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PAPI Library

Introduction

The Performance Application Programming Interface (PAPI) provides tool designers and application engineers with a consistent interface and methodology for the use of low-level performance counter hardware found across the entire compute system (i.e. CPUs, GPUs, on/off-chip memory, interconnects, I/O system, energy/power, etc.). PAPI enables users to see, in near real time, the relations between software performance and hardware events across the entire computer system.

Only the basic usage is outlined in this compendium. For a comprehensive PAPI user manual please refer to the PAPI wiki website.

PAPI Counter Interfaces

To collect performance events, PAPI provides two APIs, the high-level and low-level API.

High-Level API

The high-level API provides the ability to record performance events inside instrumented regions of serial, multi-processing (MPI, SHMEM) and thread (OpenMP, Pthreads) parallel applications. It is designed for simplicity, not flexibility. More details can be found in the PAPI wiki High-Level API description.

The following code example shows the use of the high-level API by marking a code section.

C
#include "papi.h"

int main()
{
    int retval;

    retval = PAPI_hl_region_begin("computation");
    if ( retval != PAPI_OK )
        handle_error(1);

    /* Do some computation here */

    retval = PAPI_hl_region_end("computation");
    if ( retval != PAPI_OK )
        handle_error(1);
}
Fortran
#include "fpapi.h"

program main
integer retval

call PAPIf_hl_region_begin("computation", retval)
if ( retval .NE. PAPI_OK ) then
   write (*,*) "PAPIf_hl_region_begin failed!"
end if

!do some computation here

call PAPIf_hl_region_end("computation", retval)
if ( retval .NE. PAPI_OK ) then
   write (*,*) "PAPIf_hl_region_end failed!"
end if

end program main

Events to be recorded are determined via the environment variable PAPI_EVENTS that lists comma separated events for any component (see example below). The output is generated in the current directory by default. However, it is recommended to specify an output directory for larger measurements, especially for MPI applications via environment variable PAPI_OUTPUT_DIRECTORY.

Setting performance events and output directory

export PAPI_EVENTS="PAPI_TOT_INS,PAPI_TOT_CYC"
export PAPI_OUTPUT_DIRECTORY="/scratch/measurement"

This will generate a directory called papi_hl_output in scratch/measurement that contains one or more output files in JSON format.

Low-Level API

The low-level API manages hardware events in user-defined groups called Event Sets. It is meant for experienced application programmers and tool developers wanting fine-grained measurement and control of the PAPI interface. It provides access to both PAPI preset and native events, and supports all installed components. The PAPI wiki contains also a page with more details on the low-level API.

Usage on ZIH Systems

Before you start a PAPI measurement, check which events are available on the desired architecture. For this purpose, PAPI offers the tools papi_avail and papi_native_avail. If you want to measure multiple events, please check which events can be measured concurrently using the tool papi_event_chooser. The PAPI wiki contains more details on the PAPI tools.

Hint

The PAPI tools must be run on the compute node, using an interactive shell or job.

Example: Determine the events on the partition romeo from a login node

marie@login$ module load PAPI
marie@login$ salloc -A <project> --partition=romeo
[...]
marie@login$ srun papi_avail
marie@login$ srun papi_native_avail
[...]
# Exit with Ctrl+D

Instrument your application with either the high-level or low-level API. Load the PAPI module and compile your application against the PAPI library.

Example

marie@login$ module load PAPI
marie@login$ gcc app.c -o app -lpapi
marie@login$ salloc -A <project> --partition=romeo
marie@login$ srun ./app
[...]
# Exit with Ctrl+D

Hint

The PAPI modules on ZIH systems are only installed with the default perf_event component. If you want to measure, e.g., GPU events, you have to install your own PAPI. Please see the external instructions on how to download and install PAPI. To install PAPI with additional components, you have to specify them during configure as described for the Installation of Components.