Fix T93008: Cycles: Huge memory spike when saving tile to disk

The root of the problem lies in bug in OIIO which we can work around
from our side (which does not affect pack memory usage).

Thanks Brecht for finding the root cause!

Differential Revision: https://developer.blender.org/D13186

intern/cycles/session/tile.cpp

@@ -29,6 +29,7 @@
 #include "util/path.h"
 #include "util/string.h"
 #include "util/system.h"
+#include "util/time.h"
 #include "util/types.h"
 
 CCL_NAMESPACE_BEGIN
@@ -503,9 +504,9 @@ bool TileManager::write_tile(const RenderBuffers &tile_buffers)
     }
   }
 
-  DCHECK_EQ(tile_buffers.params.pass_stride, buffer_params_.pass_stride);
+  const double time_start = time_dt();
 
-  vector<float> pixel_storage;
+  DCHECK_EQ(tile_buffers.params.pass_stride, buffer_params_.pass_stride);
 
   const BufferParams &tile_params = tile_buffers.params;
 
@@ -515,13 +516,32 @@ bool TileManager::write_tile(const RenderBuffers &tile_buffers)
   const int64_t pass_stride = tile_params.pass_stride;
   const int64_t tile_row_stride = tile_params.width * pass_stride;
 
-  const int64_t xstride = pass_stride * sizeof(float);
-  const int64_t ystride = xstride * tile_params.width;
-  const int64_t zstride = ystride * tile_params.height;
-
+  vector<float> pixel_storage;
   const float *pixels = tile_buffers.buffer.data() + tile_params.window_x * pass_stride +
                         tile_params.window_y * tile_row_stride;
 
+  /* If there is an overscan used for the tile copy pixels into single continuous block of memory
+   * without any "gaps".
+   * This is a workaround for bug in OIIO (https://github.com/OpenImageIO/oiio/pull/3176).
+   * Our task reference: T93008. */
+  if (tile_params.window_x || tile_params.window_y ||
+      tile_params.window_width != tile_params.width ||
+      tile_params.window_height != tile_params.height) {
+    pixel_storage.resize(pass_stride * tile_params.window_width * tile_params.window_height);
+    float *pixels_continuous = pixel_storage.data();
+
+    const int64_t pixels_row_stride = pass_stride * tile_params.width;
+    const int64_t pixels_continuous_row_stride = pass_stride * tile_params.window_width;
+
+    for (int i = 0; i < tile_params.window_height; ++i) {
+      memcpy(pixels_continuous, pixels, sizeof(float) * pixels_continuous_row_stride);
+      pixels += pixels_row_stride;
+      pixels_continuous += pixels_continuous_row_stride;
+    }
+
+    pixels = pixel_storage.data();
+  }
+
   VLOG(3) << "Write tile at " << tile_x << ", " << tile_y;
 
   /* The image tile sizes in the OpenEXR file are different from the size of our big tiles. The
@@ -531,6 +551,11 @@ bool TileManager::write_tile(const RenderBuffers &tile_buffers)
    *
    * The only thing we have to ensure is that the tile_x and tile_y are a multiple of the
    * image tile size, which happens in compute_render_tile_size. */
+
+  const int64_t xstride = pass_stride * sizeof(float);
+  const int64_t ystride = xstride * tile_params.window_width;
+  const int64_t zstride = ystride * tile_params.window_height;
+
   if (!write_state_.tile_out->write_tiles(tile_x,
                                           tile_x + tile_params.window_width,
                                           tile_y,
@@ -548,6 +573,8 @@ bool TileManager::write_tile(const RenderBuffers &tile_buffers)
 
   ++write_state_.num_tiles_written;
 
+  VLOG(3) << "Tile written in " << time_dt() - time_start << " seconds.";
+
   return true;
 }
 
@@ -589,6 +616,9 @@ void TileManager::finish_write_tiles()
     full_buffer_written_cb(write_state_.filename);
   }
 
+  VLOG(3) << "Tile file size is "
+          << string_human_readable_number(path_file_size(write_state_.filename)) << " bytes.";
+
   /* Advance the counter upon explicit finish of the file.
    * Makes it possible to re-use tile manager for another scene, and avoids unnecessary increments
    * of the tile-file-within-session index. */