Added report

report.md

@@ -0,0 +1,123 @@
+% Performance counters analysis for Hyper-Threading
+% Beatrice Bevilacqua, Anxhelo Xhebraj
+% March 2019
+
+Performance counters analysis for Hyper-Threading
+=================================================
+
+
+Performance Counters Frameworks
+-------------------------------
+The complexity of newer architectures has led to the necessity of
+a better knowledge of the underlying hardware in order to get peak
+performance. Following these trends new interfaces have been made
+available to developers for spotting performance bottlenecks in their
+applications such as Performance Monitoring Units (PMU).
+
+PMUs enable developers to observe and count events in the CPU such as
+branch mispredictions, cache misses and other finer grained details over
+the whole pipeline. Although powerful, dealing with such information
+remains burdensome given the diversity of the events, making it difficult
+to truly identify optimization opportunities.
+Depending by the processor family, on average 4 counters can be read
+contemporarily at any time using Model Specific Registers. In order
+to read more than 4 events, various tools multiplex such registers
+in a *time-sharing* fashion.
+
+Many tools for performance analysis based on PMUs have been developed
+ranging from *raw* event count to more sofisticated and aggregated
+measures as follows:
+
+* `msr`: direct access to the device files `/dev/cpu/*/msr`
+* [PAPI] : A Performance Application Programming Interface that
+  offers a set of APIs for using performance counters.
+  Supports multiple architectures and multiplexing.
+* [likwid] : A suite of applications and libraries for analysing
+  High Performance Computing applications. It
+  contains out of the box utilies to work with MPI,
+  power profiling and architecture topology.
+* [Intel Vtune Amplifier] : Application for performance analysis on
+  intel architectures. Gives insights regarding possible bottlenecks
+  of the application annotating its source code and provides
+  possible solutions.
+* [perf] : In a similar vein to Intel Vtune Amplifier shows which
+  functions are more critical to the application. Additionally
+  provides more high level information such as I/O and Networking.
+  It is possible to analyse raw hardware performance counters but
+  its main goal is abstracting over them.
+* [pmu-tools] : is a collection of tools for profile collection
+  and performance analysis on Intel CPUs on top of Linux perf
+
+
+`likwid`
+--------
+Given that the goal of this document is to analyze system behaviour
+through performance counters to provide insights regarding new
+possible scheduling strategies in Hyper-Threading systems, we choose
+to use the `likwid` applications and libraries for our task. The choice
+was especially driven by the presence of useful benchmarks in the `likwid`
+repository for stressing FPU and other core subsystems. Additionally
+Intel Vtune Amplifier was used to profile the benchmarks in order to
+characterize their workload.
+
+`likwid-perfctr -e` allows to query all the available events for
+the current architecture while `likwid-perfctr -a` shows the pre-configured
+event sets, called performance groups, with useful pre-selected event
+sets and derived metrics. Multiple modes of execution of performance monitoring
+are available as documented in the `likwid` wiki. Of main interest are
+**wrapper mode** and **timeline mode**. The former produces a summary of the
+events, while the latter outputs performance metrics at a specified
+frequency (specified through the `-t` flag).
+In case multiple groups need to be monitored multiplexing is performed
+at the granularity set through the `-t` flag (in timeline mode, otherwise
+`-T` for wrapper mode) and the output produced are the id of the group read
+at a given timestep and its values.
+
+ >Tests have shown that for measurements below 100 milliseconds, the
+  periodically printed results are not valid results anymore (they are higher
+  than expected) but the behavior of the results is still valid. E.g. if you
+  try to resolve the burst memory transfers, you need results for small
+  intervals. The memory bandwidth for each measurement may be higher than
+  expected (could even be higher than the theoretical maximum of the machine)
+  but the burst and non-burst traffic is clearly identifiable by highs and
+  lows of the memory bandwidth results.
+
+
+Benchmarks
+----------
+
+The benchmark available in `likwid` can be run through the `likwid-bench`
+command. For an overview of the available benchmarks run `likwid-bench -a`.
+All benchmarks perform operations over one-dimensional arrays. The benchmarks
+used in our setting are:
+
+  * `ddot_sp`: Single-precision dot product of two vectors, only scalar
+               operations
+  * `copy`: Double-precision vector copy, only scalar operations
+  * `ddot_sp_avx`: Single-precision dot product of two vectors, optimized for AVX
+  * `sum_int`: Custom benchmark similar to `sum` but working on integers
+
+All benchmarks are run with multiple configurations of number of threads (with or
+without Hyper-Threading), processor frequencies with TurboBoost disabled, working
+set size. The latter is needed in order to emulate *core-bound* executions
+(working set fitting in cache) and *memory-bound* ones.
+
+
+Details
+-------
+
+The tests were run on a Dell XPS 9750 with i7-8750H. With TurboBoost disabled
+the available frequencies range from 1.0 to 2.2 GHz. There is one socket with
+6 Physical cores and 12 Logical cores (in Hyper Threading).
+
+[PAPI]: http://icl.utk.edu/papi/
+[PAPI]: https://bitbucket.org/icl/papi.git
+[PAPI]: http://icl.utk.edu/projects/papi/wiki/PAPIC:Overview
+
+[likwid]: https://github.com/RRZE-HPC/likwid
+
+[Intel Vtune amplifier]: https://software.intel.com/en-us/vtune
+
+[perf]: http://www.brendangregg.com/perf.html
+
+[pmu-tools]: https://github.com/andikleen/pmu-tools