[libcxx-commits] [PATCH] D155531: [libc++][PSTL] Simplify the partitioning algorithm until we have better data to know how to chunk better
Nikolas Klauser via Phabricator via libcxx-commits
libcxx-commits at lists.llvm.org
Mon Aug 14 16:44:51 PDT 2023
This revision was landed with ongoing or failed builds.
This revision was automatically updated to reflect the committed changes.
Closed by commit rG051c8630124d: [libc++][PSTL] Simplify the partitioning algorithm until we have better data to… (authored by philnik).
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D155531/new/
https://reviews.llvm.org/D155531
Files:
libcxx/include/__algorithm/pstl_backends/cpu_backends/libdispatch.h
libcxx/src/pstl/libdispatch.cpp
libcxx/test/libcxx/algorithms/pstl.libdispatch.chunk_partitions.pass.cpp
Index: libcxx/test/libcxx/algorithms/pstl.libdispatch.chunk_partitions.pass.cpp
===================================================================
--- libcxx/test/libcxx/algorithms/pstl.libdispatch.chunk_partitions.pass.cpp
+++ libcxx/test/libcxx/algorithms/pstl.libdispatch.chunk_partitions.pass.cpp
@@ -28,7 +28,7 @@
auto chunks = std::__par_backend::__libdispatch::__partition_chunks(1);
assert(chunks.__chunk_count_ == 1);
assert(chunks.__first_chunk_size_ == 1);
- assert(chunks.__chunk_size_ == 0);
+ assert(chunks.__chunk_size_ == 1);
}
for (std::ptrdiff_t i = 2; i != 2ll << 20; ++i) {
Index: libcxx/src/pstl/libdispatch.cpp
===================================================================
--- libcxx/src/pstl/libdispatch.cpp
+++ libcxx/src/pstl/libdispatch.cpp
@@ -10,59 +10,20 @@
#include <__algorithm/pstl_backends/cpu_backends/libdispatch.h>
#include <__config>
#include <dispatch/dispatch.h>
-#include <thread>
_LIBCPP_BEGIN_NAMESPACE_STD
namespace __par_backend::inline __libdispatch {
-
void __dispatch_apply(size_t chunk_count, void* context, void (*func)(void* context, size_t chunk)) noexcept {
::dispatch_apply_f(chunk_count, DISPATCH_APPLY_AUTO, context, func);
}
-__chunk_partitions __partition_chunks(ptrdiff_t element_count) {
- if (element_count == 0) {
- return __chunk_partitions{1, 0, 0};
- } else if (element_count == 1) {
- return __chunk_partitions{1, 0, 1};
- }
-
+__chunk_partitions __partition_chunks(ptrdiff_t element_count) noexcept {
__chunk_partitions partitions;
- partitions.__chunk_count_ = [&] {
- ptrdiff_t cores = std::max(1u, thread::hardware_concurrency());
-
- auto medium = [&](ptrdiff_t n) { return cores + ((n - cores) / cores); };
-
- // This is an approximation of `log(1.01, sqrt(n))` which seemes to be reasonable for `n` larger than 500 and tops
- // at 800 tasks for n ~ 8 million
- auto large = [](ptrdiff_t n) { return static_cast<ptrdiff_t>(100.499 * std::log(std::sqrt(n))); };
-
- if (element_count < cores)
- return element_count;
- else if (element_count < 500)
- return medium(element_count);
- else
- return std::min(medium(element_count), large(element_count)); // provide a "smooth" transition
- }();
+ partitions.__chunk_count_ = std::max<ptrdiff_t>(1, element_count / 256);
partitions.__chunk_size_ = element_count / partitions.__chunk_count_;
- partitions.__first_chunk_size_ = partitions.__chunk_size_;
-
- const ptrdiff_t leftover_item_count = element_count - (partitions.__chunk_count_ * partitions.__chunk_size_);
-
- if (leftover_item_count == 0)
- return partitions;
-
- if (leftover_item_count == partitions.__chunk_size_) {
- partitions.__chunk_count_ += 1;
- return partitions;
- }
-
- const ptrdiff_t n_extra_items_per_chunk = leftover_item_count / partitions.__chunk_count_;
- const ptrdiff_t n_final_leftover_items = leftover_item_count - (n_extra_items_per_chunk * partitions.__chunk_count_);
-
- partitions.__chunk_size_ += n_extra_items_per_chunk;
- partitions.__first_chunk_size_ = partitions.__chunk_size_ + n_final_leftover_items;
+ partitions.__first_chunk_size_ = element_count - (partitions.__chunk_count_ - 1) * partitions.__chunk_size_;
return partitions;
}
Index: libcxx/include/__algorithm/pstl_backends/cpu_backends/libdispatch.h
===================================================================
--- libcxx/include/__algorithm/pstl_backends/cpu_backends/libdispatch.h
+++ libcxx/include/__algorithm/pstl_backends/cpu_backends/libdispatch.h
@@ -57,7 +57,7 @@
ptrdiff_t __first_chunk_size_;
};
-[[__gnu__::__const__]] _LIBCPP_EXPORTED_FROM_ABI __chunk_partitions __partition_chunks(ptrdiff_t __size);
+[[__gnu__::__const__]] _LIBCPP_EXPORTED_FROM_ABI __chunk_partitions __partition_chunks(ptrdiff_t __size) noexcept;
template <class _RandomAccessIterator, class _Functor>
_LIBCPP_HIDE_FROM_ABI void
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