Attached is a simple patch which has been extensively tested within
Facebook,
and is enabled by default in our code base.
Passing the size to the allocator allows it to optimize deallocation,
and this was seen to significantly reduce the work required in jemalloc,
with about 40% reduction in CPU cycles in the free path.
Note jemalloc >= 5.1 is required to fix a bug with 0 sizes.
cheers,
Pádraig
From eb337e751bdeb5db7d41dd584c179c2cf19485a8 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?P=C3=A1draig=20Brady?= <pbr...@fb.com>
Date: Fri, 22 Feb 2019 17:21:57 -0800
Subject: [PATCH] std::allocator::deallocate support sized-deallocation
Pass the size to the allocator so that it may optimize deallocation.
This was seen to significantly reduce the work required in jemalloc,
with about 40% reduction in CPU cycles in the free path.
Note jemalloc >= 5.1 is required to fix a bug with 0 sizes.
* libstdc++-v3/include/ext/new_allocator.h (deallocate): Pass the size
to the deallocator with -fsized-deallocation.
---
libstdc++-v3/include/ext/new_allocator.h | 17 +++++++++++++++++
1 file changed, 17 insertions(+)
diff --git a/libstdc++-v3/include/ext/new_allocator.h b/libstdc++-v3/include/ext/new_allocator.h
index e245391..061d8ce 100644
--- a/libstdc++-v3/include/ext/new_allocator.h
+++ b/libstdc++-v3/include/ext/new_allocator.h
@@ -114,6 +114,22 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
return static_cast<_Tp*>(::operator new(__n * sizeof(_Tp)));
}
+#if __cpp_sized_deallocation
+ // __p is not permitted to be a null pointer.
+ void
+ deallocate(pointer __p, size_type __t)
+ {
+#if __cpp_aligned_new
+ if (alignof(_Tp) > __STDCPP_DEFAULT_NEW_ALIGNMENT__)
+ {
+ ::operator delete(__p, __t * sizeof(_Tp),
+ std::align_val_t(alignof(_Tp)));
+ return;
+ }
+#endif
+ ::operator delete(__p, __t * sizeof(_Tp));
+ }
+#else
// __p is not permitted to be a null pointer.
void
deallocate(pointer __p, size_type)
@@ -127,6 +143,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
#endif
::operator delete(__p);
}
+#endif
size_type
max_size() const _GLIBCXX_USE_NOEXCEPT
--
2.5.5
