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Top Ten DTrace (D) Scripts


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:

Number 10: Print Utilization statistics per process


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!

Number 9: File System Flush Activity


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!

Number 8: Seek Sizes


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!

Number 7: Print Overall Paging Activity


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!

Number 6: Getting System Wide errno Information


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!

Number 5: Figuring out how much I/O is generated per process


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!

Number 4: I/O Sizes Per Process


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!

Number 3: TCP Top


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!

Number 2: Who's paging and DTrace enhanced vmstat


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***

1. Printing I/O activity in a Top-like display


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

Conclusion


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. If you have questions or comments on the article, please feel free to E-mail the author.

References