author	date	title
Beatrice Bevilacqua, Anxhelo Xhebraj	March 2019	Performance counters analysis for Hyper-Threading

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
• copy_scattered: Same as copy but accesses at page granularity (4096)

The last two benchmarks (sum_int and copy_scattered) can be found under the benchmarks/ folder of this repository. Instructions on how to compile them in likwid can be found here.

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.

Repository Structure

The repository has two branches: master and aggregated. The former contains under the data/ folder the profiling of the benchmarks run in timeline mode (-t flag) while the latter under data_aggregated/ the data of the profiling in wrapper mode (-T).

Both folders were genereated through the bench.sh file and minor variations of it for different number of threads and Hyper Threading activation.

master Details

Data naming convention

<BenchmarkName>-<#threads>-<freq>.<ext>

• #threads: 1 or 2. When the value is 2, the threads run in the same physical core in Hyper Threading

• freq: 1.0, 1.5, 2.2. The smallest frequency is not precise and ranges from 1.0 to 1.28.

• ext: stdout, stderr, header. In stderr there is the actual data with one line per group, with groups repeating for each sample. In header can be found the name of the metrics/performance counters in each group. In stdout the total running time of the benchmark and additional info. The output produced by bench.sh is not clean therefore such data has been processed through process_data.py.

Plotting

In order to easily analyze the data plot_data.py can be used. It is a Python3.6 script which uses the matplotlib library.

Usage

usage: plot_data.py [-h] [-g GROUP] [-m METRIC] [-p] file [file ...]

Plot data

positional arguments:
  file                  Path to the file without extension

optional arguments:
  -h, --help            show this help message and exit
  -g GROUP, --group GROUP
  -m METRIC, --metric METRIC
  -p, --print-groups

• First choose a file or set of files to work on
• Run python plot_data.py -p <files> to see which groups are available for analysis
• Choose one group and its id to plot
• Run python plot_data.py -g <groupId> <files>
• Choose the name of one of the metrics of the group printed out
• Run python plot_data.py -g <groupId> -m "<metricName>" <files>
• Note: Escape the metric name with double quotes

$ python plot_data.py -g 16 -m "Power DRAM [W]" data/ALU-core-1-1.0 data/ALU-core-1-1.5 data/ALU-core-1-2.2 data/Copy-mem-2-2.2 data/ALU-mem-1-1.0 data/ALU-mem-1-1.5 data/ALU-mem-1-2.2

aggregated Details

Data naming convention

<BenchmarkName>-<#threads><-HT>-<freq>.<ext>

• #threads: 1, 3, 6. Number of physical cores used in the benchmark.
• -HT: present or not. If present then in each physical cores there were two threads running in Hyper Threading otherwise just one
• freq: 1.2, 1.7, 2.2. Maximum frequency available for the benchmark. In this case the lowest frequency is precise
• ext: csv or stdout. In the csv there are the values of the metrics of the benchmark while stdout contains the running time and additional details

Plotting

In order to easily analyze the data histogram.py can be used. It is a Python3.6 script which uses the matplotlib library.

Usage

usage: histogram.py [-h] [-g GROUP] [-m METRIC] [-t TYPE] [-p]
                    file [file ...]

Plot data

positional arguments:
  file                  Path to the file without extension

optional arguments:
  -h, --help            show this help message and exit
  -g GROUP, --group GROUP
  -m METRIC, --metric METRIC
  -t TYPE, --type TYPE
  -p, --print-groups

• First choose a file or set of files to work on
• Run python histogram.py -p <files> to see which groups are available for analysis
• Choose one group and its id to plot
• Run python histogram.py -g <groupId> <files>
• Choose the name of one of the metrics of the group printed out and additionally choose also the type of aggregation across threads given that there is one value of the metric for each thread. Possible values are avg, sum, any, min, max.
• Run python plot_data.py -g <groupId> -m "<metricName>" -t <type> <files>
• Note: Escape the metric name with double quotes and when passing the file names remove their extension

$ python histogram.py -g 1 -t avg -m "Avg stall duration [cycles]" data_aggregated/Scattered*.csv data_aggregated/ALU-core*.csv

The xs represent the running time of the benchmark (right y axis) while the bars represent the metric (left y axis).

correlations

Additionally Pearson Correlation Coefficients for each varying input i.e. #Threads, Frequencies and HT have been computed through pearson_corr.py and can be found in correlations/.

Naming Convention

• <BenchmarkName>-freq.csv: for each combination of #threads and HT the correlation coefficient between the three values of the frequency and the values of the metrics at those frequencies.

• <BenchmarkName>-HT.csv: for each combination of #threads and frequencies the correlation coefficient between the two values of the HT and the values of the metrics with HT enabled or disabled.

• <BenchmarkName>-threads.csv: for each combination of frequencies and HT the correlation coefficient between the three values of the #threads and the values of the metrics at those #threads.

Plotting

$ python plot_corr.py correlations/Scattered-mem-freq.csv

Details

The tests were run on a Dell XPS 9750 with i7-8750H while charging. With TurboBoost disabled the available frequencies range from 1.2 to 2.2 GHz. There is one socket with 6 Physical cores and 12 Logical cores (in Hyper Threading).