Speed up reading sub-process output on MS-Windows

* src/w32proc.c (syms_of_ntproc) <w32-pipe-read-delay>: Set to
zero.  For the details, see this discussion:
http://lists.gnu.org/archive/html/emacs-devel/2018-06/msg00711.html.
* src/w32.c (_sys_read_ahead): Update the commentary for
w32-pipe-read-delay usage.

* doc/emacs/msdos.texi (Windows Processes): Document
w32-pipe-read-delay.

* etc/NEWS: Mention the change of the value of w32-pipe-read-delay.

2 files changed, 17 insertions, 13 deletions

diff --git a/src/w32.c b/src/w32.c
index e93aaab9ca1..c848b33b2af 100644
--- a/src/w32.c
+++ b/src/w32.c
@@ -8469,13 +8469,14 @@ _sys_read_ahead (int fd)
     {
       rc = _read (fd, &cp->chr, sizeof (char));
 
-      /* Give subprocess time to buffer some more output for us before
-         reporting that input is available; we need this because Windows 95
-         connects DOS programs to pipes by making the pipe appear to be
-         the normal console stdout - as a result most DOS programs will
-         write to stdout without buffering, ie. one character at a
-         time.  Even some W32 programs do this - "dir" in a command
-         shell on NT is very slow if we don't do this. */
+      /* Optionally give subprocess time to buffer some more output
+         for us before reporting that input is available; we may need
+         this because Windows 9X connects DOS programs to pipes by
+         making the pipe appear to be the normal console stdout -- as
+         a result most DOS programs will write to stdout without
+         buffering, i.e., one character at a time.  Even some W32
+         programs do this -- "dir" in a command shell on NT is very
+         slow if we don't do this.  */
       if (rc > 0)
         {
           int wait = w32_pipe_read_delay;
diff --git a/src/w32proc.c b/src/w32proc.c
index 28d7b6611f6..5934669c363 100644
--- a/src/w32proc.c
+++ b/src/w32proc.c
@@ -3763,14 +3763,17 @@ them blocking when trying to access unmounted drives etc.  */);
 
   DEFVAR_INT ("w32-pipe-read-delay", w32_pipe_read_delay,
               doc: /* Forced delay before reading subprocess output.
-This is done to improve the buffering of subprocess output, by
-avoiding the inefficiency of frequently reading small amounts of data.
+This may need to be done to improve the buffering of subprocess output,
+by avoiding the inefficiency of frequently reading small amounts of data.
+Typically needed only with DOS programs on Windows 9X; set to 50 if
+throughput with such programs is slow.
 
 If positive, the value is the number of milliseconds to sleep before
-reading the subprocess output.  If negative, the magnitude is the number
-of time slices to wait (effectively boosting the priority of the child
-process temporarily).  A value of zero disables waiting entirely.  */);
-  w32_pipe_read_delay = 50;
+signaling that output from a subprocess is ready to be read.
+If negative, the value is the number of time slices to wait (effectively
+boosting the priority of the child process temporarily).
+A value of zero disables waiting entirely.  */);
+  w32_pipe_read_delay = 0;
 
   DEFVAR_INT ("w32-pipe-buffer-size", w32_pipe_buffer_size,
               doc: /* Size of buffer for pipes created to communicate with subprocesses.