https://github.com/python/cpython/commit/2f5f19e783385ec5312f7054827ccf1cdb6... commit: 2f5f19e783385ec5312f7054827ccf1cdb6e14ef branch: main author: Cody Maloney <cmaloney@users.noreply.github.com> committer: vstinner <vstinner@python.org> date: 2024-07-04T09:17:00+02:00 summary: gh-120754: Reduce system calls in full-file FileIO.readall() case (#120755) This reduces the system call count of a simple program[0] that reads all the `.rst` files in Doc by over 10% (5706 -> 4734 system calls on my linux system, 5813 -> 4875 on my macOS) This reduces the number of `fstat()` calls always and seek calls most the time. Stat was always called twice, once at open (to error early on directories), and a second time to get the size of the file to be able to read the whole file in one read. Now the size is cached with the first call. The code keeps an optimization that if the user had previously read a lot of data, the current position is subtracted from the number of bytes to read. That is somewhat expensive so only do it on larger files, otherwise just try and read the extra bytes and resize the PyBytes as needeed. I built a little test program to validate the behavior + assumptions around relative costs and then ran it under `strace` to get a log of the system calls. Full samples below[1]. After the changes, this is everything in one `filename.read_text()`: ```python3 openat(AT_FDCWD, "cpython/Doc/howto/clinic.rst", O_RDONLY|O_CLOEXEC) = 3` fstat(3, {st_mode=S_IFREG|0644, st_size=343, ...}) = 0` ioctl(3, TCGETS, 0x7ffdfac04b40) = -1 ENOTTY (Inappropriate ioctl for device) lseek(3, 0, SEEK_CUR) = 0 read(3, ":orphan:\n\n.. This page is retain"..., 344) = 343 read(3, "", 1) = 0 close(3) = 0 ``` This does make some tradeoffs 1. If the file size changes between open() and readall(), this will still get all the data but might have more read calls. 2. I experimented with avoiding the stat + cached result for small files in general, but on my dev workstation at least that tended to reduce performance compared to using the fstat(). [0] ```python3 from pathlib import Path nlines = [] for filename in Path("cpython/Doc").glob("**/*.rst"): nlines.append(len(filename.read_text())) ``` [1] Before small file: ``` openat(AT_FDCWD, "cpython/Doc/howto/clinic.rst", O_RDONLY|O_CLOEXEC) = 3 fstat(3, {st_mode=S_IFREG|0644, st_size=343, ...}) = 0 ioctl(3, TCGETS, 0x7ffe52525930) = -1 ENOTTY (Inappropriate ioctl for device) lseek(3, 0, SEEK_CUR) = 0 lseek(3, 0, SEEK_CUR) = 0 fstat(3, {st_mode=S_IFREG|0644, st_size=343, ...}) = 0 read(3, ":orphan:\n\n.. This page is retain"..., 344) = 343 read(3, "", 1) = 0 close(3) = 0 ``` After small file: ``` openat(AT_FDCWD, "cpython/Doc/howto/clinic.rst", O_RDONLY|O_CLOEXEC) = 3 fstat(3, {st_mode=S_IFREG|0644, st_size=343, ...}) = 0 ioctl(3, TCGETS, 0x7ffdfac04b40) = -1 ENOTTY (Inappropriate ioctl for device) lseek(3, 0, SEEK_CUR) = 0 read(3, ":orphan:\n\n.. This page is retain"..., 344) = 343 read(3, "", 1) = 0 close(3) = 0 ``` Before large file: ``` openat(AT_FDCWD, "cpython/Doc/c-api/typeobj.rst", O_RDONLY|O_CLOEXEC) = 3 fstat(3, {st_mode=S_IFREG|0644, st_size=133104, ...}) = 0 ioctl(3, TCGETS, 0x7ffe52525930) = -1 ENOTTY (Inappropriate ioctl for device) lseek(3, 0, SEEK_CUR) = 0 lseek(3, 0, SEEK_CUR) = 0 fstat(3, {st_mode=S_IFREG|0644, st_size=133104, ...}) = 0 read(3, ".. highlight:: c\n\n.. _type-struc"..., 133105) = 133104 read(3, "", 1) = 0 close(3) = 0 ``` After large file: ``` openat(AT_FDCWD, "cpython/Doc/c-api/typeobj.rst", O_RDONLY|O_CLOEXEC) = 3 fstat(3, {st_mode=S_IFREG|0644, st_size=133104, ...}) = 0 ioctl(3, TCGETS, 0x7ffdfac04b40) = -1 ENOTTY (Inappropriate ioctl for device) lseek(3, 0, SEEK_CUR) = 0 lseek(3, 0, SEEK_CUR) = 0 read(3, ".. highlight:: c\n\n.. _type-struc"..., 133105) = 133104 read(3, "", 1) = 0 close(3) = 0 ``` Co-authored-by: Shantanu <12621235+hauntsaninja@users.noreply.github.com> Co-authored-by: Erlend E. Aasland <erlend.aasland@protonmail.com> Co-authored-by: Victor Stinner <vstinner@python.org> files: A Misc/NEWS.d/next/Core and Builtins/2024-06-19-19-54-35.gh-issue-120754.uF29sj.rst M Lib/_pyio.py M Modules/_io/fileio.c diff --git a/Lib/_pyio.py b/Lib/_pyio.py index 7d298e1674b49a..75b5ad1b1a47d2 100644 --- a/Lib/_pyio.py +++ b/Lib/_pyio.py @@ -1577,6 +1577,7 @@ def __init__(self, file, mode='r', closefd=True, opener=None): self._blksize = getattr(fdfstat, 'st_blksize', 0) if self._blksize <= 1: self._blksize = DEFAULT_BUFFER_SIZE + self._estimated_size = fdfstat.st_size if _setmode: # don't translate newlines (\r\n <=> \n) @@ -1654,14 +1655,18 @@ def readall(self): """ self._checkClosed() self._checkReadable() - bufsize = DEFAULT_BUFFER_SIZE - try: - pos = os.lseek(self._fd, 0, SEEK_CUR) - end = os.fstat(self._fd).st_size - if end >= pos: - bufsize = end - pos + 1 - except OSError: - pass + if self._estimated_size <= 0: + bufsize = DEFAULT_BUFFER_SIZE + else: + bufsize = self._estimated_size + 1 + + if self._estimated_size > 65536: + try: + pos = os.lseek(self._fd, 0, SEEK_CUR) + if self._estimated_size >= pos: + bufsize = self._estimated_size - pos + 1 + except OSError: + pass result = bytearray() while True: @@ -1737,6 +1742,7 @@ def truncate(self, size=None): if size is None: size = self.tell() os.ftruncate(self._fd, size) + self._estimated_size = size return size def close(self): diff --git a/Misc/NEWS.d/next/Core and Builtins/2024-06-19-19-54-35.gh-issue-120754.uF29sj.rst b/Misc/NEWS.d/next/Core and Builtins/2024-06-19-19-54-35.gh-issue-120754.uF29sj.rst new file mode 100644 index 00000000000000..46481d8f31aaba --- /dev/null +++ b/Misc/NEWS.d/next/Core and Builtins/2024-06-19-19-54-35.gh-issue-120754.uF29sj.rst @@ -0,0 +1 @@ +Reduce the number of system calls invoked when reading a whole file (ex. ``open('a.txt').read()``). For a sample program that reads the contents of the 400+ ``.rst`` files in the cpython repository ``Doc`` folder, there is an over 10% reduction in system call count. diff --git a/Modules/_io/fileio.c b/Modules/_io/fileio.c index b5129ffcbffdcf..d5bf328eee9c10 100644 --- a/Modules/_io/fileio.c +++ b/Modules/_io/fileio.c @@ -54,6 +54,9 @@ # define SMALLCHUNK BUFSIZ #endif +/* Size at which a buffer is considered "large" and behavior should change to + avoid excessive memory allocation */ +#define LARGE_BUFFER_CUTOFF_SIZE 65536 /*[clinic input] module _io @@ -72,6 +75,7 @@ typedef struct { unsigned int closefd : 1; char finalizing; unsigned int blksize; + Py_off_t estimated_size; PyObject *weakreflist; PyObject *dict; } fileio; @@ -196,6 +200,7 @@ fileio_new(PyTypeObject *type, PyObject *args, PyObject *kwds) self->appending = 0; self->seekable = -1; self->blksize = 0; + self->estimated_size = -1; self->closefd = 1; self->weakreflist = NULL; } @@ -482,6 +487,9 @@ _io_FileIO___init___impl(fileio *self, PyObject *nameobj, const char *mode, if (fdfstat.st_blksize > 1) self->blksize = fdfstat.st_blksize; #endif /* HAVE_STRUCT_STAT_ST_BLKSIZE */ + if (fdfstat.st_size < PY_SSIZE_T_MAX) { + self->estimated_size = (Py_off_t)fdfstat.st_size; + } } #if defined(MS_WINDOWS) || defined(__CYGWIN__) @@ -684,7 +692,7 @@ new_buffersize(fileio *self, size_t currentsize) giving us amortized linear-time behavior. For bigger sizes, use a less-than-double growth factor to avoid excessive allocation. */ assert(currentsize <= PY_SSIZE_T_MAX); - if (currentsize > 65536) + if (currentsize > LARGE_BUFFER_CUTOFF_SIZE) addend = currentsize >> 3; else addend = 256 + currentsize; @@ -707,43 +715,56 @@ static PyObject * _io_FileIO_readall_impl(fileio *self) /*[clinic end generated code: output=faa0292b213b4022 input=dbdc137f55602834]*/ { - struct _Py_stat_struct status; Py_off_t pos, end; PyObject *result; Py_ssize_t bytes_read = 0; Py_ssize_t n; size_t bufsize; - int fstat_result; - if (self->fd < 0) + if (self->fd < 0) { return err_closed(); + } - Py_BEGIN_ALLOW_THREADS - _Py_BEGIN_SUPPRESS_IPH -#ifdef MS_WINDOWS - pos = _lseeki64(self->fd, 0L, SEEK_CUR); -#else - pos = lseek(self->fd, 0L, SEEK_CUR); -#endif - _Py_END_SUPPRESS_IPH - fstat_result = _Py_fstat_noraise(self->fd, &status); - Py_END_ALLOW_THREADS - - if (fstat_result == 0) - end = status.st_size; - else - end = (Py_off_t)-1; - - if (end > 0 && end >= pos && pos >= 0 && end - pos < PY_SSIZE_T_MAX) { + end = self->estimated_size; + if (end <= 0) { + /* Use a default size and resize as needed. */ + bufsize = SMALLCHUNK; + } + else { /* This is probably a real file, so we try to allocate a buffer one byte larger than the rest of the file. If the calculation is right then we should get EOF without having to enlarge the buffer. */ - bufsize = (size_t)(end - pos + 1); - } else { - bufsize = SMALLCHUNK; + if (end > _PY_READ_MAX - 1) { + bufsize = _PY_READ_MAX; + } + else { + bufsize = (size_t)end + 1; + } + + /* While a lot of code does open().read() to get the whole contents + of a file it is possible a caller seeks/reads a ways into the file + then calls readall() to get the rest, which would result in allocating + more than required. Guard against that for larger files where we expect + the I/O time to dominate anyways while keeping small files fast. */ + if (bufsize > LARGE_BUFFER_CUTOFF_SIZE) { + Py_BEGIN_ALLOW_THREADS + _Py_BEGIN_SUPPRESS_IPH +#ifdef MS_WINDOWS + pos = _lseeki64(self->fd, 0L, SEEK_CUR); +#else + pos = lseek(self->fd, 0L, SEEK_CUR); +#endif + _Py_END_SUPPRESS_IPH + Py_END_ALLOW_THREADS + + if (end >= pos && pos >= 0 && (end - pos) < (_PY_READ_MAX - 1)) { + bufsize = (size_t)(end - pos) + 1; + } + } } + result = PyBytes_FromStringAndSize(NULL, bufsize); if (result == NULL) return NULL; @@ -783,7 +804,6 @@ _io_FileIO_readall_impl(fileio *self) return NULL; } bytes_read += n; - pos += n; } if (PyBytes_GET_SIZE(result) > bytes_read) {