DTrace is a comprehensive and flexible dynamic tracing facility built into the Solaris, FreeBSD and OS X Operating System. DTrace allows dynamic instrumentation of a running system, which can assist with answering questions like “which process is chewing up CPU 38,” or “which user is causing the cross-call activity on CPU 6,” or “which setuid binaries are being executed?”
DTrace uses a scripting language called “D,” which uses a syntax very similar to C and Awk. Several amazing DTrace scripts have been developed and distributed through the Internet, so I thought I would share my favorite DTRace scripts in a Letterman “Top 10” format:
Developer: Brendan Gregg
Download location: prustat
Prustat will list the top processes sorted by CPU, Memory, Disk or Network utilization:
$ prustat -c -t 10 5
PID %CPU %Mem %Disk %Net COMM
7176 0.88 0.70 0.00 0.00 dtrace
7141 0.00 0.43 0.00 0.00 sshd
7144 0.11 0.24 0.00 0.00 sshd
99 0.00 0.22 0.00 0.00 nscd
7146 0.00 0.19 0.00 0.00 bash
This script is super useful for getting a high level understanding
of what is happening on a server. Golden!
Developer: Solaris Internals
Download location: fsflush.d
On Solaris systems, the pagedaemon is reponsible for scanning the page cache and adjusting the MMU reference bit of each dirty page it finds. When the fsflush daemon runs, it scans the page cache looking for pages with the MMU reference bit set, and schedules these pages to be written to disk. The fsflush.d DTrace script provides a detailed breakdown of pages scanned, and the number of nanoseconds that were required to scan “SCANNED” pages:
$ fsflush.d
SCANNED EXAMINED LOCKED MODIFIED COALESCE RELEASES TIME(ns)
4254 4255 1 1 0 0 2695024
4254 4255 1 0 0 0 1921518
4254 4255 6 0 0 0 1989044
4254 4255 1 0 0 0 2401266
4254 4255 4 1 0 0 2562138
4254 4255 89 4 0 0 2425988
Now you might be wondering why “SCANNED” is less than “EXAMINED?” This is due to a bug in fsflush, and a bug report was filed to address this anomaly. Tight!
Developer: Brendan Gregg
Download location: seeksize.d
Prior to Solaris 10, determining if an application accessed data in a sequential or random pattern required reviewing mounds of truss(1m) and vxtrace(1m) data. With the introduction of DTrace and Brendan Gregg’s seeksize.d DTrace script, this question is trivial to answer:
$ seeksize.d
Sampling... Hit Ctrl-C to end.
^C
PID CMD
7312 dd if=/dev/dsk/c1t1d0s2 of=/dev/null bs=1048576
value ------------- Distribution ------------- count
-1 | 0
0 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ 1762
1 | 0
0 sched
value ------------- Distribution ------------- count
-1048576 | 0
-524288 |@@@@ 1
-262144 | 0
-131072 | 0
-65536 | 0
-32768 | 0
-16384 | 0
-8192 | 0
-4096 | 0
-2048 | 0
-1024 | 0
-512 | 0
-256 | 0
-128 | 0
-64 | 0
-32 | 0
-16 | 0
-8 | 0
-4 | 0
-2 | 0
-1 | 0
0 | 0
1 | 0
2 | 0
4 | 0
8 | 0
16 | 0
32 | 0
64 | 0
128 |@@@@ 1
256 |@@@@ 1
512 |@@@@ 1
1024 |@@@@ 1
2048 | 0
4096 | 0
8192 | 0
16384 |@@@@ 1
32768 |@@@@ 1
65536 |@@@@@@@@ 2
131072 | 0
262144 | 0
524288 |@@@@ 1
1048576 | 0
This script measures the seek distance between consecutive reads and writes, and provides a histogram with the seek distances. For applications that are using sequential access patterns (e.g., dd in this case), the distribution will be small. For applications accessing data in a random nature (e.g, sched in this example), you will see a wide distribution. Shibby!
Developer: Solaris Internals
Download location: fspaging.d
Prior to the introduction of DTrace, it was difficult to extract data on which files and disk devices were active at a specific point in time. With the introduction of fspaging.d, you can get a detailed view of which files are being accessed:
$ fspaging.d
Event Device Path RW Size Offset
get-page /lib/sparcv9/libc.so.1 8192
put-page /etc/svc/volatile/init-next.state 8192
put-page /etc/svc/volatile/network-ssh:default.log 8192
put-page /etc/svc/volatile/network-pfil:default.log 8192
This is a super useful script! Niiiiiiiiiiice!
Developer: Brendan Gregg
Download location: errinfo
When system calls have problems executing, they usually return a value to
indicate success or failure, and set the global “ERRNO” variable to a value
indicating what went wrong. To get a system wide view of which system calls are
erroring out, we can use Brendan Gregg’s DTrace script:
$ errinfo -c
Sampling... Hit Ctrl-C to end.
^C
EXEC SYSCALL ERR COUNT DESC
ttymon read 11 1 Resource temporarily unavailable
utmpd ioctl 25 2 Inappropriate ioctl for device
init ioctl 25 4 Inappropriate ioctl for device
nscd lwp_kill 3 13 No such process
fmd lwp_park 62 48 timer expired
nscd lwp_park 62 48 timer expired
svc.startd lwp_park 62 48 timer expired
This will display the process, system call, and errno number and description from /usr/src/sys/errno.h! Jeah!
Developer: Brendan Gregg
Download location: psio.pl
Several Solaris utilities provide a summary of the time spent waiting for I/O which is a meaningless metric, but fail to provide facilities to easily correlate I/O activity with a process. With the introduction of psio.pl, you can see exactly which processes are responsible for generating I/O:
$ psio.pl
UID PID PPID %I/O STIME TTY TIME CMD
root 7312 7309 70.6 16:00:59 pts/2 02:36 dd if=/dev/dsk/c1t1d0s2 of=/dev/null bs=1048576
root 0 0 0.0 10:24:18 ? 00:02 sched
root 1 0 0.0 10:24:18 ? 00:03 /sbin/init
root 2 0 0.0 10:24:18 ? 00:00 pageout
root 3 0 0.0 10:24:18 ? 00:51 fsflush
root 7 1 0.0 10:24:20 ? 00:06 /lib/svc/bin/svc.startd
root 9 1 0.0 10:24:21 ? 00:14 /lib/svc/bin/svc.configd
........
Once you find I/O intensive processes, you can use fspaging, iosnoop, and rwsnoop to get additional information:
$ iosnoop -n
MAJ MIN UID PID D BLOCK SIZE COMM PATHNAME
136 8 0 990 R 341632 8192 dtrace /lib/sparcv9/ld.so.1
136 8 0 990 R 341568 8192 dtrace /lib/sparcv9/ld.so.1
136 8 0 990 R 14218976 8192 dtrace /lib/sparcv9/libc.so.1
........
$ iosnoop -e
DEVICE UID PID D BLOCK SIZE COMM PATHNAME
dad1 0 404 R 481712 8192 vxsvc /lib/librt.so.1
dad1 0 3 W 516320 3072 fsflush /var/adm/utmpx
dad1 0 3 W 18035712 8192 fsflush /var/adm/wtmpx
........
$ rwsnoop
UID PID CMD D BYTES FILE
100 902 sshd R 42 /devices/pseudo/clone&0:ptm
100 902 sshd W 80 <unknown>
100 902 sshd R 65 /devices/pseudo/clone&0:ptm
100 902 sshd W 112 <unknown>
100 902 sshd R 47 /devices/pseudo/clone&0:ptm
100 902 sshd W 96 <unknown>
0 404 vxsvc R 1024 /etc/inet/protocols
........
Smooooooooooth!
Developer: Brendan Gregg
Download location: bitesize.d
As a Solaris administrator, we are often asked to identify application I/O sizes. This information can be acquired for a single process with truss(1m), or system wide with Brendan Gregg’s bitesize.d DTrace script:
$ bitesize.d
Sampling... Hit Ctrl-C to end.
7312 dd if=/dev/dsk/c1t1d0s2 of=/dev/null bs=1048576
value ------------- Distribution ------------- count
16 | 0
32 | 2
64 | 0
128 | 0
256 | 0
512 | 2
1024 | 0
2048 | 0
4096 | 0
8192 | 0
16384 | 0
32768 | 0
65536 | 0
131072 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ 76947
262144 | 0
If only Dorothy could see this!
Developer: Brendan Gregg
Download location: bitesize.d
Snoop(1m) and ethereal are amazing utilities, and provide a slew of options to filter data. When you don’t have time to wade through snoop data or download and install ethereal, you can use tcptop to get an overview of TCP activity on a system:
$ tcptop 5
2005 Jul 19 14:09:06, load: 0.01, TCPin: 2679 Kb, TCPout: 12 Kb
UID PID LADDR LPORT RADDR RPORT SIZE NAME
0 7138 192.168.1.3 44084 192.18.108.40 21 544 ftp
0 352 192.168.1.3 22 192.168.1.8 49805 1308 sshd
100 7134 192.168.1.3 44077 192.168.1.1 22 1618 ssh
0 7138 192.168.1.3 44089 24.98.83.96 51731 2877524 ftp
Now this is some serious bling!
Developer: Brendan Gregg
Download location: whospaging.d
With Solaris 9, the “-p” option was added to vmstat to break paging activity up into “executable,” “anonymous” and “filesystem” page types:
$ vmstat -p 5
memory page executable anonymous filesystem
swap free re mf fr de sr epi epo epf api apo apf fpi fpo fpf
1738152 832320 5 9 0 0 0 0 0 0 0 0 0 1 0 0
1683280 818800 0 2 0 0 0 0 0 0 0 0 0 0 0 0
1683280 818800 0 0 0 0 0 0 0 0 0 0 0 0 0 0
This is super useful information, but unfortunately doesn’t provide the executable responsible for the paging activity. With the introduction of whospaging.d, you can get paging activity per process:
$ whospaging.d
Who's waiting for pagein (milliseconds):
Who's on cpu (milliseconds):
svc.configd 0
sendmail 0
svc.startd 0
sshd 0
nscd 1
dtrace 3
fsflush 14
dd 1581
sched 3284
Once we get the process name that is reponsible for the paging activity, we can use dvmstat to break down the types of pages the application is paging (similar to vmstat -p, but per process!):
$ dvmstat -p 0
re maj mf fr epi epo api apo fpi fpo sy
0 0 0 13280 0 0 0 0 0 13280 0
0 0 0 13504 0 0 0 0 0 13504 0
0 0 0 13472 0 0 0 0 0 13472 0
........
Once we have an idea of which pages are being paged in or out, we can use iosnoop, rwsnoop and fspaging.d to find out which files or devices the application is writing to! Since these rockin’ scripts go hand in hand, I am placing them together. Shizam!
And without further ado, number 1 goes to … drum roll
Developer: Brendan Gregg
Download location: iotop
Download location: rwtop
After careful thought, I decided to make iotop and rwtop #1 on my top ten list. I have long dreamed of a utility that could tell me which applications were actively generating I/O to a given file, device or file system. With the introduction of iotop and rwtop, my wish came true:
$ iotop 5
2005 Jul 19 13:33:15, load: 0.24, disk_r: 95389 Kb, disk_w: 0 Kb
UID PID PPID CMD DEVICE MAJ MIN D BYTES
0 99 1 nscd dad1 136 8 R 16384
0 7037 7033 find dad1 136 8 R 2266112
0 7036 7033 dd sd7 32 58 R 15794176
0 7036 7033 dd sd6 32 50 R 15826944
0 7036 7033 dd sd5 32 42 R 15826944
0 7036 7033 dd vxio21000 100 21000 R 47448064
$ rwtop 5
2005 Jul 24 10:47:26, load: 0.18, app_r: 9 Kb, app_w: 8 Kb
UID PID PPID CMD D BYTES
100 922 920 bash R 3
100 922 920 bash W 15
100 902 899 sshd R 1223
100 926 922 ls R 1267
100 902 899 sshd W 1344
100 926 922 ls W 2742
100 920 917 sshd R 2946
100 920 917 sshd W 4819
0 404 1 vxsvc R 5120
This article introduced several scripts that have helped me debug and solve problems on the Solaris 10 servers I support. For additional details on using dtrace to troubleshoot problems, check out the reference section.
The following references were used while writing this article: