Optimize pg_readv/pg_pwritev single vector case.

For the trivial case of iovcnt == 1, kernels are measurably slower at
dealing with the more complex arguments of preadv/pwritev than the
equivalent plain old pread/pwrite.  The overheads are worth it for
iovcnt > 1, but for 1 let's just redirect to the cheaper calls.  While
we could leave it to callers to worry about that, we already have to
have our own pg_ wrappers for portability reasons so it seems
reasonable to centralize this knowledge there (thanks to Heikki for this
suggestion).  Try to avoid function call overheads by making them
inlinable, which might also allow the compiler to avoid the branch in
some cases.  For systems that don't have preadv and pwritev (currently:
Windows and [closed] Solaris), we might as well pull the replacement
functions up into the static inline functions too.

Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Discussion: https://postgr.es/m/CA+hUKGJkOiOCa+mag4BF+zHo7qo=o9CFheB8=g6uT5TUm2gkvA@mail.gmail.com

1 files changed, 69 insertions, 7 deletions

diff --git a/src/include/port/pg_iovec.h b/src/include/port/pg_iovec.h
index 689799c4258..0637d6acaba 100644
--- a/src/include/port/pg_iovec.h
+++ b/src/include/port/pg_iovec.h
@@ -17,6 +17,7 @@
 
 #include <limits.h>
 #include <sys/uio.h>
+#include <unistd.h>
 
 #else
 
@@ -36,20 +37,81 @@ struct iovec
 #define PG_IOV_MAX Min(IOV_MAX, 32)
 
 /*
- * Note that pg_preadv and pg_pwritev have a pg_ prefix as a warning that the
- * Windows implementations have the side-effect of changing the file position.
+ * Like preadv(), but with a prefix to remind us of a side-effect: on Windows
+ * this changes the current file position.
  */
-
+static inline ssize_t
+pg_preadv(int fd, const struct iovec *iov, int iovcnt, off_t offset)
+{
 #if HAVE_DECL_PREADV
-#define pg_preadv preadv
+	/*
+	 * Avoid a small amount of argument copying overhead in the kernel if
+	 * there is only one iovec.
+	 */
+	if (iovcnt == 1)
+		return pread(fd, iov[0].iov_base, iov[0].iov_len, offset);
+	else
+		return preadv(fd, iov, iovcnt, offset);
 #else
-extern ssize_t pg_preadv(int fd, const struct iovec *iov, int iovcnt, off_t offset);
+	ssize_t		sum = 0;
+	ssize_t		part;
+
+	for (int i = 0; i < iovcnt; ++i)
+	{
+		part = pg_pread(fd, iov[i].iov_base, iov[i].iov_len, offset);
+		if (part < 0)
+		{
+			if (i == 0)
+				return -1;
+			else
+				return sum;
+		}
+		sum += part;
+		offset += part;
+		if (part < iov[i].iov_len)
+			return sum;
+	}
+	return sum;
 #endif
+}
 
+/*
+ * Like pwritev(), but with a prefix to remind us of a side-effect: on Windows
+ * this changes the current file position.
+ */
+static inline ssize_t
+pg_pwritev(int fd, const struct iovec *iov, int iovcnt, off_t offset)
+{
 #if HAVE_DECL_PWRITEV
-#define pg_pwritev pwritev
+	/*
+	 * Avoid a small amount of argument copying overhead in the kernel if
+	 * there is only one iovec.
+	 */
+	if (iovcnt == 1)
+		return pwrite(fd, iov[0].iov_base, iov[0].iov_len, offset);
+	else
+		return pwritev(fd, iov, iovcnt, offset);
 #else
-extern ssize_t pg_pwritev(int fd, const struct iovec *iov, int iovcnt, off_t offset);
+	ssize_t		sum = 0;
+	ssize_t		part;
+
+	for (int i = 0; i < iovcnt; ++i)
+	{
+		part = pg_pwrite(fd, iov[i].iov_base, iov[i].iov_len, offset);
+		if (part < 0)
+		{
+			if (i == 0)
+				return -1;
+			else
+				return sum;
+		}
+		sum += part;
+		offset += part;
+		if (part < iov[i].iov_len)
+			return sum;
+	}
+	return sum;
 #endif
+}
 
 #endif							/* PG_IOVEC_H */