Are C++17 Parallel Algorithms implemented already? - c++

Question

I was trying to play around with the new parallel library features proposed in the C++17 standard, but I couldn't get it to work. I tried compiling with the up-to-date versions of g++ 8.1.1 and clang++-6.0 and -std=c++17, but neither seemed to support #include <execution>, std::execution::par or anything similar.
When looking at the cppreference for parallel algorithms there is a long list of algorithms, claiming
Technical specification provides parallelized versions of the following 69 algorithms from algorithm, numeric and memory: ( ... long list ...)
which sounds like the algorithms are ready 'on paper', but not ready to use yet?
In this SO question from over a year ago the answers claim these features hadn't been implemented yet. But by now I would have expected to see some kind of implementation. Is there anything we can use already?

Answer 1

GCC 9 has them but you have to install TBB separately
In Ubuntu 19.10, all components have finally aligned:
GCC 9 is the default one, and the minimum required version for TBB
TBB (Intel Thread Building Blocks) is at 2019~U8-1, so it meets the minimum 2018 requirement
so you can simply do:
sudo apt install gcc libtbb-dev
g++ -ggdb3 -O3 -std=c++17 -Wall -Wextra -pedantic -o main.out main.cpp -ltbb
./main.out
and use as:
#include <execution>
#include <algorithm>
std::sort(std::execution::par_unseq, input.begin(), input.end());
see also the full runnable benchmark below.
GCC 9 and TBB 2018 are the first ones to work as mentioned in the release notes: https://gcc.gnu.org/gcc-9/changes.html
Parallel algorithms and <execution> (requires Thread Building Blocks 2018 or newer).
Related threads:
How to install TBB from source on Linux and make it work
trouble linking INTEL tbb library
Ubuntu 18.04 installation
Ubuntu 18.04 is a bit more involved:
GCC 9 can be obtained from a trustworthy PPA, so it is not so bad
TBB is at version 2017, which does not work, and I could not find a trustworthy PPA for it. Compiling from source is easy, but there is no install target which is annoying...
Here are fully automated tested commands for Ubuntu 18.04:
# Install GCC 9
sudo add-apt-repository ppa:ubuntu-toolchain-r/test
sudo apt-get update
sudo apt-get install gcc-9 g++-9
# Compile libtbb from source.
sudo apt-get build-dep libtbb-dev
git clone https://github.com/intel/tbb
cd tbb
git checkout 2019_U9
make -j `nproc`
TBB="$(pwd)"
TBB_RELEASE="${TBB}/build/linux_intel64_gcc_cc7.4.0_libc2.27_kernel4.15.0_release"
# Use them to compile our test program.
g++-9 -ggdb3 -O3 -std=c++17 -Wall -Wextra -pedantic -I "${TBB}/include" -L
"${TBB_RELEASE}" -Wl,-rpath,"${TBB_RELEASE}" -o main.out main.cpp -ltbb
./main.out
Test program analysis
I have tested with this program that compares the parallel and serial sorting speed.
main.cpp
#include <algorithm>
#include <cassert>
#include <chrono>
#include <execution>
#include <random>
#include <iostream>
#include <vector>
int main(int argc, char **argv) {
using clk = std::chrono::high_resolution_clock;
decltype(clk::now()) start, end;
std::vector<unsigned long long> input_parallel, input_serial;
unsigned int seed;
unsigned long long n;
// CLI arguments;
std::uniform_int_distribution<uint64_t> zero_ull_max(0);
if (argc > 1) {
n = std::strtoll(argv[1], NULL, 0);
} else {
n = 10;
}
if (argc > 2) {
seed = std::stoi(argv[2]);
} else {
seed = std::random_device()();
}
std::mt19937 prng(seed);
for (unsigned long long i = 0; i < n; ++i) {
input_parallel.push_back(zero_ull_max(prng));
}
input_serial = input_parallel;
// Sort and time parallel.
start = clk::now();
std::sort(std::execution::par_unseq, input_parallel.begin(), input_parallel.end());
end = clk::now();
std::cout << "parallel " << std::chrono::duration<float>(end - start).count() << " s" << std::endl;
// Sort and time serial.
start = clk::now();
std::sort(std::execution::seq, input_serial.begin(), input_serial.end());
end = clk::now();
std::cout << "serial " << std::chrono::duration<float>(end - start).count() << " s" << std::endl;
assert(input_parallel == input_serial);
}
On Ubuntu 19.10, Lenovo ThinkPad P51 laptop with CPU: Intel Core i7-7820HQ CPU (4 cores / 8 threads, 2.90 GHz base, 8 MB cache), RAM: 2x Samsung M471A2K43BB1-CRC (2x 16GiB, 2400 Mbps) a typical output for an input with 100 million numbers to be sorted:
./main.out 100000000
was:
parallel 2.00886 s
serial 9.37583 s
so the parallel version was about 4.5 times faster! See also: What do the terms "CPU bound" and "I/O bound" mean?
We can confirm that the process is spawning threads with strace:
strace -f -s999 -v ./main.out 100000000 |& grep -E 'clone'
which shows several lines of type:
[pid 25774] clone(strace: Process 25788 attached
[pid 25774] <... clone resumed> child_stack=0x7fd8c57f4fb0, flags=CLONE_VM|CLONE_FS|CLONE_FILES|CLONE_SIGHAND|CLONE_THREAD|CLONE_SYSVSEM|CLONE_SETTLS|CLONE_PARENT_SETTID|CLONE_CHILD_CLEARTID, parent_tidptr=0x7fd8c57f59d0, tls=0x7fd8c57f5700, child_tidptr=0x7fd8c57f59d0) = 25788
Also, if I comment out the serial version and run with:
time ./main.out 100000000
I get:
real 0m5.135s
user 0m17.824s
sys 0m0.902s
which confirms again that the algorithm was parallelized since real < user, and gives an idea of how effectively it can be parallelized in my system (about 3.5x for 8 cores).
Error messages
Hey, Google, index this please.
If you don't have tbb installed, the error is:
In file included from /usr/include/c++/9/pstl/parallel_backend.h:14,
from /usr/include/c++/9/pstl/algorithm_impl.h:25,
from /usr/include/c++/9/pstl/glue_execution_defs.h:52,
from /usr/include/c++/9/execution:32,
from parallel_sort.cpp:4:
/usr/include/c++/9/pstl/parallel_backend_tbb.h:19:10: fatal error: tbb/blocked_range.h: No such file or directory
19 | #include <tbb/blocked_range.h>
| ^~~~~~~~~~~~~~~~~~~~~
compilation terminated.
so we see that <execution> depends on an uninstalled TBB component.
If TBB is too old, e.g. the default Ubuntu 18.04 one, it fails with:
#error Intel(R) Threading Building Blocks 2018 is required; older versions are not supported.

Answer 2

You can refer https://en.cppreference.com/w/cpp/compiler_support to check all C++ feature implementation status. For your case, just search "Standardization of Parallelism TS", and you will find only MSVC and Intel C++ compilers support this feature now.

Answer 3

Intel has released a Parallel STL library which follows the C++17 standard:
https://github.com/intel/parallelstl
It is being merged into GCC.

Answer 4

Gcc does not yet implement the Parallelism TS (see https://gcc.gnu.org/onlinedocs/libstdc++/manual/status.html#status.iso.2017)
However libstdc++ (with gcc) has an experimental mode for some equivalent parallel algorithms. See https://gcc.gnu.org/onlinedocs/libstdc++/manual/parallel_mode.html
Getting it to work:
Any use of parallel functionality requires additional compiler and
runtime support, in particular support for OpenMP. Adding this support
is not difficult: just compile your application with the compiler flag
-fopenmp. This will link in libgomp, the GNU Offloading and Multi Processing Runtime Library, whose presence is mandatory.
Code example
#include <vector>
#include <parallel/algorithm>
int main()
{
std::vector<int> v(100);
// ...
// Explicitly force a call to parallel sort.
__gnu_parallel::sort(v.begin(), v.end());
return 0;
}

Answer 5

Gcc now support execution header, but not standard clang build from https://apt.llvm.org