I made the change below, it leaves the code alone, just grows the
buffer more each time, by a factor 1.5x so copying time is no longer
quadratic in the output size.
I think I'll do this in the 1.6 branch too. Backtraces there have
been slow to the point of unusable for me in some parsing stuff I've
been doing with say 50k or so strings in various parameters. (The
backtrace goes via an output string so that it can truncate big args
like that.)
Index: strports.c
===================================================================
RCS file: /cvsroot/guile/guile/guile-core/libguile/strports.c,v
retrieving revision 1.108
diff -u -u -r1.108 strports.c
--- strports.c 23 May 2005 19:57:21 -0000 1.108
+++ strports.c 1 Aug 2005 23:47:06 -0000
@@ -65,7 +65,30 @@
has been written to, but this is only updated after a flush.
read_pos and write_pos in principle should be equal, but this is only true
when rw_active is SCM_PORT_NEITHER.
-*/
+
+ ENHANCE-ME - output blocks:
+
+ The current code keeps an output string as a single block. That means
+ when the size is increased the entire old contents must be copied. It'd
+ be more efficient to begin a new block when the old one is full, so
+ there's no re-copying of previous data.
+
+ To make seeking efficient, keeping the pieces in a vector might be best,
+ though appending is probably the most common operation. The size of each
+ block could be progressively increased, so the bigger the string the
+ bigger the blocks.
+
+ When `get-output-string' is called the blocks have to be coalesced into a
+ string, the result could be kept as a single big block. If blocks were
+ strings then `get-output-string' could notice when there's just one and
+ return that with a copy-on-write (though repeated calls to
+ `get-output-string' are probably unlikely).
+
+ Another possibility would be to extend the port mechanism to let SCM
+ strings come through directly from `display' and friends. That way if a
+ big string is written it can be kept as a copy-on-write, saving time
+ copying and maybe saving some space. */
+
scm_t_bits scm_tc16_strport;
@@ -117,7 +140,14 @@
#define SCM_WRITE_BLOCK 80
/* ensure that write_pos < write_end by enlarging the buffer when
- necessary. update read_buf to account for written chars. */
+ necessary. update read_buf to account for written chars.
+
+ The buffer is enlarged by 1.5 times, plus SCM_WRITE_BLOCK. Adding just a
+ fixed amount is no good, because there's a block copy for each increment,
+ and that copying would take quadratic time. In the past it was found to
+ be very slow just adding 80 bytes each time (eg. about 10 seconds for
+ writing a 100kbyte string). */
+
static void
st_flush (SCM port)
{
@@ -125,7 +155,7 @@
if (pt->write_pos == pt->write_end)
{
- st_resize_port (pt, pt->write_buf_size + SCM_WRITE_BLOCK);
+ st_resize_port (pt, pt->write_buf_size * 3 / 2 + SCM_WRITE_BLOCK);
}
pt->read_pos = pt->write_pos;
if (pt->read_pos > pt->read_end)
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