string port slow output on big string

[Kevin Ryde <user42@zip.com.au>]

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I tried writing a biggish string to a string port, and it was very
slow.  Eg.

    (use-modules (ice-9 time))
    (let ((str (make-string 100000 #\x)))
      (call-with-output-string (lambda (port)
                                 (time (display str port))))
      #f)

gives on my poor 333mhz

    clock utime stime cutime cstime gctime
     7.63  7.58  0.05   0.00   0.00   4.17

I struck this trying to use regexp-substitute/global on a file slurped
into memory.  It was 130k, which is a decent size, but it's well
within the realm of reason.


I think strports.c st_write ends up doing a realloc and copy every 80
bytes of the block it's writing.  It knows the size, but it lets
st_flush just grow by 80 bytes at a time.

The change below speeds it up from 7 seconds to 10 ms for me.

But I don't know if the read side bits of this change are right.  Is
it supposed to update read_pos, read_end and read_buf_size to be the
end of the string, or something?


(Of course what would be even nicer would be to avoid big reallocing
altogether, like keep a list of chunks and only join them when a
get-string call wants the entire block.  But that can wait.)



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--- strports.c.~1.105.~	2005-01-28 08:25:34.000000000 +1100
+++ strports.c	2005-02-14 11:20:05.000000000 +1100
@@ -142,18 +142,14 @@
   scm_t_port *pt = SCM_PTAB_ENTRY (port);
   const char *input = (char *) data;
 
-  while (size > 0)
-    {
-      int space = pt->write_end - pt->write_pos;
-      int write_len = (size > space) ? space : size;
-      
-      memcpy ((char *) pt->write_pos, input, write_len);
-      pt->write_pos += write_len;
-      size -= write_len;
-      input += write_len;
-      if (write_len == space)
-	st_flush (port);
-    }
+  /* if not enough room for "size" then make that amount and an additional
+     SCM_WRITE_BLOCK */
+  if (size > pt->write_end - pt->write_pos)
+    st_resize_port (pt, pt->write_pos - pt->write_buf
+                        + size + SCM_WRITE_BLOCK);
+
+  memcpy ((char *) pt->write_pos, input, size);
+  pt->write_pos += size;
 }
 
 static void

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