[PATCH] io.c (io_fwrite): copy to hidden buffer when writing

[Eric Wong <e@80x24.org>]

This avoids garbage from IO#write for [Bug #13085] when
called in a read-write loop while protecting the VM
from race conditions forced by the user.

Memory usage from benchmark/bm_io_copy_stream_write.rb
is reduced greatly:

  target 0: a (ruby 2.5.0dev (2017-01-05 trunk 57270) [x86_64-linux])
  target 1: b (ruby 2.5.0dev (2017-01-05) [x86_64-linux])

  Memory usage (last size) (B)
  name	a	b
  io_copy_stream_write	81899520.000	6561792.000

  Memory consuming ratio (size) with the result of `a' (greater is better)
  name	b
  io_copy_stream_write	12.481

Despite the extra deep data copy, there is a small speedup in
execution time due to GC avoidance:

  Execution time (sec)
  name	a	b
  io_copy_stream_write	0.393	0.296

  Speedup ratio: compare with the result of `a' (greater is better)
  name	b
  io_copy_stream_write	1.328

This patch increases memory bandwidth use by pessimistically
copying the data into a temporary hidden buffer.  Using a
lightweight frozen copy (as before this patch) is ineffective
in read-write loops, since the read operation will trigger
a heavy copy behind our back due to the CoW operation.

It is also impossible to safely release memory from the
lightweight CoW string, because we have no idea how many
lightweight duplicates exist by the time we reacquire GVL.

So, we now make a heavy copy up front which we recycle
immediately upon completion.

Ideally, Ruby should have a way of detecting Strings which are
not visible to other threads and be able to optimize away the
internal copy.  Or, we give up on the idea of implicit data
sharing between threads since its dangerous anyways.
---
 io.c | 28 +++++++++++++++++++++-------
 1 file changed, 21 insertions(+), 7 deletions(-)

diff --git a/io.c b/io.c
index 1074689..b7a85b9 100644
--- a/io.c
+++ b/io.c
@@ -1423,20 +1423,34 @@ static long
 io_fwrite(VALUE str, rb_io_t *fptr, int nosync)
 {
     int converted = 0;
+    VALUE tmp = str;
+    const char *ptr;
+    long len;
+
 #ifdef _WIN32
     if (fptr->mode & FMODE_TTY) {
 	long len = rb_w32_write_console(str, fptr->fd);
 	if (len > 0) return len;
     }
 #endif
-    str = do_writeconv(str, fptr, &converted);
-    if (converted)
-	OBJ_FREEZE(str);
-    else
-	str = rb_str_new_frozen(str);
+    tmp = do_writeconv(str, fptr, &converted);
+    if (converted) {
+	rb_obj_hide(tmp);
+    }
+    else if (!OBJ_FROZEN_RAW(str)) {
+	tmp = rb_str_tmp_new(0);
+	rb_str_buf_append(tmp, str);
+    }
+
+    RSTRING_GETMEM(tmp, ptr, len);
+    len = io_binwrite(tmp, ptr, len, fptr, nosync);
 
-    return io_binwrite(str, RSTRING_PTR(str), RSTRING_LEN(str),
-		       fptr, nosync);
+    if (tmp != str) {
+	rb_str_resize(tmp, 0);
+	rb_gc_force_recycle(tmp);
+    }
+
+    return len;
 }
 
 ssize_t
-- 
EW