Difference between revisions of "Strace: what a process does"

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strace is a [[command]] to show what a command or process is doing.
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<metadesc>How to strace a process. How to understand strace's output. How to follow sub-processes, how to format the string length so you get a sensible meaning, how to find the file descriptors and how to see where the process spends most of its time.</metadesc>
  
−
= Analyzing strace's output =
+
strace is a [[command]] that shows a process' calls to the kernel (aka syscalls). This helps you to do excellent troubleshooting and gain good performance tuning insights.
−
To analyze strace's output you must know that the first keyword in a line of output from strace is always a syscall like open or read. Then, in parantheses, the arguments follow, then the result. The meaning of the syscall and its parameters and results can be found with the [[command]]
+
 
 +
<pic border=1 src="http://www.linuxintro.org/strace-mindmap.jpg" width=50% align=right caption="strace cheat sheet" link="http://www.linuxintro.org/strace-mindmap.jpg"  />
 +
 
 +
= Understand strace's output =
 +
Let's start with a real-world example: You want to [[troubleShoot sound]] on your computer. You know that speaker-test does some test noise. To find out what exactly happens, issue:
 +
# strace -s 99 -ff speaker-test
 +
You will find a lot of lines like
 +
 
 +
<pre>
 +
[pid 15287] write(6, "W", 1)            = 1
 +
[pid 15287] recvmsg(7, {msg_name(0)=NULL, msg_iov(1)=[{"L\0\0\0\2L\0\0\2+U\0\0\0\0\0\31vqU\0\0\0\0\0\0\0\0001TU \240\326\0\4\252\354TU \240\326\0\4\262%r\0\0\0\0\0\20\0\0r\0\0\0\0\0\2\260\220R\0\0\0\0\0\0\0\0R\0\0\0\0\0\2\260\220", 83}], msg_controllen=24, {cmsg_len=24, cmsg_level=SOL_SOCKET, cmsg_type=SCM_CREDENTIALS{pid=2289, uid=0, gid=0}}, msg_flags=0}, 0) = 83
 +
[pid 15287] write(6, "W", 1)            = 1
 +
</pre>
 +
 
 +
'''Looks complicated, right?''' But fear not, every line follows the same pattern:
 +
;[pid ''process id''] syscall(parameters) = return code
 +
 
 +
You will only find the first column with the '''process id''' if you use the -ff option (follow sub-processes). For the '''syscall''' (write or recvmsg in the above example) it is easy to obtain information - just type
 
  man 2 ''syscall''
 
  man 2 ''syscall''
 +
in the above example
 +
man 2 write
 +
and
 +
man 2 recvmsg
 +
And you will find abundant information what the syscall does. If you understand 50% you are doing well :)
 +
The '''return code''' will also be documented in the syscall's man page.
 +
 +
'''So what happens here?''' Obviously speaker-test calls the kernel's ''recvmsg'' method. The interesting thing we get from <tt>man 2 recvmsg</tt> is that the first parameter is sockfd, the file descriptor of a socket. So it hands over a socket as a parameter. Let's look at process' file descriptors:
 +
<pre>
 +
cd /proc/15287/fd
 +
ls -l
 +
total 0
 +
lrwx------ 1 root root 64 Apr  4 22:55 0 -> /dev/pts/4
 +
lrwx------ 1 root root 64 Apr  4 22:55 1 -> /dev/pts/4
 +
lrwx------ 1 root root 64 Apr  4 22:55 2 -> /dev/pts/4
 +
lr-x------ 1 root root 64 Apr  4 22:55 3 -> pipe:[297487]
 +
l-wx------ 1 root root 64 Apr  4 22:55 4 -> pipe:[297487]
 +
lr-x------ 1 root root 64 Apr  4 22:55 5 -> pipe:[297488]
 +
l-wx------ 1 root root 64 Apr  4 22:55 6 -> pipe:[297488]
 +
lrwx------ 1 root root 64 Apr  4 22:55 7 -> socket:[297495]
 +
</pre>
 +
 +
aha! file descriptor 7 is the unix socket 297495. Let's look what this is connected to:
 +
 +
<pre>
 +
ss -p | grep 297495
 +
u_str  ESTAB      0      0      /run/user/0/pulse/native 297496                * 297495  users:(("pulseaudio",2289,28))
 +
u_str  ESTAB      0      0                    * 297495                * 297496  users:(("speaker-test",15286,7))
 +
</pre>
 +
 +
We see - the socket 297495 is connected to the socket 297496. What is written into 297495 will be readable from 297496 and the other way round. This couple of sockets connects the speaker-test process with the pulseaudio process. 2289 is pulseaudio's process id, and 15286 is the root process id of speaker-test. This system is using pulseaudio for sound which is an important information for troubleshooting sound. In my example I had been messing around with pulseaudio's configuration and I could fix the problem quickly once I understood I was using it.
 +
 +
= strace start options =
 +
You can start a new process with strace or you can attach strace to an already running process. To start a new process just put strace in front of its command:
 +
strace -s 99 -ff ls
 +
 +
To attach strace to an already running process, in this example firefox, find out the process' id:
 +
ps -A | grep firefox
 +
2728 pts/1    00:00:02 firefox
 +
 +
Then call strace with the id as parameter:
 +
strace -s 99 -ff -p 2728
  
−
== output ==
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= limitations =
−
Here is an example output from strace that we are going to analyze:
+
strace will show you all syscalls from a process, and that's it. This also mean that if your process hangs in an endless loop that does not do any syscall, strace will not show anything.
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clock_gettime(CLOCK_MONOTONIC, {4433, 764347636}) = 0
 
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poll([{fd=12, events=POLLIN|POLLPRI|POLLRDNORM|POLLRDBAND}], 1, 0) = 1 ([{fd=12, revents=POLLIN|POLLRDNORM}])
 
−
<abbr title="Syscall. Find out more with the command man 2 recv.">recv</abbr>(<abbr title="As we can tell from man 2 recv, the following is a file descriptor. Find out more with the command cat /prov/''PID''/fd/12">12</abbr>, "\37\36&\364T<\264\6Og\20\2\30%-\31C\3757\350{\260\0203\351\23&O\365(y\212"..., 16384, 0) = 1448
 
  
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== analysis ==
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= Examples =
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Every line follows the syntax
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* [http://scn.sap.com/community/linux/blog/2014/04/11/dispwork-running-but-not-connected-to-message-server# find out why SAP does not start]
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syscall(arguments) = return value
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* find out why [[shellinabox]] shows [[forkpty failed]]
 +
* you can find out [[is my ulimit exceeded]]
 +
* I only got [[guacAmole]] to run because I strace'd it.
  
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The meaning of the syscall, its expected arguments and its return value can be determined from its man page. For example to understand the first line call [http://linux.die.net/man/2/clock_gettime man 2 clock_gettime]. It retrieves the current time.
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= interprocess communication =
 +
Processes will typically communicate:
 +
* via network
 +
* via sockets
 +
* via pipes
 +
* via shared memory
 +
* by writing into files
  
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Wow, that was easy. Now to the next line, poll. According to [http://linux.die.net/man/2/poll man 2 poll] it waits for an event on a file descriptor, like "data to be read". The file descriptor is the first argument, 12. What's this? Let's assume the process ID we are tracing is 7179. Then we can find out its file descriptors like this:
+
== via sockets ==
−
  /proc/7179/fd # ll
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For example when using strace to [[troubleshoot sound]] you may see that speaker-test receives messages from file descriptor 7:
−
  total 0
+
<pre>
−
lr-x------ 1 root root 64 Apr 19 08:02 0 -> /dev/null
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strace -s 99 -ffp 32681
−
lrwx------ 1 root root 64 Apr 19 08:02 1 -> socket:[156171]
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[pid 32682] recvmsg(7, {msg_name(0)=NULL, msg_iov(1)=[{"L\0\0\0\2L\0\0\0}U\0\0\0\0\0\22\10\277U\0\0\0\0\0\0\0\0001TU b|\0\17(\347TU b|\0\0175\342r\0\0\0\0\0\20\0\0r\0\0\0\0\0\2\260\220R\0\0\0\0\0\0\0\0R\0\0\0\0\0\2\260\220", 83}], msg_controllen=24, {cmsg_len=24, cmsg_level=SOL_SOCKET, cmsg_type=SCM_CREDENTIALS{pid=2289, uid=0, gid=0}}, msg_flags=0}, 0) = 83
−
l-wx------ 1 root root 64 Apr 19 08:02 2 -> /var/log/apache2/error_log
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</pre>
−
  lr-x------ 1 root root 64 Apr 19 08:02 4 -> pipe:[156208]
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Now let's find out what file descriptor 7 of process 32681 is:
−
  l-wx------ 1 root root 64 Apr 19 08:02 5 -> pipe:[156208]
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<pre>
−
  l-wx------ 1 root root 64 Apr 19 08:02 6 -> /var/log/apache2/access_log
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  ll /proc/32681/fd/7
−
  lrwx------ 1 root root 64 Apr 19 08:02 7 -> anon_inode:[eventpoll]
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  lrwx------ 1 root root 64 Apr 4 18:17 /proc/32681/fd/7 -> socket:[198064]
−
lrwx------ 1 root root 64 Apr 19 08:02 12 -> socket:[155097]
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</pre>
 +
So it is unix socket 198064. Let's find out what process it is:
 +
<pre>
 +
  # ss -p | grep 198064
 +
  u_str  ESTAB      0      0                    * 198064                * 198065  users:(("speaker-test",32681,7))
 +
  u_str  ESTAB      0      0      /run/user/0/pulse/native 198065                * 198064 users:(("pulseaudio",2289,28))
 +
</pre>
  
−
Ok, to find out what socket:[155097] is we use the command
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== via pipes ==
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# lsof | grep 155097
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Let's construct a situation where processes communicate via [[pipe]]s. The following [[command]] will create two processes, cat writing random data to a pipe and grep reading it:
−
httpd2-pr 7158            root    3u    IPv4    155097      0t0        TCP *:http (LISTEN)
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  cat /dev/urandom | grep "To be or not to be, that is the question"
−
  httpd2-pr 7179          wwwrun    3u    IPv4    155097      0t0        TCP *:http (LISTEN)
 
  
−
So this is a socket listening to the http port.
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Let's find the process IDs:
 +
<pre>
 +
ps -A | grep -E "cat|grep"
 +
  8654 pts/1    00:10:30 cat
 +
  8655 pts/1    00:00:04 grep
 +
10949 pts/2    00:00:00 grep
 +
</pre>
 +
Let's look at cat's file descriptors:
 +
<pre>
 +
cd /proc/8654/fd
 +
ll
 +
total 0
 +
lrwx------ 1 root root 64 Apr  6 11:40 0 -> /dev/pts/1
 +
l-wx------ 1 root root 64 Apr  6 11:40 1 -> pipe:[70679]
 +
lrwx------ 1 root root 64 Apr  6 11:39 2 -> /dev/pts/1
 +
lr-x------ 1 root root 64 Apr  6 11:40 3 -> /dev/urandom
 +
</pre>
 +
So cat is communicating with a pipe numbered 70679. Let's see if we can find the other process that is reading from it:
 +
<pre>
 +
lsof | grep 70679
 +
cat        8654            root    1w    FIFO        0,8      0t0      70679 pipe
 +
grep      8655            root    0r    FIFO        0,8      0t0      70679 pipe
 +
</pre>
 +
Works. We can use the command [[lsof]] to find the processes involved into pipe communication.
  
 
= Search for a syscall =
 
= Search for a syscall =
 
With strace, you can search for a special syscall. For example,
 
With strace, you can search for a special syscall. For example,
 
  strace -e open ''command''
 
  strace -e open ''command''
 +
 
will show you all files that have been touched (opened) by ''command''. This can be used e.g. to [[find out where configuration changes are stored]].
 
will show you all files that have been touched (opened) by ''command''. This can be used e.g. to [[find out where configuration changes are stored]].
  
 
= Performance analysis =
 
= Performance analysis =
 
With strace, you can find out the biggest time-consuming syscalls during a program run:
 
With strace, you can find out the biggest time-consuming syscalls during a program run:
 +
<pre>
 
  strace -c ls -R
 
  strace -c ls -R
 
  Entries  Repository  Root
 
  Entries  Repository  Root
Line 74: Line 168:
 
  ------ ----------- ----------- --------- --------- ----------------
 
  ------ ----------- ----------- --------- --------- ----------------
 
  100.00    0.614296                198036        19 total
 
  100.00    0.614296                198036        19 total
−
 
+
</pre>
 
Now you go
 
Now you go
 
  [http://linux.die.net/man/2/getdents64 man 2 getdents64]
 
  [http://linux.die.net/man/2/getdents64 man 2 getdents64]
Line 87: Line 181:
  
 
[[Category:low-level]]
 
[[Category:low-level]]
 +
[[Category:TroubleShooting]]
 
[[Category:command]]
 
[[Category:command]]
 +
[[Category:mindmap]]

Latest revision as of 08:20, 22 July 2023


strace is a command that shows a process' calls to the kernel (aka syscalls). This helps you to do excellent troubleshooting and gain good performance tuning insights.

strace cheat sheet

Understand strace's output

Let's start with a real-world example: You want to troubleShoot sound on your computer. You know that speaker-test does some test noise. To find out what exactly happens, issue: 

# strace -s 99 -ff speaker-test

You will find a lot of lines like 

[pid 15287] write(6, "W", 1)            = 1
[pid 15287] recvmsg(7, {msg_name(0)=NULL, msg_iov(1)=[{"L\0\0\0\2L\0\0\2+U\0\0\0\0\0\31vqU\0\0\0\0\0\0\0\0001TU \240\326\0\4\252\354TU \240\326\0\4\262%r\0\0\0\0\0\20\0\0r\0\0\0\0\0\2\260\220R\0\0\0\0\0\0\0\0R\0\0\0\0\0\2\260\220", 83}], msg_controllen=24, {cmsg_len=24, cmsg_level=SOL_SOCKET, cmsg_type=SCM_CREDENTIALS{pid=2289, uid=0, gid=0}}, msg_flags=0}, 0) = 83
[pid 15287] write(6, "W", 1)            = 1

Looks complicated, right? But fear not, every line follows the same pattern:

[pid process id] syscall(parameters) = return code

You will only find the first column with the process id if you use the -ff option (follow sub-processes). For the syscall (write or recvmsg in the above example) it is easy to obtain information - just type 

man 2 syscall

in the above example 

man 2 write

and 

man 2 recvmsg

And you will find abundant information what the syscall does. If you understand 50% you are doing well :) The return code will also be documented in the syscall's man page.

So what happens here? Obviously speaker-test calls the kernel's recvmsg method. The interesting thing we get from man 2 recvmsg is that the first parameter is sockfd, the file descriptor of a socket. So it hands over a socket as a parameter. Let's look at process' file descriptors: 

 cd /proc/15287/fd
 ls -l
 total 0
 lrwx------ 1 root root 64 Apr  4 22:55 0 -> /dev/pts/4
 lrwx------ 1 root root 64 Apr  4 22:55 1 -> /dev/pts/4
 lrwx------ 1 root root 64 Apr  4 22:55 2 -> /dev/pts/4
 lr-x------ 1 root root 64 Apr  4 22:55 3 -> pipe:[297487]
 l-wx------ 1 root root 64 Apr  4 22:55 4 -> pipe:[297487]
 lr-x------ 1 root root 64 Apr  4 22:55 5 -> pipe:[297488]
 l-wx------ 1 root root 64 Apr  4 22:55 6 -> pipe:[297488] 
 lrwx------ 1 root root 64 Apr  4 22:55 7 -> socket:[297495]

aha! file descriptor 7 is the unix socket 297495. Let's look what this is connected to: 

 ss -p | grep 297495
 u_str  ESTAB      0      0      /run/user/0/pulse/native 297496                * 297495  users:(("pulseaudio",2289,28))
 u_str  ESTAB      0      0                    * 297495                * 297496  users:(("speaker-test",15286,7))

We see - the socket 297495 is connected to the socket 297496. What is written into 297495 will be readable from 297496 and the other way round. This couple of sockets connects the speaker-test process with the pulseaudio process. 2289 is pulseaudio's process id, and 15286 is the root process id of speaker-test. This system is using pulseaudio for sound which is an important information for troubleshooting sound. In my example I had been messing around with pulseaudio's configuration and I could fix the problem quickly once I understood I was using it.

strace start options

You can start a new process with strace or you can attach strace to an already running process. To start a new process just put strace in front of its command: 

strace -s 99 -ff ls

To attach strace to an already running process, in this example firefox, find out the process' id: 

ps -A | grep firefox
2728 pts/1    00:00:02 firefox

Then call strace with the id as parameter: 

strace -s 99 -ff -p 2728

limitations

strace will show you all syscalls from a process, and that's it. This also mean that if your process hangs in an endless loop that does not do any syscall, strace will not show anything.

Examples

interprocess communication

Processes will typically communicate:

  • via network
  • via sockets
  • via pipes
  • via shared memory
  • by writing into files

via sockets

For example when using strace to troubleshoot sound you may see that speaker-test receives messages from file descriptor 7: 

 strace -s 99 -ffp 32681
 [pid 32682] recvmsg(7, {msg_name(0)=NULL, msg_iov(1)=[{"L\0\0\0\2L\0\0\0}U\0\0\0\0\0\22\10\277U\0\0\0\0\0\0\0\0001TU b|\0\17(\347TU b|\0\0175\342r\0\0\0\0\0\20\0\0r\0\0\0\0\0\2\260\220R\0\0\0\0\0\0\0\0R\0\0\0\0\0\2\260\220", 83}], msg_controllen=24, {cmsg_len=24, cmsg_level=SOL_SOCKET, cmsg_type=SCM_CREDENTIALS{pid=2289, uid=0, gid=0}}, msg_flags=0}, 0) = 83

Now let's find out what file descriptor 7 of process 32681 is: 

 ll /proc/32681/fd/7
 lrwx------ 1 root root 64 Apr  4 18:17 /proc/32681/fd/7 -> socket:[198064]

So it is unix socket 198064. Let's find out what process it is: 

 # ss -p | grep 198064
 u_str  ESTAB      0      0                    * 198064                * 198065  users:(("speaker-test",32681,7))
 u_str  ESTAB      0      0      /run/user/0/pulse/native 198065                * 198064  users:(("pulseaudio",2289,28))

via pipes

Let's construct a situation where processes communicate via pipes. The following command will create two processes, cat writing random data to a pipe and grep reading it: 

cat /dev/urandom | grep "To be or not to be, that is the question"

Let's find the process IDs: 

 ps -A | grep -E "cat|grep"
  8654 pts/1    00:10:30 cat
  8655 pts/1    00:00:04 grep
 10949 pts/2    00:00:00 grep

Let's look at cat's file descriptors: 

cd /proc/8654/fd
ll
total 0
lrwx------ 1 root root 64 Apr  6 11:40 0 -> /dev/pts/1
l-wx------ 1 root root 64 Apr  6 11:40 1 -> pipe:[70679]
lrwx------ 1 root root 64 Apr  6 11:39 2 -> /dev/pts/1
lr-x------ 1 root root 64 Apr  6 11:40 3 -> /dev/urandom

So cat is communicating with a pipe numbered 70679. Let's see if we can find the other process that is reading from it: 

 lsof | grep 70679
 cat        8654            root    1w     FIFO        0,8      0t0      70679 pipe
 grep       8655            root    0r     FIFO        0,8      0t0      70679 pipe

Works. We can use the command lsof to find the processes involved into pipe communication.

Search for a syscall

With strace, you can search for a special syscall. For example, 

strace -e open command

will show you all files that have been touched (opened) by command. This can be used e.g. to find out where configuration changes are stored.

Performance analysis

With strace, you can find out the biggest time-consuming syscalls during a program run: 

 strace -c ls -R
 Entries  Repository  Root
 % time     seconds  usecs/call     calls    errors syscall
 ------ ----------- ----------- --------- --------- ----------------
  67.09    0.412153          14     29664           getdents64
  27.70    0.170168          11     14849        14 open
   4.24    0.026043           0    123740           write
   0.72    0.004443           0     14837           close
   0.20    0.001204           0     14836           fstat
   0.05    0.000285         285         1           execve
   0.00    0.000000           0        12           read
   0.00    0.000000           0         4         3 stat
   0.00    0.000000           0        33           mmap
   0.00    0.000000           0        18           mprotect
   0.00    0.000000           0         4           munmap
   0.00    0.000000           0        12           brk
   0.00    0.000000           0         2           rt_sigaction
   0.00    0.000000           0         1           rt_sigprocmask
   0.00    0.000000           0         2           ioctl
   0.00    0.000000           0         1         1 access
   0.00    0.000000           0         3           mremap
   0.00    0.000000           0         1           fcntl
   0.00    0.000000           0         1           getrlimit
   0.00    0.000000           0         1           statfs
   0.00    0.000000           0         1           arch_prctl
   0.00    0.000000           0         3         1 futex
   0.00    0.000000           0         1           set_tid_address
   0.00    0.000000           0         8           fadvise64
   0.00    0.000000           0         1           set_robust_list
 ------ ----------- ----------- --------- --------- ----------------
 100.00    0.614296                198036        19 total

Now you go 

man 2 getdents64

to find out what that syscall is about.

See also

Retrieved from "http://linuxintro.org/index.php?title=Strace:_what_a_process_does&oldid=19864"