The Bandwidth Benchmark

This is a collection of simple streaming kernels.

Apart from the micro-benchmark functionality this is also a blueprint for other micro-benchmark applications.

It contains C modules for:

• Aligned data allocation
• Query and control affinity settings
• Accurate wall clock timing

Moreover the benchmark showcases a simple generic Makefile that can be used in other projects.

You may want to have a look at https://github.com/RRZE-HPC/TheBandwidthBenchmark/wiki for a collection of results that were created using TheBandwidthBenchmark.

Overview

The benchmark is heavily inspired by John McCalpin's https://www.cs.virginia.edu/stream/ benchmark.

It contains the following streaming kernels with corresponding data access pattern (Notation: S - store, L - load, WA - write allocate). All variables are vectors, s is a scalar:

• init (S1, WA): Initilize an array: a = s. Store only.
• sum (L1): Vector reduction: s += a. Load only.
• copy (L1, S1, WA): Classic memcopy: a = b.
• update (L1, S1): Update vector: a = a * scalar. Also load + store but without write allocate.
• triad (L2, S1, WA): Stream triad: a = b + c * scalar.
• daxpy (L2, S1): Daxpy: a = a + b * scalar.
• striad (L3, S1, WA): Schoenauer triad: a = b + c * d.
• sdaxpy (L3, S1): Schoenauer triad without write allocate: a = a + b * c.

Build

1. Configure the tool chain and additional options in config.mk:

# Supported: GCC, CLANG, ICC, ICX
TOOLCHAIN ?= CLANG
ENABLE_OPENMP ?= false
ENABLE_LIKWID ?= false

OPTIONS  =  -DSIZE=120000000ull
OPTIONS +=  -DNTIMES=10
OPTIONS +=  -DARRAY_ALIGNMENT=64
#OPTIONS +=  -DVERBOSE_AFFINITY
#OPTIONS +=  -DVERBOSE_DATASIZE
#OPTIONS +=  -DVERBOSE_TIMER

The verbosity options enable detailed output about affinity settings, allocation sizes and timer resolution. If you uncomment DVERBOSE_AFFINITY the processor every thread is currently scheduled on and the complete affinity mask for every thread is printed.

Notice: OpenMP involves significant overhead through barrier cost, especially on systems with many memory domains. The default problem size is set to almost 4GB to have enough work vs overhead. If you suspect that the result should be better you may try to further increase the problem size. To compare to original stream results on X86 systems you have to ensure that streaming store instructions are used. For the ICC tool chain this is now the default (Option -qopt-streaming-stores=always).

• Build with:

make

You can build multiple tool chains in the same directory, but notice that the Makefile is only acting on the one currently set. Intermediate build results are located in the ./build/<TOOLCHAIN> directory.

• Clean up intermediate build results for active tool chain, data files and plots with:

make clean

Clean all build results for all tool chains:

make distclean

• Optional targets:

Generate assembler:

make asm

The assembler files will also be located in the ./build/<TOOLCHAIN> directory.

Reformat all source files using clang-format. This only works if clang-format is in your path.

make format

Support for clang language server

The Makefile will generate a .clangd configuration to correctly set all options for the clang language server. This is only important if you use an editor with LSP support and want to edit or explore the source code. It is required to use GNU Make 4.0 or newer. While older make versions will work, the generation of the .clangd configuration for the clang language server will not work. The default Make version included in MacOS is 3.81! Newer make versions can be easily installed on MacOS using the Homebrew package manager.

An alternative is to use Bear, a tool that generates a compilation database for clang tooling. This method also will enable to jump to any definition without a previously opened buffer. You have to build TheBandwidthBenchmark one time with Bear as a wrapper:

bear -- make

Usage

To run the benchmark call:

./bwBench-<TOOLCHAIN> [mode (optional)]

Apart from the default parallel work sharing mode with fixed problem size TheBandwidthBenchmark also supports two modes with varying problem sizes: sequential (call with seq mode option) and throughput (call with tp mode option). These are intended for scanning the complete memory hierarchy instead of only the main memory domain. See below for details on how to use those modes.

NOTICE: The seq and tp modes may take up to 30m or more, depending on the system.

In default mode the benchmark will output the results similar to the stream benchmark. Results are validated. For threaded execution it is recommended to control thread affinity.

We recommend to use likwid-pin for setting the number of threads used and to control thread affinity:

likwid-pin -C 0-3 ./bwbench-GCC

Example output for threaded execution:

-------------------------------------------------------------
[pthread wrapper]
[pthread wrapper] MAIN -> 0
[pthread wrapper] PIN_MASK: 0->1  1->2  2->3
[pthread wrapper] SKIP MASK: 0x0
        threadid 140271463495424 -> core 1 - OK
        threadid 140271455102720 -> core 2 - OK
        threadid 140271446710016 -> core 3 - OK
OpenMP enabled, running with 4 threads
----------------------------------------------------------------------------
Function      Rate(MB/s)  Rate(MFlop/s)  Avg time     Min time     Max time
Init:          22111.53    -             0.0148       0.0145       0.0165
Sum:           46808.59    46808.59      0.0077       0.0068       0.0140
Copy:          30983.06    -             0.0207       0.0207       0.0208
Update:        43778.69    21889.34      0.0147       0.0146       0.0148
Triad:         34476.64    22984.43      0.0282       0.0278       0.0305
Daxpy:         45908.82    30605.88      0.0214       0.0209       0.0242
STriad:        37502.37    18751.18      0.0349       0.0341       0.0388
SDaxpy:        46822.63    23411.32      0.0281       0.0273       0.0325
----------------------------------------------------------------------------
Solution Validates

Scaling runs

Apart from the highest sustained memory bandwidth also the scaling behavior within memory domains is a important system property.

There is a helper script downloadable at https://github.com/RRZE-HPC/TheBandwidthBenchmark/wiki/util/extractResults.pl that creates a text result file from multiple runs that can be used as input to plotting applications as gnuplot and xmgrace. This involves two steps: Executing the benchmark runs and creating the data file.

To run the benchmark for different thread counts within a memory domain execute (this assumes bash or zsh):

for nt in 1 2 4 6 8 10; do likwid-pin -q -C E:M0:$nt:1:2 ./bwbench-ICC > dat/emmy-$nt.txt; done

It is recommended to just use one thread per core in case the processor supports hyperthreading. Use whatever stepping you like, here a stepping of two was used. The -q option suppresses output from likwid-pin. Above line uses the expression based syntax, on systems with hyperthreading enabled (check with, e.g., likwid-topology) you have to skip the other hardware threads on each core. For above system with 2 hardware threads per core this results in -C E:M0:$nt:1:2, on a system with 4 hardware threads per core you would need -C E:M0:$nt:1:4. The string before the dash (here emmy) can be arbitrary, but the the extraction script expects the thread count after the dash. Also the file ending has to be .txt. Please check with a text editor on some result files if everything worked as expected.

To extract the results and output in a plot table format execute:

./extractResults.pl ./dat

The script will pick up all result files in the directory specified and create a column format output file. In this case:

#nt     Init    Sum     Copy    Update  Triad   Daxpy   STriad  SDaxpy
1       4109    11900   5637    8025    7407    9874    8981    11288
2       8057    22696   11011   15174   14821   18786   17599   21475
4       15602   39327   21020   28197   27287   33633   31939   37146
6       22592   45877   29618   37155   36664   40259   39911   41546
8       28641   46878   35763   40111   40106   41293   41022   41950
10      33151   46741   38187   40269   39960   40922   40567   41606

Please be aware the single core memory bandwidth as well as the scaling behavior depends on the frequency settings.

Sequential vs Throughput mode: Sweeping over a range of problem size

TheBandwidthBenchmark comes in 2 additional variants: Sequential and Throughput. These 2 modes performs a sweep over different array sizes ranging from N = 100 till the array size N specified in config.mk.

• Sequential - Runs TheBandwidthBenchmark in sequential mode for all kernels. Command to run in sequential mode:

./bwBench-<TOOLCHAIN> seq

• Throughput (Multi-threaded) - Runs TheBandwidthBenchmark in multi-threaded mode for all kernels. Requires flag ENABLE_OPENMP=true in config.mk. Command to run in throughput mode:

./bwBench-<TOOLCHAIN> tp

Each of these modes output the results for each individual kernel. The output files will be created in the ./dat directory.

Visualizing the data from the Sequential/Throughput modes

Required: Gnuplot 5.2+

The user can visualize the outputs from ./dat directory using the provided gnuplot scripts. The scripts are located in ./gnuplot_script directory where a bash file takes care of generating and executing the gnuplot commands. The plots from gnuplot can then be found in ./plot directory.

There are 2 ways you can visualize the output:

• Plotting Array Size (N) vs Bandwidth (MB/s) - this mode creates plot with the Array Size (N) on x-axis and Bandwidth (MB/s) on y-axis. The Array size (N) will be the same for each kernel. Use this makefile command to generate this type of plot:

make plot

• Plotting Dataset Size (MB) vs Bandwidth (MB/s) - this mode creates plot with the Dataset Size (MB) on x-axis and Bandwidth (MB/s) on y-axis. The Dataset size (MB) will be the different for each kernel. For example the total dataset for Init kernel will be 4x times less than the total dataset size for the STriad kernel.

make plot_dataset

The script also generates a combined plot with bandwidths from all the kernels into one plot.

Caveats

A few known caveats to the user, based on the experience with the compilers.

• Intel oneAPI DPC++/C++ Compiler 2023.2.0 (icx/icpx compiler):

  • NonTemporal Stores (aka Streaming Stores): We leave the choice to the user whether to use NT stores or not.

    • If the user wants to use NT stores using -qopt-streaming-stores=always compiler flag, then the user has to avoid using the -ffreestanding compiler flag. This will not generate NT instructions, but generates calls to __libirc_nontemporal_store@PLT in the assembly.
    • For the Througput mode with OpenMP, the icx/icpx compiler does not respect the nontemporal() clause with the OpenMP simd directive.

    It's recommended not to use NT stores if the user wants to observe cache hierarchy when using the Sequential or Throughput mode.