[PATCH] libstdc++: basic_filebuf: don't flush more often than necessary.

[Charles-Francois Natali <cf.natali@gmail.com>]

`basic_filebuf::xsputn` would bypass the buffer when passed a chunk of
size 1024 and above, seemingly as an optimisation.

This can have a significant performance impact if the overhead of a
`write` syscall is non-negligible, e.g. on a slow disk, on network
filesystems, or simply during IO contention because instead of flushing
every `BUFSIZ` (by default), we can flush every 1024 char.
The impact is even greater with custom larger buffers, e.g. for network
filesystems, because the code could issue `write` for example 1000X more
often than necessary with respect to the buffer size.
It also introduces a significant discontinuity in performance when
writing chunks of size 1024 and above.

See this reproducer which writes down a fixed number of chunks to a file
open with `O_SYNC` - to replicate high-latency `write` - for varying
size of chunks:

```
$ cat test_fstream_flush.cpp

int
main(int argc, char* argv[])
{
  assert(argc == 3);

  const auto* path = argv[1];
  const auto chunk_size = std::stoul(argv[2]);

  const auto fd =
    open(path, O_CREAT | O_TRUNC | O_WRONLY | O_SYNC | O_CLOEXEC, 0666);
  assert(fd >= 0);

  auto filebuf = __gnu_cxx::stdio_filebuf<char>(fd, std::ios_base::out);
  auto stream = std::ostream(&filebuf);

  const auto chunk = std::vector<char>(chunk_size);

  for (auto i = 0; i < 1'000; ++i) {
    stream.write(chunk.data(), chunk.size());
  }

  return 0;
}
```

```
$ g++ -o /tmp/test_fstream_flush test_fstream_flush.cpp -std=c++17
$ for i in $(seq 1021 1025); do echo -e "\n$i"; time /tmp/test_fstream_flush /tmp/foo $i; done

1021

real    0m0.997s
user    0m0.000s
sys     0m0.038s

1022

real    0m0.939s
user    0m0.005s
sys     0m0.032s

1023

real    0m0.954s
user    0m0.005s
sys     0m0.034s

1024

real    0m7.102s
user    0m0.040s
sys     0m0.192s

1025

real    0m7.204s
user    0m0.025s
sys     0m0.209s
```

See the huge drop in performance at the 1024-boundary.

An `strace` confirms that from size 1024 we effectively defeat
buffering:
1023-sized writes
```
$ strace -P /tmp/foo -e openat,write,writev /tmp/test_fstream_flush /tmp/foo 1023 2>&1 | head -n5
openat(AT_FDCWD, "/tmp/foo", O_WRONLY|O_CREAT|O_TRUNC|O_SYNC|O_CLOEXEC, 0666) = 3
writev(3, [{iov_base="\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., iov_len=8184}, {iov_base="\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., iov_len=1023}], 2) = 9207
writev(3, [{iov_base="\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., iov_len=8184}, {iov_base="\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., iov_len=1023}], 2) = 9207
writev(3, [{iov_base="\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., iov_len=8184}, {iov_base="\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., iov_len=1023}], 2) = 9207
writev(3, [{iov_base="\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., iov_len=8184}, {iov_base="\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., iov_len=1023}], 2) = 9207
```

vs 1024-sized writes
```
$ strace -P /tmp/foo -e openat,write,writev /tmp/test_fstream_flush /tmp/foo 1024 2>&1 | head -n5
openat(AT_FDCWD, "/tmp/foo", O_WRONLY|O_CREAT|O_TRUNC|O_SYNC|O_CLOEXEC, 0666) = 3
writev(3, [{iov_base=NULL, iov_len=0}, {iov_base="\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., iov_len=1024}], 2) = 1024
writev(3, [{iov_base="", iov_len=0}, {iov_base="\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., iov_len=1024}], 2) = 1024
writev(3, [{iov_base="", iov_len=0}, {iov_base="\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., iov_len=1024}], 2) = 1024
writev(3, [{iov_base="", iov_len=0}, {iov_base="\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., iov_len=1024}], 2) = 1024
```

Instead, it makes sense to only bypass the buffer if the amount of data
to be written is larger than the buffer capacity.

Closes https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63746
---
 libstdc++-v3/include/bits/fstream.tcc         |  9 +--
 .../27_io/basic_filebuf/sputn/char/63746.cc   | 55 +++++++++++++++++++
 2 files changed, 58 insertions(+), 6 deletions(-)
 create mode 100644 libstdc++-v3/testsuite/27_io/basic_filebuf/sputn/char/63746.cc

diff --git a/libstdc++-v3/include/bits/fstream.tcc b/libstdc++-v3/include/bits/fstream.tcc
index 7ccc887b8..2e9369628 100644
--- a/libstdc++-v3/include/bits/fstream.tcc
+++ b/libstdc++-v3/include/bits/fstream.tcc
@@ -757,23 +757,20 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     {
       streamsize __ret = 0;
       // Optimization in the always_noconv() case, to be generalized in the
-      // future: when __n is sufficiently large we write directly instead of
-      // using the buffer.
+      // future: when __n is larger than the available capacity we write
+      // directly instead of using the buffer.
       const bool __testout = (_M_mode & ios_base::out
 			      || _M_mode & ios_base::app);
       if (__check_facet(_M_codecvt).always_noconv()
 	  && __testout && !_M_reading)
 	{
-	  // Measurement would reveal the best choice.
-	  const streamsize __chunk = 1ul << 10;
 	  streamsize __bufavail = this->epptr() - this->pptr();
 
 	  // Don't mistake 'uncommitted' mode buffered with unbuffered.
 	  if (!_M_writing && _M_buf_size > 1)
 	    __bufavail = _M_buf_size - 1;
 
-	  const streamsize __limit = std::min(__chunk, __bufavail);
-	  if (__n >= __limit)
+	  if (__n >= __bufavail)
 	    {
 	      const streamsize __buffill = this->pptr() - this->pbase();
 	      const char* __buf = reinterpret_cast<const char*>(this->pbase());
diff --git a/libstdc++-v3/testsuite/27_io/basic_filebuf/sputn/char/63746.cc b/libstdc++-v3/testsuite/27_io/basic_filebuf/sputn/char/63746.cc
new file mode 100644
index 000000000..36448e049
--- /dev/null
+++ b/libstdc++-v3/testsuite/27_io/basic_filebuf/sputn/char/63746.cc
@@ -0,0 +1,55 @@
+// Copyright (C) 2013-2022 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+// { dg-require-fileio "" }
+
+#include <fstream>
+#include <testsuite_hooks.h>
+
+class testbuf : public std::filebuf {
+public:
+  char_type* pub_pprt() const
+  {
+    return this->pptr();
+  }
+
+  char_type* pub_pbase() const
+  {
+    return this->pbase();
+  }
+};
+
+void test01()
+{
+  using namespace std;
+
+  // Leave capacity to avoid flush.
+  const streamsize chunk_size = BUFSIZ - 1 - 1;
+  const char data[chunk_size] = {};
+
+  testbuf a_f;
+  VERIFY( a_f.open("tmp_63746_sputn", ios_base::out) );
+  VERIFY( chunk_size == a_f.sputn(data, chunk_size) );
+  VERIFY( (a_f.pub_pprt() - a_f.pub_pbase()) == chunk_size );
+  VERIFY( a_f.close() );
+}
+
+int main()
+{
+  test01();
+  return 0;
+}
-- 
2.30.2