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.

1 files changed, 7 insertions, 0 deletions

diff --git a/doc/emacs/msdos.texi b/doc/emacs/msdos.texi
index 679bdd3e83b..c69c7d37f9b 100644
--- a/doc/emacs/msdos.texi
+++ b/doc/emacs/msdos.texi
@@ -808,6 +808,13 @@ communications with subprocesses to programs that exhibit unusual
 behavior with respect to buffering pipe I/O.
 
 @ifnottex
+@vindex w32-pipe-read-delay
+  If you need to invoke MS-DOS programs as Emacs subprocesses, you may
+see low rate of reading data from such programs.  Setting the variable
+@code{w32-pipe-read-delay} to a non-zero value may improve throughput
+in these cases; we suggest the value of 50 for such situations.  The
+default is zero.
+
 @findex w32-shell-execute
   The function @code{w32-shell-execute} can be useful for writing
 customized commands that run MS-Windows applications registered to