diff --git a/ChangeLog b/ChangeLog
index 73cdbcd986..ffedc38209 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,10 @@
+Sun Jul 20 12:44:23 2014  Eric Wong  <e@80x24.org>
+
+	* include/ruby/io.h (rb_io_buffer_t): pack structure
+	  Reduces rb_io_t from 200 to 192 bytes, allowing rb_io_t to
+	  occupy one less cache line.
+	  [Feature #10050]
+
 Sun Jul 20 12:41:53 2014  Eric Wong  <e@80x24.org>
 
 	* include/ruby/io.h (rb_io_t): shrink to 200 bytes from 216 on 64-bit
diff --git a/include/ruby/io.h b/include/ruby/io.h
index bdde9844e2..8f0367707e 100644
--- a/include/ruby/io.h
+++ b/include/ruby/io.h
@@ -51,12 +51,12 @@ extern "C" {
 
 RUBY_SYMBOL_EXPORT_BEGIN
 
-typedef struct {
+typedef PACKED_STRUCT(struct {
     char *ptr;                  /* off + len <= capa */
     int off;
     int len;
     int capa;
-} rb_io_buffer_t;
+}) rb_io_buffer_t;
 
 typedef struct rb_io_t {
     FILE *stdio_file;		/* stdio ptr for read/write if available */