PerfTools.md

Introduction

perf consists of two parts: the kernel space implementation and the userland tools. This wiki entry focusses on the latter. These tools are installed on taurus, and others and provides support for sampling applications and reading performance counters.

Installation

On taurus load the module via

module load perf/r31

Configuration

Admins can change the behaviour of the perf tools kernel part via the following interfaces

File Name	Description
/proc/sys/kernel/perf_event_max_sample_rate	describes the maximal sample rate for perf record and native access. This is used to limit the performance influence of sampling.
/proc/sys/kernel/perf_event_mlock_kb	defines the number of pages that can be used for sampling via perf record or the native interface
/proc/sys/kernel/perf_event_paranoid	defines access rights:
	-1 - Not paranoid at all
	0 - Disallow raw tracepoint access for unpriv
	1 - Disallow cpu events for unpriv
	2 - Disallow kernel profiling for unpriv
/proc/sys/kernel/kptr_restrict	Defines whether the kernel address maps are restricted

Perf Stat

perf stat provides a general performance statistic for a program. You can attach to a running (own) process, monitor a new process or monitor the whole system. The latter is only available for root user, as the performance data can provide hints on the internals of the application.

For Users

Run perf stat <Your application>. This will provide you with a general overview on some counters.

Performance counter stats for 'ls':= 
          2,524235 task-clock                #    0,352 CPUs utilized           
                15 context-switches          #    0,006 M/sec                   
                 0 CPU-migrations            #    0,000 M/sec                   
               292 page-faults               #    0,116 M/sec                   
         6.431.241 cycles                    #    2,548 GHz                     
         3.537.620 stalled-cycles-frontend   #   55,01% frontend cycles idle    
         2.634.293 stalled-cycles-backend    #   40,96% backend  cycles idle    
         6.157.440 instructions              #    0,96  insns per cycle         
                                             #    0,57  stalled cycles per insn 
         1.248.527 branches                  #  494,616 M/sec                   
            34.044 branch-misses             #    2,73% of all branches         
       0,007167707 seconds time elapsed

• Generally speaking task clock tells you how parallel your job has been/how many cpus were used.
• Context switches are an information about how the scheduler treated the application. Also interrupts cause context switches. Lower is better.
• CPU migrations are an information on whether the scheduler moved the application between cores. Lower is better. Please pin your programs to CPUs to avoid migrations. This can be done with environment variables for OpenMP and MPI, with likwid-pin, numactl and taskset.
• Page faults describe how well the Translation Lookaside Buffers fit for the program. Lower is better.
• Cycles tells you how many CPU cycles have been spent in executing the program. The normalized value tells you the actual average frequency of the CPU(s) running the application.
• stalled-cycles-... tell you how well the processor can execute your code. Every stall cycle is a waste of CPU time and energy. The reason for such stalls can be numerous. It can be wrong branch predictions, cache misses, occupation of CPU resources by long running instructions and so on. If these stall cycles are to high you might want to review your code.
• The normalized instructions number tells you how well your code is running. More is better. Current x86 CPUs can run 3 to 5 instructions per cycle, depending on the instruction mix. A count of less then 1 is not favorable. In such a case you might want to review your code.
• branches and branch-misses tell you how many jumps and loops are performed in your code. Correctly predicted branches should not hurt your performance, branch-misses on the other hand hurt your performance very badly and lead to stall cycles.
• Other events can be passed with the -e flag. For a full list of predefined events run perf list
• PAPI runs on top of the same infrastructure as perf stat, so you might want to use their meaningful event names. Otherwise you can use raw events, listed in the processor manuals.

For Admins

Administrators can run a system wide performance statistic, e.g., with perf stat -a sleep 1 which measures the performance counters for the whole computing node over one second.

Perf Record

perf record provides the possibility to sample an application or a system. You can find performance issues and hot parts of your code. By default perf record samples your program at a 4000 Hz. It records CPU, Instruction Pointer and, if you specify it, the call chain. If your code runs long (or often) enough, you can find hot spots in your application and external libraries. Use perf report to evaluate the result. You should have debug symbols available, otherwise you won't be able to see the name of the functions that are responsible for your load. You can pass one or multiple events to define the sampling event.

What is a sampling event? Sampling reads values at a specific sampling frequency. This frequency is usually static and given in Hz, so you have for example 4000 events per second and a sampling frequency of 4000 Hz and a sampling rate of 250 microseconds. With the sampling event, the concept of a static sampling frequency in time is somewhat redefined. Instead of a constant factor in time (sampling rate) you define a constant factor in events. So instead of a sampling rate of 250 microseconds, you have a sampling rate of 10,000 floating point operations.

Why would you need sampling events? Passing an event allows you to find the functions that produce cache misses, floating point operations, ... Again, you can use events defined in perf list and raw events.

Use the -g flag to receive a call graph.

For Users

Just run perf record ./myapp or attach to a running process.

Using Perf with MPI

Perf can also be used to record data for indivdual MPI processes. This requires a wrapper script (perfwrapper) with the following content. Also make sure that the wrapper script is executable (chmod +x).

#!/bin/bash
perf record -o perf.data.$SLURM_JOB_ID.$SLURM_PROCID $@

To start the MPI program type srun ./perfwrapper ./myapp on your command line. The result will be n independent perf.data files that can be analyzed individually with perf report.

For Admins

This tool is very effective, if you want to help users find performance problems and hot-spots in their code but also helps to find OS daemons that disturb such applications. You would start perf record -a -g to monitor the whole node.

Perf Report

perf report is a command line UI for evaluating the results from perf record. It creates something like a profile from the recorded samplings. These profiles show you what the most used have been. If you added a callchain, it also gives you a callchain profile.<br /> *Disclaimer: Sampling is not an appropriate way to gain exact numbers. So this is merely a rough overview and not guaranteed to be absolutely correct.*<span style="font-size: 1em;"> </span>

On Taurus

On Taurus, users are not allowed to see the kernel functions. If you have multiple events defined, then the first thing you select in perf report is the type of event. Press right

Available samples
96 cycles
11 cache-misse

Hints:

• The more samples you have, the more exact is the profile. 96 or 11 samples is not enough by far.
• Repeat the measurement and set -F 50000 to increase the sampling frequency.
• The higher the frequency, the higher the influence on the measurement.

If you'd select cycles, you would get such a screen:

Events: 96  cycles
+  49,13%  test_gcc_perf  test_gcc_perf      [.] main.omp_fn.0
+  34,48%  test_gcc_perf  test_gcc_perf      [.] 
+   6,92%  test_gcc_perf  test_gcc_perf      [.] omp_get_thread_num@plt
+   5,20%  test_gcc_perf  libgomp.so.1.0.0   [.] omp_get_thread_num
+   2,25%  test_gcc_perf  test_gcc_perf      [.] main.omp_fn.1
+   2,02%  test_gcc_perf  [kernel.kallsyms]  [k] 0xffffffff8102e9ea

Increased sample frequency:

Events: 7K cycles
+  42,61%  test_gcc_perf  test_gcc_perf      [.] p
+  40,28%  test_gcc_perf  test_gcc_perf      [.] main.omp_fn.0
+   6,07%  test_gcc_perf  test_gcc_perf      [.] omp_get_thread_num@plt
+   5,95%  test_gcc_perf  libgomp.so.1.0.0   [.] omp_get_thread_num
+   4,14%  test_gcc_perf  test_gcc_perf      [.] main.omp_fn.1
+   0,69%  test_gcc_perf  [kernel.kallsyms]  [k] 0xffffffff8102e9ea
+   0,04%  test_gcc_perf  ld-2.12.so         [.] check_match.12442
+   0,03%  test_gcc_perf  libc-2.12.so       [.] printf
+   0,03%  test_gcc_perf  libc-2.12.so       [.] vfprintf
+   0,03%  test_gcc_perf  libc-2.12.so       [.] __strchrnul
+   0,03%  test_gcc_perf  libc-2.12.so       [.] _dl_addr
+   0,02%  test_gcc_perf  ld-2.12.so         [.] do_lookup_x
+   0,01%  test_gcc_perf  libc-2.12.so       [.] _int_malloc
+   0,01%  test_gcc_perf  libc-2.12.so       [.] free
+   0,01%  test_gcc_perf  libc-2.12.so       [.] __sigprocmask
+   0,01%  test_gcc_perf  libgomp.so.1.0.0   [.] 0x87de
+   0,01%  test_gcc_perf  libc-2.12.so       [.] __sleep
+   0,01%  test_gcc_perf  ld-2.12.so         [.] _dl_check_map_versions
+   0,01%  test_gcc_perf  ld-2.12.so         [.] local_strdup
+   0,00%  test_gcc_perf  libc-2.12.so       [.] __execvpe

Now you select the most often sampled function and zoom into it by pressing right. If debug symbols are not available, perf report will show which assembly instruction is hit most often when sampling. If debug symbols are available, it will also show you the source code lines for these assembly instructions. You can also go back and check which instruction caused the cache misses or whatever event you were passing to perf record.

Perf Script

If you need a trace of the sampled data, you can use perf script command, which by default prints all samples to stdout. You can use various interfaces (e.g., python) to process such a trace.

Perf Top

perf top is only available for admins, as long as the paranoid flag is not changed (see configuration).

It behaves like the top command, but gives you not only an overview of the processes and the time they are consuming but also on the functions that are processed by these.