What stream does perf use

consoleio-redirectionoutputperf

What stream does the perf command use!? I've been trying to capture it with

(perf stat -x, -ecache-misses ./a.out>/dev/null) 2> results

following https://stackoverflow.com/q/13232889/50305, but to no avail. Why can I not capture the input… it's like letting some fish get away!!

Best Answer

Older versions of perf ~2.6.x

I'm using perf version: 2.6.35.14-106.

Capture all the output

I don't have the -x switch on my Fedora 14 system so I'm not sure if that's your actual problem or not. I'll investigate on a newer Ubuntu 12.10 system later on but this worked for me:

$ (perf stat -ecache-misses ls ) > stat.log 2>&1
$
$ more stat.log 
maccheck.txt
sample.txt
stat.log

 Performance counter stats for 'ls':

              13209  cache-misses            

        0.018231264  seconds time elapsed

I only want perf's output

You could try this, the output from ls will get redirected to /dev/null. The output form perf (both STDERR and STDOUT) goes to the file, stat.log.

$ (perf stat -ecache-misses ls > /dev/null ) > stat.log 2>&1
[saml@grinchy 89576]$ more stat.log 

 Performance counter stats for 'ls':

              12949  cache-misses            

        0.022831281  seconds time elapsed

Newer versions of perf 3.x+

I'm using perf version: 3.5.7

Capturing only perf's output

With the newer versions of perf there are dedicated options for controlling where messages get sent. You have the choice of either sending them to a file via the -o|--output option. Simply give either of those switches a filename to capture the output.

-o file, --output file
    Print the output into the designated file.

The alternative is to redirect the output to a alternate file descriptor, 3, for example. All you need to do is direct this alternate file handle prior to streaming to it.

--log-fd
    Log output to fd, instead of stderr. Complementary to --output, and 
    mutually exclusive with it. --append may be used here. Examples: 
       3>results perf stat --log-fd 3  — $cmd
       -or-
       3>>results perf stat --log-fd 3 --append — $cmd

So if we wanted to collect the perf output for the ls command you could use this command:

$ 3>results.log perf stat --log-fd 3 ls > /dev/null
$ 
$ more results.log

 Performance counter stats for 'ls':

          2.498964 task-clock                #    0.806 CPUs utilized          
                 0 context-switches          #    0.000 K/sec                  
                 0 CPU-migrations            #    0.000 K/sec                  
               258 page-faults               #    0.103 M/sec                  
           880,752 cycles                    #    0.352 GHz                    
           597,809 stalled-cycles-frontend   #   67.87% frontend cycles idle   
           652,087 stalled-cycles-backend    #   74.04% backend  cycles idle   
         1,261,424 instructions              #    1.43  insns per cycle        
                                             #    0.52  stalled cycles per insn [55.31%]
     <not counted> branches                
     <not counted> branch-misses           

       0.003102139 seconds time elapsed

If you use the --append version then the contents of multiple commands will be appended to the same log file, results.log in our case.

Installing perf

Installation is pretty trivial:

Fedora

$ yum install perf

Ubuntu/Debian

$ apt-get install linux-tool-common linux-tools

References

Related Solutions

Bash – Process substitution with input redirection

You were really close:

tr "o" "a" < <(echo "Foo")

The substitution <() makes a file descriptor and just pastes the path to the shell. For comprehension just execute:

<(echo blubb)

You will see the error:

-bash: /dev/fd/63: Permission denied

That's why it just pastes /dev/fd/63 into the shell and /dev/fd/63 is not excutable, because it's a simple pipe. In the tr-example above, it's echo "Foo" that writes to the pipe and via input redirection < it's the tr command that reads from the file descriptor.

Bash – How to capture ordered STDOUT/STDERR and add timestamp/prefixes

You might use coprocesses. Simple wrapper that feeds both outputs of a given command to two sed instances (one for stderr the other for stdout), which do the tagging.

#!/bin/bash
exec 3>&1
coproc SEDo ( sed "s/^/STDOUT: /" >&3 )
exec 4>&2-
coproc SEDe ( sed "s/^/STDERR: /" >&4 )
eval $@ 2>&${SEDe[1]} 1>&${SEDo[1]}
eval exec "${SEDo[1]}>&-"
eval exec "${SEDe[1]}>&-"

Note several things:

It is a magic incantation for many people (including me) - for a reason (see the linked answer below).
There is no guarantee it won't occasionally swap couple of lines - it all depends on scheduling of the coprocesses. Actually, it is almost guaranteed that at some point in time it will. That said, if keeping the order strictly the same, you have to process the data from both stderr and stdin in the same process, otherwise the kernel scheduler can (and will) make a mess of it.

If I understand the problem correctly, it means that you would need to instruct the shell to redirect both streams to one process (which can be done AFAIK). The trouble starts when that process starts deciding what to act upon first - it would have to poll both data sources and at some point get into state where it would be processing one stream and data arrive to both streams before it finishes. And that is exactly where it breaks down. It also means, that wrapping the output syscalls like stderred is probably the only way to achieve your desired outcome (and even then you might have a problem once something becomes multithreaded on a multiprocessor system).

As far as coprocesses be sure to read Stéphane's excellent answer in How do you use the command coproc in Bash? for in depth insight.