gh-120754: Reduce system calls in full-file readall case

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 a over 10% reduction in system call count.

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 size_estimated;
     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->size_estimated = -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->size_estimated = (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;
@@ -707,41 +715,48 @@ 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
+    end = self->size_estimated;
+    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) + 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);
+            pos = _lseeki64(self->fd, 0L, SEEK_CUR);
 #else
-    pos = lseek(self->fd, 0L, SEEK_CUR);
+            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;
+            _Py_END_SUPPRESS_IPH
+            Py_END_ALLOW_THREADS
 
-    if (end > 0 && end >= pos && pos >= 0 && end - pos < PY_SSIZE_T_MAX) {
-        /* 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 >= pos && pos >= 0 && end - pos < PY_SSIZE_T_MAX) {
+                bufsize = bufsize - Py_SAFE_DOWNCAST(pos, Py_off_t, size_t);
+            }
+        }
     }
 
     result = PyBytes_FromStringAndSize(NULL, bufsize);
@@ -783,7 +798,6 @@ _io_FileIO_readall_impl(fileio *self)
             return NULL;
         }
         bytes_read += n;
-        pos += n;
     }
 
     if (PyBytes_GET_SIZE(result) > bytes_read) {