[gegl] buffer: in gegl-tile-alloc, limit number of tiles per block

[Ell <ell src gnome org>]

commit fff84b3a42050a4aeeedde9f46966ef99979288b
Author: Ell <ell_se yahoo com>
Date:   Fri Jul 19 19:54:49 2019 +0300

    buffer: in gegl-tile-alloc, limit number of tiles per block
    
    In gegl-tile-alloc, limit the number of tiles in a single block
    (currently, to 1024 tiles).  This sets an upper limit for block
    overhead even when using a big tile cache, and, in particular,
    avoids regularly falling back to single-tile allocations when the
    cache size is much higher than memory size (see previous commit).

 gegl/buffer/gegl-tile-alloc.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)
---
diff --git a/gegl/buffer/gegl-tile-alloc.c b/gegl/buffer/gegl-tile-alloc.c
index 38c8e824e..5745bec9c 100644
--- a/gegl/buffer/gegl-tile-alloc.c
+++ b/gegl/buffer/gegl-tile-alloc.c
@@ -41,6 +41,7 @@
 
 #define GEGL_TILE_BLOCK_BUFFER_OFFSET GEGL_ALIGN (sizeof (GeglTileBlock))
 #define GEGL_TILE_BLOCK_SIZE_RATIO    0.01
+#define GEGL_TILE_BLOCK_MAX_BUFFERS   1024
 #define GEGL_TILE_BLOCKS_PER_TRIM     10
 #define GEGL_TILE_SENTINEL_BLOCK      ((GeglTileBlock *) ~(guintptr) 0)
 
@@ -141,6 +142,7 @@ gegl_tile_block_new (GeglTileBlock * volatile *block_ptr,
   GeglTileBuffer **next_buffer;
   gsize            block_size;
   gsize            buffer_size;
+  gsize            n_buffers;
   gint             n_blocks;
   gint             i;
 
@@ -150,9 +152,14 @@ gegl_tile_block_new (GeglTileBlock * volatile *block_ptr,
                        GEGL_TILE_BLOCK_SIZE_RATIO);
   block_size -= block_size % buffer_size;
 
-  if (block_size <= buffer_size)
+  n_buffers = block_size / buffer_size;
+  n_buffers = MIN (n_buffers, GEGL_TILE_BLOCK_MAX_BUFFERS);
+
+  if (n_buffers <= 1)
     return NULL;
 
+  block_size = n_buffers * buffer_size;
+
   block = gegl_try_malloc (GEGL_TILE_BLOCK_BUFFER_OFFSET + block_size);
 
   if (! block)
@@ -170,7 +177,7 @@ gegl_tile_block_new (GeglTileBlock * volatile *block_ptr,
 
   buffer = block->head;
 
-  for (i = block_size / buffer_size; i; i--)
+  for (i = n_buffers; i; i--)
     {
       buffer->block = block;