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<!-- ISBN=0672311623 //-->
<!-- TITLE=SAMS TEACH YOURSELF LINUX IN 24 HOURS //-->
<!-- AUTHOR=BILL BALL, STEPHEN SMOOGEN //-->
<!-- PUBLISHER=MACMILLAN //-->
<!-- IMPRINT=SAMS //-->
<!-- PUBLICATION DATE=1998 //-->
<!-- CHAPTER=20 //-->
<!-- PAGES=0291-0312 //-->
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<P><CENTER>
<a href="0299-0301.html">Previous</A> | <a href="../ewtoc.html">Table of Contents</A> | <a href="0305-0307.html">Next</A>
</CENTER></P>
<A NAME="PAGENUM-302"><P>Page 302</P></A>
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<PRE>
# free
total used free shared buffers cached
Mem: 30892 28004 2888 14132 3104 10444
-/+ buffers: 14456 16436
Swap: 34268 7964 26304
</PRE>
<!-- END CODE //-->
<P>This shows a 32MB system with 34MB swap space. Notice that nearly all the
system memory is being used, and nearly 8MB of swap space has been used.
</P>
<P>By default, the free command displays memory in kilobytes, or 1024-byte notation. You
can use the -b option to display your memory in bytes, or the
-m option to display memory in megabytes. You can also use the
free command to constantly monitor how much memory is being used through the
-s command. This is handy as a real-time monitor if you
specify a .01-second update and run the free command in a terminal window under X11.
</P>
<H4><A NAME="ch20_ 12">
Virtual Memory Reporting with the vmstat Command
</A></H4>
<P>The vmstat is a general-purpose monitoring program, which offers real-time display of
not only memory usage, virtual memory statistics, but disk activity, system usage, and
central processing unit (CPU) activity. If you call
vmstat without any command-line options, you'll get a one-time
snapshot, for example:
</P>
<!-- CODE //-->
<PRE>
# vmstat
procs memory swap io system cpu
r b w swpd free buff cache si so bi bo in cs us sy id
0 0 0 7468 1060 4288 10552 1 1 10 1 134 68 3 2 96
</PRE>
<!-- END CODE //-->
<P>If you specify a time interval in seconds on the
vmstat command line, you'll get a continuously scrolling report. Having a constant display of what is going on with your computer can
help you if you're trying to find out why your computer suddenly slows down, or why
there's a lot of disk activity.
</P>
<H4><A NAME="ch20_ 13">
Viewing Your Shell's "Ulimit"ations
</A></H4>
<P>You've already seen how you can limit the size of core dump files previously in this
hour. There are other settings you can set in your shell. If you're using the
bash or pdksh (ksh) shell, you can use the ulimit command's
-a option to print your current settings, for example:
</P>
<!-- CODE //-->
<PRE>
# ulimit -a
core file size (blocks) 1000000
data seg size (kbytes) unlimited
file size (blocks) unlimited
max memory size (kbytes) unlimited
stack size (kbytes) 8192
cpu time (seconds) unlimited
max user processes 256
pipe size (512 bytes) 8
open files 256
virtual memory (kbytes) 2105343
</PRE>
<!-- END CODE //-->
<P>If you're using the tcsh or csh shell, you can use the
limit command to list the current settings:
</P>
<A NAME="PAGENUM-303"><P>Page 303</P></A>
<!-- CODE //-->
<PRE>
$ limit
cputime unlimited
filesize unlimited
datasize unlimited
stacksize 8192 kbytes
coredumpsize 1000000 kbytes
memoryuse unlimited
descriptors 256
memorylocked unlimited
maxproc 256
</PRE>
<!-- END CODE //-->
<P>These limits are different from limits for the root operator. The limits shown in
this example are known as soft limits. To view the shell's hard limits, log in as the root
operator, and use the -a limit option (use -Ha for bash or ksh's
ulimit command), for example:
</P>
<!-- CODE //-->
<PRE>
$ su
Password:
# limit -h
cputime unlimited
filesize unlimited
datasize unlimited
stacksize 8192 kbytes
coredumpsize unlimited
memoryuse unlimited
descriptors 256
memorylocked unlimited
maxproc 256
</PRE>
<!-- END CODE //-->
<P>As you can see, viewing the limits as the root operator in the tcsh shell shows a
much different situation. This is another good reason not to run as the root operator! As
a sysadmin, you can use these settings to limit the amount of memory or number of
processes available to each user. This is extremely handy if you have a number of people working
on your computer at the same time, and you want to conserve system memory. For
using Linux on a standalone computer under your normal login and working conditions,
you'll find the default limits quite reasonable.
</P>
<H4><A NAME="ch20_ 14">
Reclaiming Memory with the kill Command
</A></H4>
<P>As a desperate measure if you need to quickly reclaim memory, you can stop
running programs by using the kill command. In order to kill a specific program, you should
use the ps command to list current running processes, and then stop any or all of them with
the kill command. By default, the ps command lists processes you own and which you can
kill, for example:
</P>
<!-- CODE //-->
<PRE>
# ps
PID TTY STAT TIME COMMAND
367 p0 S 0:00 bash
581 p0 S 0:01 rxvt
582 p1 S 0:00 (bash)
747 p0 S 0:00 (applix)
809 p0 S 0:18 netscape index.html
</PRE>
<!-- END CODE //-->
<A NAME="PAGENUM-304"><P>Page 304</P></A>
<!-- CODE SNIP //-->
<PRE>
810 p0 S 0:00 (dns helper)
945 p0 R 0:00 ps
</PRE>
<!-- END CODE SNIP //-->
<P>The ps command will list the currently running programs and the program's
process number, or PID. You can use this information to kill a process with
</P>
<!-- CODE SNIP //-->
<PRE>
# kill -9 809
</PRE>
<!-- END CODE SNIP //-->
<P>However, if you need to reclaim memory efficiently, you should use the
ps command's -m option, which also lists the memory usage of each process, for example:
</P>
<!-- CODE //-->
<PRE>
# ps -m
PID TTY MAJFLT MINFLT TRS DRS SIZE SWAP RSS SHRD LIB DT COMMAND
747 p0 0 3 16 208 364 140 224 224 0 0 (applix)
582 p1 151 274 124 184 436 128 308 268 0 10 (bash)
959 p0 89 20 28 376 404 0 404 320 0 21 ps -m
367 p0 305 826 220 316 600 64 536 428 0 27 bash
810 p0 313 38 164 696 968 108 860 596 0 47 (dns helpe
581 p0 212 508 28 960 1280 292 988 304 0 171 rxvt
809 p0 2615 1205 3900 3692 8684 1092 7592 4644 0 699 netscape
</PRE>
<!-- END CODE //-->
<P>By using this information, you can see that if you want to reclaim the most memory,
you should stop the Netscape Web browser, as it is using nearly 9MB of system
memory. Although you wouldn't normally use the
kill command to stop programs, the kill command can be helpful to stop runaway, or nonresponsive, programs. The
kill command works by sending a signal to the Linux kernel, along with the PID, so the kernel can act on
the process. There are various signals you can use, although, as I've pointed out, the -9,
or SIGKILL option is the most abrupt and drastic. You can see a list of different signals by
using the kill command's -l option, for example:
</P>
<!-- CODE //-->
<PRE>
# kill -l
1) SIGHUP 2) SIGINT 3) SIGQUIT 4) SIGILL
5) SIGTRAP 6) SIGIOT 7) SIGBUS 8) SIGFPE
9) SIGKILL 10) SIGUSR1 11) SIGSEGV 12) SIGUSR2
13) SIGPIPE 14) SIGALRM 15) SIGTERM 17) SIGCHLD
18) SIGCONT 19) SIGSTOP 20) SIGTSTP 21) SIGTTIN
22) SIGTTOU 23) SIGURG 24) SIGXCPU 25) SIGXFSZ
26) SIGVTALRM 27) SIGPROF 28) SIGWINCH 29) SIGIO
30) SIGPWR
</PRE>
<!-- END CODE //-->
<P>For more details on these signals, and the kill command, see its manual page.
</P>
<P>The ps command has nearly two dozen command-line options, and you can also list
all running processes. See the ps manual page for more information. You can also use the
top command, discussed next, to find and kill processes.
</P>
<H3><A NAME="ch20_ 15">
Getting System Load Information with the top and
xload Commands
</A></H3>
<P>The top command, found under the /usr/bin directory, is a system monitor that
displays
</P>
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