Introducing DTrace

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Bryan Cantrill

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Nov 15, 2003, 12:58:26 AM11/15/03
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All,

The DTrace cat has been pawing its way out of the metaphorical bag for quite
some time now, with Gavin Maltby posting some especially enticing examples
here on comp.unix.solaris. Now that Solaris Express 11/03 is publicly
available for download, we on the DTrace engineering team wanted to take a
moment to post some slightly more elaborate examples of this new technology.

Before we get started, note that there is quite a bit of publicly available
documentation on DTrace at BigAdmin:

http://www.sun.com/bigadmin/content/dtrace

Also, note that this post isn't meant to be a comprehensive tutorial -- it's
really more of a quick demonstration of how DTrace might be used to solve
problems that you may come across in your day-to-day workings with Solaris.

On with the show: let's say you're looking at mpstat(1) output on your
multiuser server. You might see something like this:

CPU minf mjf xcal intr ithr csw icsw migr smtx srw syscl usr sys wt idl
12 1 27 3504 338 206 765 27 114 65 1 337 9 19 41 31
13 1 19 5725 98 68 723 22 108 120 0 692 3 17 20 61
14 0 57 3873 224 192 670 22 75 86 1 805 7 10 35 48
15 36 7 1551 42 28 689 6 68 59 0 132 2 9 35 54
16 14 7 7209 504 457 1031 37 125 244 0 459 6 30 4 60
17 5 5 4960 150 108 817 37 98 154 0 375 6 26 6 62
18 5 6 6085 1687 1661 741 60 76 248 0 434 3 33 0 64
19 0 15 10037 72 41 876 23 100 291 1 454 2 19 9 71
20 12 5 5890 746 711 992 32 122 216 2 960 10 33 4 53
21 60 5 1567 729 713 467 15 80 59 0 376 2 35 10 53
22 0 6 4378 315 291 751 17 84 142 1 312 3 16 1 80
23 0 6 12119 33 3 874 20 82 384 1 513 4 24 11 62

And well you may wonder (as perhaps you often have) -- what the hell is
causing all of those cross calls, anyway? (Cross calls appear in the "xcal"
column; see mpstat(1).)

Using DTrace, investigating this is a snap:

# dtrace -n xcalls'{@[execname] = count()}'
dtrace: description 'xcalls' matched 4 probes
[ letting this run for a few seconds ]
^C

mozilla-bin 1
lockd 1
in.mpathd 2
nsrmmd 5
grep 6
chmod 6
cat 6
nwadmin 13
ls 24
in.tftpd 28
nsrindexd 34
fsflush 38
cut 42
find 42
mkdir 66
rm 76
ipop3d 78
scp 79
inetd 96
dtrace 111
nawk 118
imapd-simmonmt 126
rmdir 132
sshd 138
rpc.rstatd 159
mv 398
save 1292
gzip 1315
get_all2 1678
sched 1712
nfsd 3709
tar 27054

So the bulk of these are coming from tar(1) processes. To explore the cross
calls coming from tar specifically, we could add a predicate to the above:

# dtrace -n xcalls'/execname == "tar"/{@[pid] = count()}'
dtrace: description 'xcalls' matched 4 probes
[ again letting this run for a few seconds ]
^C

938339 507
938415 784
938229 2438
938390 3282
938363 6464
937067 13984

But now we notice a strange thing: none of these processes seems to be
around by the time we're back on the shell, e.g.:

# pflags 937067
pflags: cannot examine 937067: no such process or core file

So is someone launching a bunch of short-lived tar processes? To explore
this as a rough first cut, we could just print the pid whenever a tar
process is launched:

# dtrace -n syscall::exec\*:return'/execname == "tar"/{trace(pid)}'
CPU ID FUNCTION:NAME
18 112 exece:return 954538
21 112 exece:return 954519
17 112 exece:return 954584
21 112 exece:return 954579
21 112 exece:return 954607
22 112 exece:return 954632
21 112 exece:return 954658
20 112 exece:return 954684
20 112 exece:return 954705
15 112 exece:return 954727
14 112 exece:return 954763
16 112 exece:return 954775
12 112 exece:return 954798
18 112 exece:return 954818
22 112 exece:return 954873
22 112 exece:return 954889
23 112 exece:return 954893
14 112 exece:return 954916
14 112 exece:return 954971
...

Seeing that this is happening with some frequency (the above has output
every second or so), we want to explore more generally what is being
exec(2)'d on the machine:

# dtrace -n syscall::exec\*:entry'{@[copyinstr(arg0)] = count()}'
[ letting this run for a bit ]
^C

/usr/lib/mail.local 1
/usr/bin/true 1
/usr/bin/pgrep 1
/usr/bin/wc 1
/usr/bin/touch 1
/usr/bin/sleep 1
/usr/sbin/modinfo 4
/bin/awk 4
/bin/sh 4
/usr/bin/chmod 14
/usr/bin/cat 14
/usr/bin/tar 14
./do1 14
/usr/bin/cut 15
/usr/bin/grep 15
/usr/bin/find 15
/usr/bin/ls 15
/usr/bin/mv 15
/usr/bin/rm 16
/usr/bin/mkdir 17
/usr/bin/gzip 29
/usr/bin/nawk 44
/usr/bin/rmdir 48
/home/ellenp/rmdir 48
/home1/netscape/rmdir 48
/usr/dt/bin/rmdir 96
/usr/openwin/bin/rmdir 96

There's a bunch that's interesting here -- but for now we're interested
in those tar processes. In particular, when someone is exec'ing a tar
process, we want to know who exactly they are. To do this, we write
the following DTrace script, and call it "whotar.d":

syscall::exec*:entry
/copyinstr(arg0) == "/usr/bin/tar"/
{
@[execname] = count();
}

Running this points us straight to the culprit:

# dtrace -s whotar.d
[ again letting this run for a bit ]
^C

get_all2 22

So get_all2 is the problem. We can now use traditional tools to hone in
on this:

# ptree `pgrep get_all2`
100578 /usr/sbin/inetd -s
900870 in.rlogind
900872 -csh
901382 /bin/sh get_all3
901383 /bin/sh ./get_all2 80/80acc080 90/00fccc90
959045 /bin/sh ./get_all2 80/80acc080 90/00fccc90
959046 tar xvf - explorer.80c23d8e.ptgw02is-2003.10.27.09.35/README
959047 gzip -dc /net/pamer.central/proactive/rawdata/8e/80c23d8e
901384 /bin/sh ./get_all2 90/80bdbb90 a0/1274eda0
959565 /bin/sh ./get_all2 90/80bdbb90 a0/1274eda0
959566 tar xvf - explorer.8089ed9d.curly-2002.05.06.10.00/README exp
959567 gzip -dc /net/pamer.central/proactive/rawdata/9d/8089ed9d
901385 /bin/sh ./get_all2 a0/80aabda0 b0/1d6c05b0
959619 mv disks/diskinfo disks/format.out disks/mount.out etc/release
901386 /bin/sh ./get_all2 b0/80b264b0 c0/335419c0
959612 /bin/sh ./get_all2 b0/80b264b0 c0/335419c0
959613 tar xvf - explorer.82a86abd.seq22-2002.05.31.21.49/README exp
959614 gzip -dc /net/pamer.central/proactive/rawdata/bd/82a86abd

So that explains where the tar processes (and thus the cross calls) are coming
from! And just how short-lived _are_ these tar processes? We can answer this
question with DTrace, natch. To do this, we'll use an associative array and
the "timestamp" variable. We are going to linearly quantize the result from 0
seconds to 60 seconds. We'll also use a "tick-1sec" probe to automatically
stop after a minute. The script (which we call "longtar.d"):

syscall::exec*:return
/execname == "tar"/
{
start[pid] = timestamp;
}

fbt::exit:entry
/start[pid]/
{
@["tar time, in seconds"] =
lquantize((timestamp - start[pid]) / 1000000000, 0, 60);
start[pid] = 0;
}

tick-1sec
/n++ >= 60/
{
exit(0);
}

Running this:

# dtrace -s longtar.d
dtrace: script 'longtar.d' matched 4 probes
CPU ID FUNCTION:NAME
23 38110 :tick-1sec

tar time, in seconds
value ------------- Distribution ------------- count
< 0 | 0
0 |@ 1
1 |@@@@@ 4
2 |@@@@@@@@@@@ 8
3 |@@@@@@@ 5
4 |@@@@@@@ 5
5 |@@ 2
6 | 0
7 |@@@@ 3
8 | 0

This highlights just how difficult this problem would be to find using
traditional (process-centric) tools: each tar process is only alive
for (roughly) two seconds! And yet, taken together, the tar processes
are accounting for the vast majority of cross calls on the system.

This is a good example of using DTrace because the problem here is _not_ a
single application -- it's an insurgent army of tar processes that are
slipping away before they can be noticed. Indeed, running prstat(1) on this
machine doesn't show tar -- or _any_ of the get_all2 progeny processes -- as
top resource-consuming processes. Using DTrace, however, one can quickly
root-cause aberrant systemic behavior of this nature.

Of course, this is but a fraction of what DTrace can do. We encourage you to
check out the AnswerBook at BigAdmin:

http://www.sun.com/bigadmin/content/dtrace

Or better yet, go to:

http://wwws.sun.com/software/solaris/solaris-express/get.html

There you can download Solaris Express 11/03 and try DTrace yourself!

Enjoy,
Bryan Cantrill, Mike Shapiro and Adam Leventhal
DTrace Engineering Team

----------------------------------------------------------------------------
Bryan Cantrill, Solaris Kernel Development. b...@eng.sun.com (650) 786-3652

Thommy M.

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Nov 20, 2003, 4:40:26 PM11/20/03
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b...@kiowa.eng.sun.com (Bryan Cantrill) writes:

> 959612 /bin/sh ./get_all2 b0/80b264b0 c0/335419c0
> 959613 tar xvf - explorer.82a86abd.seq22-2002.05.31.21.49/README exp
> 959614 gzip -dc /net/pamer.central/proactive/rawdata/bd/82a86abd
>
> So that explains where the tar processes (and thus the cross calls) are coming
> from!


So that explains why it takes some time to get the explorer dump to the database... ;-)

/Thommy M.

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