[PATCH v2] libstdc++: basic_filebuf: don't flush more often than necessary.
`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(fd, std::ios_base::out);
auto stream = std::ostream();
const auto chunk = std::vector(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
real0m0.997s
user0m0.000s
sys 0m0.038s
1022
real0m0.939s
user0m0.005s
sys 0m0.032s
1023
real0m0.954s
user0m0.005s
sys 0m0.034s
1024
real0m7.102s
user0m0.040s
sys 0m0.192s
1025
real0m7.204s
user0m0.025s
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
Signed-off-by: Charles-Francois Natali
---
libstdc++-v3/include/bits/fstream.tcc | 9 ++---
.../27_io/basic_filebuf/sputn/char/63746.cc | 38 +++
2 files changed, 41 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 =
[PATCH] libstdc++: basic_filebuf: don't flush more often than necessary.
`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(fd, std::ios_base::out);
auto stream = std::ostream();
const auto chunk = std::vector(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
real0m0.997s
user0m0.000s
sys 0m0.038s
1022
real0m0.939s
user0m0.005s
sys 0m0.032s
1023
real0m0.954s
user0m0.005s
sys 0m0.034s
1024
real0m7.102s
user0m0.040s
sys 0m0.192s
1025
real0m7.204s
user0m0.025s
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
