dtrace_topics_intro.pdf

1
# dtrace -n 'syscall:::entry { @[exe
dtrace: description 'syscall:::entry
^C
iscsitgtd 1
nscd 1
operapluginclean 3
screen-4.0.2 3
devfsadm 4
httpd 10
sendmail 10
xload 10
evince 12
operapluginwrapp 20
xclock 20
xntpd 25
FvwmIconMan 32
fmd 81
FvwmPager 170
dtrace 432
gnome-terminal 581
fvwm2 1045
x64 1833
akd 2574
opera 2923
Xorg 4723
soffice.bin 5037
DTrace Topics:
Introduction
Brendan Gregg
Sun Microsystems
April 2007
1

2
DTrace Topics: Introduction
• This presentation is an introduction to DTrace, and
is part of the “DTrace Topics” collection.
> Difficulty:
> Audience: Everyone
• These slides cover:
> What is DTrace
> What is DTrace for
> Who uses DTrace
> DTrace Essentials
> Usage Features

3
What is DTrace
• DTrace is a dynamic troubleshooting and analysis
tool first introduced in the Solaris 10 and
OpenSolaris operating systems.
• DTrace is many things, in particular:
> A tool
> A programming language interpreter
> An instrumentation framework
• DTrace provides observability across the entire
software stack from one tool. This allows you to
examine software execution like never before.

4
DTrace example #1
• Tracing new processes system-wide,
System calls are only one layer of the software stack.
# dtrace -n 'syscall::exece:return { trace(execname); }'
dtrace: description 'syscall::exece:return ' matched 1 probe
CPU ID FUNCTION:NAME
0 76044 exece:return man
0 76044 exece:return sh
0 76044 exece:return neqn
0 76044 exece:return tbl
0 76044 exece:return nroff
0 76044 exece:return col
0 76044 exece:return mv
0 76044 exece:return more

5
The Entire Software Stack
• How did you analyze these?
Kernel
Memory
allocation Scheduler
Device Drivers
Syscall Interface
Libraries
User Executable
Dynamic Languages
Hardware
Examples:
Java, JavaScript, ...
/usr/bin/*
/usr/lib/*
VFS, DNLC, UFS,
ZFS, TCP, IP, ...
sd, st, hme, eri, ...
man -s2
disk data controller
File Systems

6
• It was possible, but difficult:
Kernel
Memory
Device Drivers
Syscall Interface
Libraries
User Executable
Dynamic Languages
Hardware
Previously:
debuggers
truss -ua.out
apptrace, sotruss
prex; tnf*
lockstat
mdb
truss
kstat, PICs, guesswork
File Systems

7
• DTrace is all seeing:
Kernel
Memory
Device Drivers
Syscall Interface
Libraries
User Executable
Dynamic Languages
Hardware
DTrace visibility:
Yes, with providers
Yes
Yes
Yes
Yes
No. Indirectly, yes
File Systems

8
What DTrace is like
• DTrace has the combined capabilities of numerous
previous tools and more:
Plus a programming language similar to C and awk.
Tool Capability
truss -ua.out tracing user functions
apptrace tracing library calls
truss tracing system calls
prex; tnf* tracing some kernel functions
lockstat profiling the kernel
mdb -k accessing kernel VM
mdb -p accessing process VM

9
Syscall Example
• Using truss:
$ truss date
execve("/usr/bin/date", 0x08047C9C, 0x08047CA4) argc = 1
resolvepath("/usr/lib/ld.so.1", "/lib/ld.so.1", 1023) = 12
resolvepath("/usr/bin/date", "/usr/bin/date", 1023) = 13
xstat(2, "/usr/bin/date", 0x08047A58) = 0
open("/var/ld/ld.config", O_RDONLY) = 3
fxstat(2, 3, 0x08047988) = 0
mmap(0x00000000, 152, PROT_READ, MAP_SHARED, 3, 0) = 0xFEFB0000
close(3) = 0
mmap(0x00000000, 4096, PROT_READ|PROT_WRITE|PROT_EXEC, MAP_PRIVATE|MAP_ANON, -1
sysconfig(_CONFIG_PAGESIZE) = 4096
[...]
Only examine 1 process
Output is
limited to
provided
options
truss slows down the target

10
Syscall Example
• Using DTrace:
# dtrace -n 'syscall:::entry { printf("%16s %x %x", execname, arg0, arg1); }'
dtrace: description 'syscall:::entry ' matched 233 probes
1 75943 read:entry Xorg f 8047130
1 76211 setitimer:entry Xorg 0 8047610
1 76143 writev:entry Xorg 22 80477f8
1 76255 pollsys:entry Xorg 8046da0 1a
1 75943 read:entry Xorg 22 85121b0
1 76035 ioctl:entry soffice.bin 6 5301
1 76035 ioctl:entry soffice.bin 6 5301
1 76255 pollsys:entry soffice.bin 8047530 2
[...]
You choose the output
Watch every process
Minimum performance cost

11
What is DTrace for
• Troubleshooting software bugs
> Proving what the problem is, and isn't.
> Measuring the magnitude of the problem.
• Detailed observability
> Observing devices, such as disk or network activity.
> Observing applications, whether they are from Solaris,
3rd
party, or in-house.
• Capturing profiling data for performance analysis
> If there is latency somewhere, DTrace can find it

12
What isn't DTrace
• DTrace isn't a replacement for kstat or SMNP
> kstat already provides inexpensive long term monitoring.
• DTrace isn't sentient, it needs to borrow your brain
to do the thinking
• DTrace isn't “dTrace”

13
Who is DTrace for
• Application Developers
> Fetch in-flight profiling data without restarting the apps,
even on customer production servers.
> Detailed visibility of all the functions that they wrote, and
the rest of the software stack.
> Add static probes as a stable debug interface.
• Application Support
> Provides a comprehensive insight into application
behavior.
> Analyze faults and root-cause performance issues.
> Prove where issues are, and measure their magnitude.

14
Who is DTrace for
• System Administrators
> Troubleshoot, analyze, investigate where never before.
> See more of your system - fills in many observability
gaps.
• Database Administrators
> Analyze throughput performance issues across all
system components.
• Security Administrators
> Customized short-term auditing
> Malware deciphering

15
Who is DTrace for
• Kernel Engineers
> Fetch kernel trace data from almost every function.
> Function arguments are auto-casted providing access to
all struct members.
> Fetch nanosecond timestamps for function execution.
> Troubleshoot device drivers, including during boot.
> Add statically defined trace points for debugging.

16
How to use DTrace
• DTrace can be used by either:
> Running prewritten one-liners and scripts
– DTrace one-liners are easy to use and ofter useful,
http://www.solarisinternals.com/dtrace
– The DtraceToolkit contains over 100 scripts ready to run,
http://www.opensolaris.org/os/community/dtrace/dtracetoolkit
> Writing your own one-liners and scripts
– Encouraged – the possibilities are endless
– It helps to know C
– It can help to know operating system fundamentals

17
DTrace wins
• Finding unnecessary work
> Having deep visibility often finds work being performed
that isn't needed. Eliminating these can produce the
biggest DTrace wins – 2x, 20x, etc.
• Solving performance issues
> Being able to measure where the latencies are, and
show what their costs are. These can produce typical
performance wins – 5%, 10%, etc.

18
DTrace wins
• Finding bugs
> Many bugs are found though static debug frameworks;
DTrace is a dynamic framework that allows custom and
comprehensive debug info to be fetched when needed.
• Proving performance issues
> Many valuable DTrace wins have no immediate percent
improvement, they are about gathering evidence to
prove the existence and magnitude of issues.

19
Example scenario: The past
• Take a performance issue on a complex customer
system,
• With previous observability tools, customers could
often find problems but not take the measurements
needed to prove that they found the problem.
> What is the latency cost for this issue? As a percent?
Customer:
“Why is our system slow?”

20
• The “blame wheel”
Application Vendor:
“The real problem
may be the database.”
Database Vendor:
“The real problem
may be the OS.”
OS Vendor:
“The real problem may be the application.”

21
• The lack of proof can mean stalemate.
Customer:
“I think I've found the issue
in the application code.”
Application Vendor:
“That issue is costly to fix.
We are happy to fix it, so long as
you can prove that this is the issue.”

22
Example scenario: The future
A happy ending
• With DTrace, all players can examine all of the
software themselves.
– Example: “80% of the average transaction time is spent in the
application waiting for user-level locks.”
Customer:
“I measured the problem,
it is in the application.”
Application Vendor:
“I'd better fix that right away.”

23
Example scenario: The future
An alternate happy ending for application vendors
– Example: “80% of our average transaction time is consumed by
a bug in libc.”
OS Vendor:
“We'd better fix that right away.”
Application Vendor:
“We measured the problem
and found it was in the OS.”

24
Answers to initial questions
• DTrace is not available for Solaris 9.
• You need to be root, or have the correct privileges,
to run /usr/sbin/dtrace.
• There is a GUI called chime.
• DTrace is safe for production use, provided you
don't deliberately try to cause harm.
• DTrace has low impact when in use, and zero
impact when not.

25
What's next:
• We just covered:
> What is DTrace
> Who uses DTrace
• Next up is:
> DTrace Essentials
> Usage Features

26
Terminology
• Example #1
[...]
consumer probe action

27
Consumer
• Consumers of libdtrace(3LIB),
dtrace command line and scripting interface
lockstat kernel lock statistics
plockstat user-level lock statistics
intrstat run-time interrupt statistics
• libdtrace is currently a private interface and not to
be used directly (nor is there any great reason to);
the supported interface is dtrace(1M).
> NOTE: You are still encouraged to use libkstat(3LIB) and
proc(4) directly, rather than wrapping /usr/bin consumers.

28
Privileges
• Non-root users need certain DTrace privileges to be
able to use DTrace.
• These privileges are from the Solaris 10 “Least
Privilege” feature.
$ id
uid=1001(user1) gid=1(other)
$ /usr/sbin/dtrace -n 'syscall::exece:return'
dtrace: failed to initialize dtrace: DTrace requires additional privileges

29
Probes
• Data is generated from instrumentation points called
“probes”.
• DTrace provides thousands of probes.
• Probe examples:
Probe Name Description
syscall::read:entry A read() syscall began
proc:::exec-success A process created successfully
io:::start An I/O was issued (disk/vol/NFS)
io:::done An I/O completed

30
Probe Names
• Probe names are a four-tuple:
> Provider A library of related probes.
> Module The module the function belongs to,
either a kernel module or user segment.
> Function The function name that contains the probe.
> Name The name of the probe.
syscall::exece:return
Provider Module Function Name

31
Listing Probes
• dtrace -l lists all currently available probes that
you have privilege to see, with one probe per line:
• Here the root user sees 69,879 available probes.
• The probe count changes – it is dynamic (DTrace).
# dtrace -l
ID PROVIDER MODULE FUNCTION NAME
1 dtrace BEGIN
2 dtrace END
3 dtrace ERROR
4 sched FX fx_yield schedctl-yi
[...]
# dtrace -l | wc -l
69880

32
Tracing Probes
• dtrace -n takes a probe name and enables tracing:
• The default output contains:
– CPU CPU id that event occured on (if this
changes, the output may be shuffled)
– ID DTrace probe id
– FUNCTION:NAME Part of the probe name
# dtrace -n syscall::exece:return
dtrace: description 'syscall::exece:return' matched 1 probe
0 76044 exece:return
0 76044 exece:return
^C

33
Providers
• Examples of providers:
Provider Description
syscall system call entries and returns
proc process and thread events
sched kernel scheduling events
sysinfo system statistic events
vminfo virtual memory events
io system I/O events
profile fixed rate sampling
pid user-level tracing
fbt raw kernel tracing

34
Providers
• Example of probes:
Provider Example probe
syscall syscall::read:entry
proc proc:::exec-success
sched sched:::on-cpu
sysinfo sysinfo:::readch
vminfo vminfo:::maj_fault
io io:::start
profile profile:::profile-1000hz
pid pid172:libc:fopen:entry
pid172:a.out:main:entry
fbt fbt::bdev_strategy:entry

35
Providers
• Providers are documented in the DTrace Guide, as
separate chapters.
• Providers are dynamic, the number of available
probes can vary.
• Some providers are “unstable interface”, such as
fbt and sdt.
> This means that their probes, while useful, may vary in
name and arguments between Solaris versions.
> Try to use stable providers instead (if possible).

36
Provider Documentation
• Some providers assume a little background
knowledge, other providers assume a lot. Knowing
where to find supporting documentation is
important.
• Where do you find documentation on:
> Syscalls?
> User Libraries?
> Application Code?
> Kernel functions?

37
Provider Documentation
• Additional documentation may be found here:
Target Provider Additional Docs
syscalls syscall man(2)
libraries pid:lib* man(3C)
app code pid:a.out source code?
raw kernel fbt Solaris Internals 2nd
Ed,
http://cvs.opensolaris.org

38
Actions
• When a probe fires, an action executes.
• Actions are written in the D programming language.
• Actions can:
> print output
> save data to variables, and perform calculations
> walk kernel or process memory
• With destruction actions allowed, actions can:
> raise signals on processes
> execute shell commands
> write to some areas of memory

39
trace() Example
• The trace() action accepts one argument and prints
it when the probe fired.
0 76044 exece:return col
[...]

40
printf() Example
• DTrace ships with a powerful printf(), to print
formatted output.
# dtrace -n 'syscall::exece:return { printf("%6d %sn", pid, execname); }'
0 74415 exece:return 4301 sh
0 74415 exece:return 4304 neqn
0 74415 exece:return 4305 nroff
[...]

41
Default Variables
• Numerous predefined variables can be used, eg:
> pid, tid Process ID, Thread ID
> timestamp Nanosecond timestamp since boot
> probefunc Probe function name (3rd
field)
> execname Process name
> arg0, ... Function arguments and return value
> errno Last syscall failure error code
> curpsinfo Struct contating current process info, eg,
curpsinfo->pr_psargs – process + args
• Pointers and structs! DTrace can walk memory
using C syntax, and has kernel types predefined.

42
curthread
• curthread is a pointer to current kthread_t
From here you can walk kernel memory and answer
endless questions about OS internals.
• Eg, the current process user_t is,
curthread->t_procp->p_user
• You might not ever use curthread, but it is good to
know that you can. (And there are other ways to get
inside the kernel). Opinion:
curthread is like the down staircase
in nethack, angband, moria, ...

43
Variable Types
• DTrace supports the following variable types:
> Integers
> Structs
> Pointers
> Strings
> Associative arrays
> Aggregates
• Including types from /usr/include/sys, eg uint32_t.

44
Aggregations
• A great feature of DTrace is to process data as it is
captured, such as using aggregations.
• Eg, frequency counting syscalls:
@num is the aggregation variable, probefunc is the key,
and count() is the aggregating function.
# dtrace -n 'syscall:::entry { @num[probefunc] = count(); }'
^C
[...]
writev 170
write 257
read 896
pollsys 959
ioctl 1253

45
Aggregating Functions
• These include:
> count() count events, useful for frequency counts
> sum(value) sum the value
> avg(value) average the value
> min(value) find the value minimum
> max(value) find the value maximum
> quantize(value) print power-2 distribution plots

46
Quantize
• Very cool function, here we quantize write sizes:
• Here we see that ls processes usually write
between 32 and 127 bytes. Makes sense?
# dtrace -n 'sysinfo:::writech { @dist[execname] = quantize(arg0); }'
dtrace: description 'sysinfo:::writech ' matched 4 probes
^C
[...]
ls
value ------------- Distribution ------------- count
4 | 0
8 | 2
16 | 0
32 |@@@@@@@@@@@@@@@@@@@ 118
64 |@@@@@@@@@@@@@@@@@@@@@ 127
128 | 0
[...]

47
ls -l
ls writes one line at a time, each around 80 chars long.
# ls -l /etc
dttotal 793
lrwxrwxrwx 1 root root 12 Mar 21 03:28 TIMEZONE -> default/init
drwxr-xr-x 4 root sys 6 Apr 16 06:59 X11
drwxr-xr-x 2 adm adm 3 Mar 20 09:25 acct
drwxr-xr-x 3 root root 3 Apr 16 23:11 ak
lrwxrwxrwx 1 root root 12 Mar 21 03:28 aliases -> mail/aliases
drwxr-xr-x 5 root sys 5 Feb 20 23:29 amd64
drwxr-xr-x 7 root bin 18 Mar 20 09:20 apache
drwxr-xr-x 4 root bin 7 Feb 20 23:12 apache2
drwxr-xr-x 2 root sys 5 Feb 20 23:27 apoc
-rw-r--r-- 1 root bin 1012 Mar 20 09:33 auto_home
-rw-r--r-- 1 root bin 1066 Mar 20 09:33 auto_master
lrwxrwxrwx 1 root root 16 Mar 21 03:28 autopush -> ../sbin/autopu
[...]

48
Predicates
• DTrace predicates are used to filter probes, so that
the action fires when a conditional is true.
probename /predicate/ { action }
• Eg, syscalls for processes called “bash”:
# dtrace -n 'syscall:::entry /execname == "bash"/ { @num[probefunc] =
count(); }'
^C
exece 2
[...]
read 29
write 31
lwp_sigmask 42
sigaction 62

49
Scripting
• If your one-liners get too long, write scripts. Eg,
bash-syscalls.d:
• Getting it running:
#!/usr/sbin/dtrace -s
syscall:::entry
/execname == "bash"/
{
@num[probefunc] = count();
}
# chmod 755 bash-syscalls.d
# ./bash-syscalls.d
dtrace: script './bash-syscalls.d' matched 233 probes
[...]

50
What's next:
• We just covered:
> What is DTrace
> Who uses DTrace
> DTrace Essentials
• Next up is:
> Usage Features

51
Measuring Time
• Access to high resolution timestamps is of particular
use for performance analysis.
> timestamp time since boot in nanoseconds
> vtimestamp thread on-CPU timestamp
• Measuring these for application and operating
system function calls will answer:
> timestamp where is the latency?
> vtimestamp why are the CPUs busy?

52
Printing Stacks
• Printing user and kernel stack traces explains both
why and the how something happened.
• Why is bash calling read()? Using ustack():
# dtrace -n 'syscall::read:entry /execname == "bash"/ { ustack(); }'
dtrace: description 'syscall::read:entry ' matched 1 probe
0 74314 read:entry
libc.so.1`_read+0x7
bash`rl_getc+0x22
bash`rl_read_key+0xad
bash`readline_internal_char+0x5f
bash`0x80b1171
bash`0x80b118c
bash`readline+0x3a
[...] Ahh, readline()

53
End of Intro
• DTrace is a big topic, but you don't need to know it
all to get value from DTrace.
• To learn more, browse “DTrace Topics”,
http://www.solarisinternals.com/dtrace.
Here you will find:
> A wiki version of this presentation
> The PDF for this presentation
> dozens of other DTrace Topics (eg, one-liners!)
• Also see the “Solaris Performance and Tools” book,
http://www.sun.com/books/catalog/solaris_perf_tools.xml

54
Sampling
• DTrace isn't just about tracing events, DTrace can
also sample at customized rates.
• Eg, sampling 5-level user stack traces from Xorg:
# dtrace -n 'profile-1001 /execname == "Xorg"/ { @[ustack(5)] = count(); }'
dtrace: description 'profile-1001 ' matched 1 probe
^C
libfb.so`fbSolid+0x2c6
libfb.so`fbFill+0xb8
libfb.so`fbPolyFillRect+0x1d5
nvidia_drv.so`0xfe09e87b
Xorg`miColorRects+0x124
41
nvidia_drv.so`_nv000592X+0x3d
0x1016c00
87
nvidia was on-CPU
87 times

55
See Also
• DTrace home:
http://www.opensolaris.org/os/community/dtrace
> Main site of links
> DTrace-discuss mailing list
• Team DTrace blogs:
> http://blogs.sun.com/bmc
> http://blogs.sun.com/mws
> http://blogs.sun.com/ahl
• DTraceToolkit:
> http://www.opensolaris.org/os/community/dtrace/dtracetoolkit

56
dtrace:::END
Brendan Gregg
brendan@sun.com
56

dtrace_topics_intro.pdf

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