top的用法

top

The top command is probably the most useful one for an Oracle DBA managing a database on Linux. Say the system is slow and you want to find out who is gobbling up all the CPU and/or memory. To display the top processes, you use the command top.

Note that unlike other commands, top does not produce an output and sits still. It refreshes the screen to display new information. So, if you just issue top and leave the screen up, the most current information is always up. To stop and exit to shell, you can press Control-C.

$ top

18:46:13  up 11 days, 21:50,  5 users,  load average: 0.11, 0.19, 0.18 
151 processes: 147 sleeping, 4 running, 0 zombie, 0 stopped 
CPU states:  cpu    user    nice  system    irq  softirq  iowait    idle 
           total   12.5%    0.0%    6.7%   0.0%     0.0%    5.3%   75.2% 
Mem:  1026912k av,  999548k used,   27364k free,       0k shrd,  116104k buff 
                    758312k actv,  145904k in_d,   16192k in_c 
Swap: 2041192k av,  122224k used, 1918968k free                  590140k cached 
 
  PID USER     PRI  NI  SIZE  RSS SHARE STAT %CPU %MEM   TIME CPU COMMAND 
  451 oracle    15   0  6044 4928  4216 S     0.1  0.4   0:20   0 tnslsnr 
 8991 oracle    15   0  1248 1248   896 R     0.1  0.1   0:00   0 top 
    1 root      19   0   440  400   372 S     0.0  0.0   0:04   0 init 
    2 root      15   0     0    0     0 SW    0.0  0.0   0:00   0 keventd 
    3 root      15   0     0    0     0 SW    0.0  0.0   0:00   0 kapmd 
    4 root      34  19     0    0     0 SWN   0.0  0.0   0:00   0 ksoftirqd/0 
    7 root      15   0     0    0     0 SW    0.0  0.0   0:01   0 bdflush 
    5 root      15   0     0    0     0 SW    0.0  0.0   0:33   0 kswapd 
    6 root      15   0     0    0     0 SW    0.0  0.0   0:14   0 kscand 
    8 root      15   0     0    0     0 SW    0.0  0.0   0:00   0 kupdated 
    9 root      25   0     0    0     0 SW    0.0  0.0   0:00   0 mdrecoveryd
... output snipped ...

Let's examine the different types of information produced. The first line:

18:46:13  up 11 days, 21:50,  5 users,  load average: 0.11, 0.19, 0.18

shows the current time (18:46:13), that system has been up for 11 days; that the system has been working for 21 hours 50 seconds. The load average of the system is shown (0.11, 0.19, 0.18) for the last 1, 5 and 15 minutes respectively. (By the way, you can also get this information by issuing the uptime command.)

If the load average is not required, press the letter "l" (lowercase L); it will turn it off. To turn it back on press l again. The second line:

151 processes: 147 sleeping, 4 running, 0 zombie, 0 stopped

shows the number of processes, running, sleeping, etc. The third and fourth lines:

CPU states:  cpu    user    nice  system    irq  softirq  iowait    idle 
           total   12.5%    0.0%    6.7%   0.0%     0.0%    5.3%   75.2%

show the CPU utilization details. The above line shows that user processes consume 12.5% and system consumes 6.7%. The user processes include the Oracle processes. Press "t" to turn these three lines off and on. If there are more than one CPU, you will see one line per CPU.

The next two lines:

Mem:  1026912k av, 1000688k used,  26224k free,    0k shrd,  113624k buff 
                    758668k actv,  146872k in_d,  14460k in_c
Swap: 2041192k av, 122476k used,   1918716k free             591776k cached

show the memory available and utilized. Total memory is "1026912k av", approximately 1GB, of which only 26224k or 26MB is free. The swap space is 2GB; but it's almost not used. To turn it off and on, press "m".

The rest of the display shows the processes in a tabular format. Here is the explanation of the columns:

While the top is being displayed, you can press a few keys to format the display as you like. Pressing the uppercase M key sorts the output by memory usage. (Note that using lowercase m will turn the memory summary lines on or off at the top of the display.) This is very useful when you want to find out who is consuming the memory. Here is sample output:

PID USER     PRI  NI  SIZE  RSS SHARE STAT %CPU %MEM   TIME CPU COMMAND 
31903 oracle    15   0 75760  72M 72508 S     0.0  7.2   0:01   0 ora_smon_PRODB2 
31909 oracle    15   0 68944  66M 64572 S     0.0  6.6   0:03   0 ora_mmon_PRODB2 
31897 oracle    15   0 53788  49M 48652 S     0.0  4.9   0:00   0 ora_dbw0_PRODB2

Now that you learned how to interpret the output, let's see how to use command line parameters.

The most useful is -d, which indicates the delay between the screen refreshes. To refresh every second, use top -d 1.

The other useful option is -p. If you want to monitor only a few processes, not all, you can specify only those after the -p option. To monitor processes 13609, 13608 and 13554, issue:

top -p 13609 -p 13608 -p 13554

This will show results in the same format as the top command, but only those specific processes.

20:51:14  up 11 days, 23:55,  4 users,  load average: 0.88, 0.39, 0.27 
113 processes: 110 sleeping, 2 running, 1 zombie, 0 stopped 
CPU states:  cpu    user    nice  system    irq  softirq  iowait    idle 
           total    1.0%    0.0%    5.6%   2.2%     0.0%   91.2%    0.0% 
Mem:  1026912k av, 1008832k used,   18080k free,       0k shrd,   30064k buff 
                    771512k actv,  141348k in_d,   13308k in_c 
Swap: 2041192k av,   66776k used, 1974416k free                  812652k cached 
 
  PID USER     PRI  NI  SIZE  RSS SHARE STAT %CPU %MEM   TIME CPU COMMAND 
16143 oracle    15   0 39280  32M 26608 D     4.0  3.2   0:02   0 oraclePRODB2...
    5 root      15   0     0    0     0 SW    1.6  0.0   0:33   0 kswapd
... output snipped ...

select s.sid, s.username, s.program
from v$session s, v$process p
where spid = 16143
and p.addr = s.paddr
/

       SID USERNAME PROGRAM
------------------- -----------------------------
       159 SYS      rman@prolin2 (TNS V1-V3)