[PATCH] D147340: [Support] Improve Windows performance of buffered raw_ostream
Andrew Ng via Phabricator via llvm-commits
llvm-commits at lists.llvm.org
Fri Mar 31 10:15:22 PDT 2023
andrewng created this revision.
andrewng added reviewers: mstorsjo, rnk.
Herald added a subscriber: hiraditya.
Herald added a project: All.
andrewng requested review of this revision.
Herald added a project: LLVM.
The "preferred" buffer size for raw_ostream is set to BUFSIZ which on
Windows is only 512. This results in more calls to write and this
overhead can have a significant negative impact on performance,
especially when Anti-Virus is also involved.
Therefore increase the "preferred" buffer size to 16KB for Windows.
One example of where this helps is the LLD --Map option which dumps out
the symbol map for a link. In a link of UE4, this change has been seen
to improve the performance of the symbol map writing by more than a
factor of 6.
https://reviews.llvm.org/D147340
Files:
llvm/lib/Support/raw_ostream.cpp
Index: llvm/lib/Support/raw_ostream.cpp
===================================================================
--- llvm/lib/Support/raw_ostream.cpp
+++ llvm/lib/Support/raw_ostream.cpp
@@ -84,8 +84,15 @@
}
size_t raw_ostream::preferred_buffer_size() const {
+#ifdef _WIN32
+ // On Windows BUFSIZ is only 512 which results in more calls to write. This
+ // overhead can cause significant performance degradation. Therefore use a
+ // better default.
+ return (16 * 1024);
+#else
// BUFSIZ is intended to be a reasonable default.
return BUFSIZ;
+#endif
}
void raw_ostream::SetBuffered() {
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