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

diff --git a/configure b/configure
index bf3ea690db31e2b4c13a723c76c8b7fc99413940..217704e9cad6b1f4607fcdebba4093f6307cab58 100755 (executable)
--- a/configure
+++ b/configure
@@ -16057,7 +16057,7 @@ cat >>confdefs.h <<_ACEOF
 _ACEOF
 
 
-# We can't use AC_REPLACE_FUNCS to replace these functions, because it
+# We can't use AC_CHECK_FUNCS to detect these functions, because it
 # won't handle deployment target restrictions on macOS
 ac_fn_c_check_decl "$LINENO" "preadv" "ac_cv_have_decl_preadv" "#include <sys/uio.h>
 "
@@ -16070,16 +16070,6 @@ fi
 cat >>confdefs.h <<_ACEOF
 #define HAVE_DECL_PREADV $ac_have_decl
 _ACEOF
-if test $ac_have_decl = 1; then :
-
-else
-  case " $LIBOBJS " in
-  *" preadv.$ac_objext "* ) ;;
-  *) LIBOBJS="$LIBOBJS preadv.$ac_objext"
- ;;
-esac
-
-fi
 
 ac_fn_c_check_decl "$LINENO" "pwritev" "ac_cv_have_decl_pwritev" "#include <sys/uio.h>
 "
@@ -16092,16 +16082,6 @@ fi
 cat >>confdefs.h <<_ACEOF
 #define HAVE_DECL_PWRITEV $ac_have_decl
 _ACEOF
-if test $ac_have_decl = 1; then :
-
-else
-  case " $LIBOBJS " in
-  *" pwritev.$ac_objext "* ) ;;
-  *) LIBOBJS="$LIBOBJS pwritev.$ac_objext"
- ;;
-esac
-
-fi
 
 
 # This is probably only present on macOS, but may as well check always

diff --git a/configure.ac b/configure.ac
index fed7167e65d801714267f99092d58131f422ff65..e49de9e4f04e04dadba627d1658f1957dd517406 100644 (file)
--- a/configure.ac
+++ b/configure.ac
@@ -1816,10 +1816,10 @@ AC_CHECK_DECLS(posix_fadvise, [], [], [#include <fcntl.h>])
 AC_CHECK_DECLS(fdatasync, [], [], [#include <unistd.h>])
 AC_CHECK_DECLS([strlcat, strlcpy, strnlen])
 
-# We can't use AC_REPLACE_FUNCS to replace these functions, because it
+# We can't use AC_CHECK_FUNCS to detect these functions, because it
 # won't handle deployment target restrictions on macOS
-AC_CHECK_DECLS([preadv], [], [AC_LIBOBJ(preadv)], [#include <sys/uio.h>])
-AC_CHECK_DECLS([pwritev], [], [AC_LIBOBJ(pwritev)], [#include <sys/uio.h>])
+AC_CHECK_DECLS([preadv], [], [], [#include <sys/uio.h>])
+AC_CHECK_DECLS([pwritev], [], [], [#include <sys/uio.h>])
 
 # This is probably only present on macOS, but may as well check always
 AC_CHECK_DECLS(F_FULLFSYNC, [], [], [#include <fcntl.h>])

diff --git a/src/include/port/pg_iovec.h b/src/include/port/pg_iovec.h
index 689799c42580150eeb85bc7a5130d85768f89bc2..0637d6acaba41b072c2145895f5d82119ead839e 100644 (file)
--- 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 */

diff --git a/src/port/meson.build b/src/port/meson.build
index a0d0a9583a0d7d769e93a3fb1f5c687d02416f94..576a48b48c790f9a9e8b202f674f2e7d344e3a11 100644 (file)
--- a/src/port/meson.build
+++ b/src/port/meson.build
@@ -66,8 +66,6 @@ replace_funcs_neg = [
   ['getpeereid'],
   ['inet_aton'],
   ['mkdtemp'],
-  ['preadv', 'HAVE_DECL_PREADV'],
-  ['pwritev', 'HAVE_DECL_PWRITEV'],
   ['strlcat'],
   ['strlcpy'],
   ['strnlen'],

diff --git a/src/port/preadv.c b/src/port/preadv.c
deleted file mode 100644 (file)
index e762283..0000000
--- a/src/port/preadv.c
+++ /dev/null
@@ -1,43 +0,0 @@
-/*-------------------------------------------------------------------------
- *
- * preadv.c
- *       Implementation of preadv(2) for platforms that lack one.
- *
- * Portions Copyright (c) 1996-2023, PostgreSQL Global Development Group
- *
- * IDENTIFICATION
- *       src/port/preadv.c
- *
- *-------------------------------------------------------------------------
- */
-
-
-#include "c.h"
-
-#include <unistd.h>
-
-#include "port/pg_iovec.h"
-
-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;
-}

diff --git a/src/port/pwritev.c b/src/port/pwritev.c
deleted file mode 100644 (file)
index 519de45..0000000
--- a/src/port/pwritev.c
+++ /dev/null
@@ -1,43 +0,0 @@
-/*-------------------------------------------------------------------------
- *
- * pwritev.c
- *       Implementation of pwritev(2) for platforms that lack one.
- *
- * Portions Copyright (c) 1996-2023, PostgreSQL Global Development Group
- *
- * IDENTIFICATION
- *       src/port/pwritev.c
- *
- *-------------------------------------------------------------------------
- */
-
-
-#include "c.h"
-
-#include <unistd.h>
-
-#include "port/pg_iovec.h"
-
-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;
-}

diff --git a/src/tools/msvc/Mkvcbuild.pm b/src/tools/msvc/Mkvcbuild.pm
index 84f648c174a57af528159e2ccc34f7860cec8b13..d26d1a84e81aadc4b6176fe224207fc7a3059563 100644 (file)
--- a/src/tools/msvc/Mkvcbuild.pm
+++ b/src/tools/msvc/Mkvcbuild.pm
@@ -104,7 +104,7 @@ sub mkvcbuild
          inet_net_ntop.c kill.c open.c
          snprintf.c strlcat.c strlcpy.c dirmod.c noblock.c path.c
          dirent.c getopt.c getopt_long.c
-         preadv.c pwritev.c pg_bitutils.c
+         pg_bitutils.c
          pg_strong_random.c pgcheckdir.c pgmkdirp.c pgsleep.c pgstrcasecmp.c
          pqsignal.c mkdtemp.c qsort.c qsort_arg.c bsearch_arg.c quotes.c system.c
          strerror.c tar.c