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std::sort report

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  • Component: libstdc++

  • Version: 12.3.0

  • Summary: High memory usage for parallel std::sort

  • Description: The memory usage of parallel std::sort is very high compared to the sequential version and even other parallel implementations. The attached code is a simple test case to compare the memory usage of parallel std::sort, tbb::parallel_sort and sequential std::sort. The test case has been replicated in several systems with versions of GCC 10, 11 and 12.

An example of the results (and max. memory usage according to /usr/bin/time) is shown in the following table:

Executable Size Time Max Resident Memory
./pstl_sort.out 33554432 0:00.23 423776k
./tbb_sort.out 33554432 0:00.44 143952k
./seq_sort.out 33554432 0:03.32 134836k
./pstl_sort.out 1073741824 0:05.68 14236656k
./tbb_sort.out 1073741824 0:13.02 4207680k
./seq_sort.out 1073741824 2:07.38 4198124k

In the example, the parallel std::sort (pstl_sort) uses ~3 times more memory than the tbb::parallel_sort (tbb_sort) and the sequential std::sort (seq_sort). It also runs faster, though.

Despite the docs saying that `std::sort`` "require additional O(n) memory space for parallel execution", it seems that the memory overhead is higher than what would be expected.

Is that high memory usage a deliberate trade-off for performance? Is the algorithm still in development to improve memory usage?

  • Notes of the benchmark:

    • System specs in the example:

    • CPU: AMD EPYC 7551

    • RAM: 256 GB DDR4

    • OS: Debian 10.10

    • Compilation with:

g++ -std=c++17 -O3 -pedantic -Wall -Wextra -Werror -o pstl_sort.out main.cpp -ltbb -DPSTL_SORT
g++ -std=c++17 -O3 -pedantic -Wall -Wextra -Werror -o tbb_sort.out main.cpp -ltbb -DTBB_SORT
g++ -std=c++17 -O3 -pedantic -Wall -Wextra -Werror -o seq_sort.out main.cpp -ltbb

Issue in the GCC Bugzilla

Bug 113504 has been successfully created.

Email sent to: gcc-bugs@gcc.gnu.org, saifi.khan@nishan.io

Redirected to LLVM's GitHub https://github.com/llvm/llvm-project/tree/main/pstl.