yes: output data more efficiently

yes(1) may be used to generate repeating patterns of text
for test inputs etc., so adjust to be more efficient.

Profiling the case where yes(1) is outputting small items
through stdio (which was the default case), shows the overhead
of continuously processing small items in main() and in stdio:

    $ yes >/dev/null & perf top -p $!
    31.02%  yes           [.] main
    27.36%  libc-2.20.so  [.] _IO_file_xsputn@@GLIBC_2.2.5
    14.51%  libc-2.20.so  [.] fputs_unlocked
    13.50%  libc-2.20.so  [.] strlen
    10.66%  libc-2.20.so  [.] __GI___mempcpy
     1.98%  yes           [.] fputs_unlocked@plta

Sending more data per stdio call improves the situation,
but still, there is significant stdio overhead due to memory copies,
and the repeated string length checking:

    $ yes "`echo {1..1000}`" >/dev/null & perf top -p $!
    42.26%  libc-2.20.so  [.] __GI___mempcpy
    17.38%  libc-2.20.so  [.] strlen
     5.21%  [kernel]      [k] __srcu_read_lock
     4.58%  [kernel]      [k] __srcu_read_unlock
     4.27%  libc-2.20.so  [.] _IO_file_xsputn@@GLIBC_2.2.5
     2.50%  libc-2.20.so  [.] __GI___libc_write
     2.45%  [kernel]      [k] system_call
     2.40%  [kernel]      [k] system_call_after_swapgs
     2.27%  [kernel]      [k] vfs_write
     2.09%  libc-2.20.so  [.] _IO_do_write@@GLIBC_2.2.5
     2.01%  [kernel]      [k] fsnotify
     1.95%  libc-2.20.so  [.] _IO_file_write@@GLIBC_2.2.5
     1.44%  yes           [.] main

We can avoid all stdio overhead by building up the buffer
_once_ and outputting that, and the profile below shows
the bottleneck moved to the kernel:

    $ src/yes >/dev/null & perf top -p $!
    15.42%  [kernel]      [k] __srcu_read_lock
    12.98%  [kernel]      [k] __srcu_read_unlock
     9.41%  libc-2.20.so  [.] __GI___libc_write
     9.11%  [kernel]      [k] vfs_write
     8.35%  [kernel]      [k] fsnotify
     8.02%  [kernel]      [k] system_call
     5.84%  [kernel]      [k] system_call_after_swapgs
     4.54%  [kernel]      [k] __fget_light
     3.98%  [kernel]      [k] sys_write
     3.65%  [kernel]      [k] selinux_file_permission
     3.44%  [kernel]      [k] rw_verify_area
     2.94%  [kernel]      [k] __fsnotify_parent
     2.76%  [kernel]      [k] security_file_permission
     2.39%  yes           [.] main
     2.17%  [kernel]      [k] __fdget_pos
     2.13%  [kernel]      [k] sysret_check
     0.81%  [kernel]      [k] write_null
     0.36%  yes           [.] write@plt

Note this change also ensures that yes(1) will only write
complete lines for lines shorter than BUFSIZ.

* src/yes.c (main): Build up a BUFSIZ buffer of lines,
and output that, rather than having stdio process each item.
* tests/misc/yes.sh: Add a new test for various buffer sizes.
* tests/local.mk: Reference the new test.
Fixes http://bugs.gnu.org/20029

diff --git a/src/yes.c b/src/yes.c
index b35b13f7546acac7db79988bc7a99dd075aa8e5d..d6dd4b1ef9ccec0cc6e376d30482d849bb8a52e8 100644 (file)
--- a/src/yes.c
+++ b/src/yes.c
@@ -58,6 +58,10 @@ Repeatedly output a line with all specified STRING(s), or 'y'.\n\
 int
 main (int argc, char **argv)
 {
+  char buf[BUFSIZ];
+  char *pbuf = buf;
+  int i;
+
   initialize_main (&argc, &argv);
   set_program_name (argv[0]);
   setlocale (LC_ALL, "");
@@ -77,9 +81,44 @@ main (int argc, char **argv)
       argv[argc++] = bad_cast ("y");
     }
 
-  while (true)
+  /* Buffer data locally once, rather than having the
+     large overhead of stdio buffering each item.   */
+  for (i = optind; i < argc; i++)
+    {
+      size_t len = strlen (argv[i]);
+      if (BUFSIZ < len || BUFSIZ - len <= pbuf - buf)
+        break;
+      memcpy (pbuf, argv[i], len);
+      pbuf += len;
+      *pbuf++ = i == argc - 1 ? '\n' : ' ';
+    }
+  if (i < argc)
+    pbuf = NULL;
+  else
+    {
+      size_t line_len = pbuf - buf;
+      size_t lines = BUFSIZ / line_len;
+      while (--lines)
+        {
+          memcpy (pbuf, pbuf - line_len, line_len);
+          pbuf += line_len;
+        }
+    }
+
+  /* The normal case is to continuously output the local buffer.  */
+  while (pbuf)
+    {
+      if (write (STDOUT_FILENO, buf, pbuf - buf) == -1)
+        {
+          error (0, errno, _("standard output"));
+          return EXIT_FAILURE;
+        }
+    }
+
+  /* If the data doesn't fit in BUFSIZ then it's large
+     and not too inefficient to output through stdio.  */
+  while (! pbuf)
     {
-      int i;
       for (i = optind; i < argc; i++)
         if (fputs (argv[i], stdout) == EOF
             || putchar (i == argc - 1 ? '\n' : ' ') == EOF)

diff --git a/tests/local.mk b/tests/local.mk
index 9a5208082e2a9c1f332ef2090c91d857f9c61e8f..56cba699a69c72af6c38a8b5d95da1caf1e2a2c2 100644 (file)
--- a/tests/local.mk
+++ b/tests/local.mk
@@ -402,6 +402,7 @@ all_tests =                                 \
   tests/misc/uniq.pl                           \
   tests/misc/uniq-perf.sh                      \
   tests/misc/xattr.sh                          \
+  tests/misc/yes.sh                            \
   tests/tail-2/wait.sh                         \
   tests/tail-2/retry.sh                                \
   tests/tail-2/symlink.sh                      \

diff --git a/tests/misc/yes.sh b/tests/misc/yes.sh
new file mode 100755 (executable)
index 0000000..4ed300a
--- /dev/null
+++ b/tests/misc/yes.sh
@@ -0,0 +1,32 @@
+#!/bin/sh
+# Validate yes buffer handling
+
+# Copyright (C) 2015 Free Software Foundation, Inc.
+
+# This program is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+. "${srcdir=.}/tests/init.sh"; path_prepend_ ./src
+print_ver_ yes
+
+# Check various buffer sizes, with the most important
+# size being BUFSIZ used for the local buffer to yes(1).
+# Note a \n is added, so actual sizes required internally
+# are 1 more than the size used here.
+for size in 1 1999 4095 4096 8191 8192 16383 16384; do
+  printf "%${size}s\n" '' > out.1
+  yes "$(printf %${size}s '')" | head -n2 | uniq > out.2
+  compare out.1 out.2 || fail=1
+done
+
+Exit $fail