Bug #44723 Larger read_buffer_size values can cause performance

         decrease for INSERTs


Bulk inserts (multiple row, CREATE ... SELECT, INSERT ... SELECT) into
MyISAM tables were performed inefficiently. This was mainly affecting
use cases where read_buffer_size was considerably large (>256K) and low
number of rows was inserted (e.g. 30-100).

The problem was that during I/O cache initialization (this happens
before each bulk insert) allocated I/O buffer was unnecessarily
initialized to '\0'.

This was happening because of mess in flag values. MyISAM informs I/O
cache to wait for free space (if out of disk space) by passing
MY_WAIT_IF_FULL flag. Since MY_WAIT_IF_FULL and MY_ZEROFILL have the
same values, memory allocator was initializing memory to '\0'.

The performance gain provided with this patch may only be visible with
non-debug binaries, since safemalloc always initializes allocated memory
to 0xA5A5...

mysys/mf_iocache.c

@@ -233,10 +233,13 @@ int init_io_cache(IO_CACHE *info, File file, uint cachesize,
       buffer_block = cachesize;
       if (type == SEQ_READ_APPEND)
 	buffer_block *= 2;
-      if ((info->buffer=
-	   (byte*) my_malloc(buffer_block,
-			     MYF((cache_myflags & ~ MY_WME) |
-				 (cachesize == min_cache ? MY_WME : 0)))) != 0)
+      /*
+        Unset MY_WAIT_IF_FULL bit if it is set, to prevent conflict with
+        MY_ZEROFILL.
+      */
+      myf flag = MYF((cache_myflags & ~ (MY_WME | MY_WAIT_IF_FULL)) |
+                (cachesize == min_cache ? MY_WME : 0));
+      if ((info->buffer= (byte*) my_malloc(buffer_block, flag)) != 0)
       {
 	info->write_buffer=info->buffer;
 	if (type == SEQ_READ_APPEND)