==== LIKWID ====

=== LIKWID - "Like I Knew What I'm Doing" ===
LIKWID is developed by FAU for Performance Optimization, Modeling, and Architecture Analysis
 * Bringing the thinking back to the performance engineer.
 * Open-source
 * Command-line and Software Lib interface. No GUI
 * x86 Architecture (Intel & AMD) with initial support for Nvidia GPUs 

=== Quick Start in SCC ===
LIKIWID Toolset is available in SCC as a module, thus before using LIKWID a user need to load their preferred LIKWID version module to set the environment correctly.

<code>
(base) gwdu101:25 17:17:18 ~ > module show likwid
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------
  /opt/sw/modules/21.12/cascadelake/Core/likwid/5.2.0.lua:
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------
...

(base) gwdu101:25 17:17:24 ~ > 
(base) gwdu101:25 17:18:33 ~ > module load likwid
(base) gwdu101:25 17:18:43 ~ >
</code>

The following tasks can be performed by LIKWID:
=== Node architecture information ===
<code>
$ likwid-topology
$ likwid-powermeter
</code>
Examples of node-architecture for SCC's amp016:

{{:en:services:application_services:high_performance_computing:likwid-topology-cache_g.png?400|}}
{{:en:services:application_services:high_performance_computing:likwid-topology-cache_g2.png?400|}}

=== Affinity control and data placement ===
<code>
$ likwid-pin
$ likwid-mpirun
</code>

=== Query and alter system settings ===
<code>
$ likwid-features
$ likwid-setFrequencies
</code>

=== Application performance profiling (perf-counter) ===
 * Using the available hardware counters to measure events that characterises the interaction between software and hardware
 * Uses a light-weight marker API for code instrumentation
<code>
$ likwid-perfctr
</code>

=== Micro-benchmarking - Application and framework to enable: ===
  * Quantify sustainable performance limits
  * Separate influences considering isolated instruction code snippets
  * Reverse-egineer processor features
  * Discover hardware bugs
<code>
$ likwid-bench
$ likwid-memsweeper
</code>

=== likwid-topology ===
  * Thread topology: How processor IDs map on physical compute resources
  * Cache topology: How processors share the cache hierarchy
  * Cache properties: Detailed information about all cache levels
  * NUMA topology: NUMA domains and memory sizes
  * GPU topology: GPU information

=== likwid-pin ===
  * Explicitly supports pthread and the OpenMP implementations of Intel and GNU gcc
  * Only used with "pthread_create" API call which are dynamically linked with the static placement of threads.

=== likwid-perfctr ===
  * a lightweight command-line application to configure and read out hardware performance data
  * Can be used as a wrapper (no modification in the application) or by adding "Marker_API" functions inside the code
  * There are preconfigured performance groups with useful event sets and derived metrics
  * Since ***likwid-perfctr*** measures all events on the specified CPUs, it is necessary for processes and threads to dedicated resources.
  * This can be done by pinning the application manually or using the built-in functionality

=== Performance Groups ===

  * An outstanding feature of LIKWID
  * Organizes and combines micro-architecture events and counters with e.g. run-time and clock speed
  * Provides a set of derived metrics for efficient analysis
  * They are read on the fly without compilation by command-line selection
  * Are found in the path "${INSTALL_PREFIX}/share/likwid"

Examples of using "likwid-perfctr" on SCC's amp016 node
  * Use option "-a" to see available performance groups:

{{:en:services:application_services:high_performance_computing:likwid-perfctr-perfgroup.png?400|}}

  * Use "likwid-perfctr -g CLOCK" to measure the clock speed.

{{:en:services:application_services:high_performance_computing:likwid-perfctr-g-clock.png?400|}}

  * Use "likwid-perfctr -g FLOPS_DP" to measure the Arithmetic Intensity in double precision. 

{{:en:services:application_services:high_performance_computing:likwid-perfctr-g-flop_dp.png?400|}}

  * Use "likwid-perfctr -g MEM" to measure the bandwidth of primary memory.

{{:en:services:application_services:high_performance_computing:likwid-perfctr-g-mem.png?400|}}

=== Marker API ===
  * Enables measurements of user-defined code regions.
  * The Marker API offers 6 functions (for C/C++) to measure named regions
  * Activated by "-DLIKWID_PERFORM" to compiler calls
    <code>
    LIKWID_MARKER_INIT //global initialization
    LIKWID_MARKER_THREADINIT //individual thread initialization
    LIKWID_MARKER_START('compute') //Start a code region named 'compute'
    LIKWID_MARKER_STOP('compute') //Stop the code region named 'compute'
    LIWKID_MARKER_SWITCH //Switches perfromance group or event set in a round-robin fashion
    LIKWID_MARKER_CLOSE //global finalization
    </code>
    
==== Additional information ====
  * [LIKWID](https://github.com/RRZE-HPC/likwid/wiki)