a one-line summary for each memory pool. To detect memory leakage, look for any pool whose memory allocation or usage steadily increases.

Monitoring memory is useful for tracking down memory leakage. Memory leakage occurs when memory remains allocated after the structure that holds its address was deallocated. There is no way to access the memory once its address is gone. Therefore, the memory remains with no way to remove it.

Suppose that the onstat -g ses command produces the following sample output:

session                                      #RSAM     total       used
id      user      tty      pid     hostname  threads   memory      memory
24      informix   -         0     -         0         8192          5880
23      informix   -     13453     bison     1         6701056    6608512
8       informix   -         0     -         0         8192          5880
7       informix   -         0     -         0         16384        14344
6       informix   -         0     -         0         8192          5880
4       informix   -         0     -         0         16384        14344
3       informix   -         0     -         0         8192          5880
2       informix   -         0     -         0         8192          5880
ps auxw | grep 13453

Suppose also that your DB–Access session is hooked to session 23 (the italicized line in the preceding sample output).

ps auxw | grep 13453

You can now obtain information about memory-pool usage with the
-g mem option of onstat:

onstat -g mem

Suppose that the preceding onstat command generated the following sample output (some output lines are omitted for brevity):

Pool Summary:
name         class      addr
resident         R      a002018
res-buff         R      a118018
global           V      a18a018
mt               V      a18e018
smartblob        V      a192018
...
23               V      a3e0018
24               V      a3e2018
23.RTN.SAPI      V      a40c018
23.CMD.SAPI      V      a3ee018
...

Figure 88 shows the lines of the onstat -g mem output that indicate user memory allocations.

Figure 88. Memory Pools in onstat -g mem Output

Each memory duration has a separate memory pool. The three letters before "SAPI" identify each memory pool. The following table shows the memory-pool names for regular and advanced memory durations.

After you determine a specific session identifier or memory-pool name that exhibits a problem, you can find out which specific kind of memory is affected with the -g ufr option of onstat. The -g ufr option of onstat shows memory fragments by usage. For example, the following onstat command captures a snapshot every 30 seconds of memory pools that session 23 uses:

onstat -g ufr 23 -r 30

Sample output for the preceding command follows:

Memory usage for pool name 23:
size         memid
2152         log
2016         ostcb
2600         sqtcb
8472         gentcb
1664         osenv
6392648      sqscb
792          filetable
112          rdahead
120          overhead
96           scb
416          sapi
3296         fragman
18808        opentable
280          hashfiletab
10056        temprec
592          GenPg
224          ru
56           sort
94848        ralloc

In the preceding sample output, the main memory consumer is sqscb.

Any memory leakage from a DataBlade API function would show up in the memid column entry labelled sapi. For more information on the onstat command, see the IBM Informix: Administrator's Reference.