Fixes issue #19929: Call os.read with 32768 within subprocess.Popen

communicate rather than 4096 for efficiency.  A microbenchmark shows
Linux and OS X both using ~50% less cpu time this way.

2 files changed, 5 insertions, 1 deletions

diff --git a/Lib/subprocess.py b/Lib/subprocess.py
index e7f39fef06..e79e5fd614 100644
--- a/Lib/subprocess.py
+++ b/Lib/subprocess.py
@@ -1601,7 +1601,7 @@ class Popen(object):
                                     selector.unregister(key.fileobj)
                                     key.fileobj.close()
                         elif key.fileobj in (self.stdout, self.stderr):
-                            data = os.read(key.fd, 4096)
+                            data = os.read(key.fd, 32768)
                             if not data:
                                 selector.unregister(key.fileobj)
                                 key.fileobj.close()
diff --git a/Misc/NEWS b/Misc/NEWS
index 633ef5040e..f81db69a3e 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -24,6 +24,10 @@ Library
 - Issue #19343: Expose FreeBSD-specific APIs in resource module.  Original
   patch by Koobs.
 
+- Issue #19929: Call os.read with 32768 within subprocess.Popen.communicate
+  rather than 4096 for efficiency.  A microbenchmark shows Linux and OS X
+  both using ~50% less cpu time this way.
+
 - Issue #19506: Use a memoryview to avoid a data copy when piping data
   to stdin within subprocess.Popen.communicate.  5-10% less cpu usage.