[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() {


-------------- next part --------------
A non-text attachment was scrubbed...
Name: D147340.510075.patch
Type: text/x-patch
Size: 602 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20230331/53c5c021/attachment.bin>


More information about the llvm-commits mailing list