Changes between Version 5 and Version 6 of rfc26_blockcache

Timestamp:

Jun 4, 2015, 7:01:10 AM (9 years ago)

Author:

Even Rouault

Comment:

rephrase

rfc26_blockcache

@@ -14 +14 @@
 GDALRasterBand::InitBlockInfo, as raised in #3224
 
-For example, a band of a dataset at level 21 with a GoogleMaps tiling requires 2097152x2097152 tiles of 256x256 pixels. This means that GDAL will try to allocate an array of 32768x32768 = 1 billion elements (32768 = 2097152 / 64). The size of this array is 4 GB on a 32-bit build, so it cannot be allocated at all. And it is 8 GB on a 64-bit build (even if the size is generally only virtual memory but not actually allocated as physical pages of memory due to over-commit mechanism of the operating system). At dataset closing, this means that those 1 billion cells will have to be explored to discover remaining cached blocks. In reality, all above figures must be multiplied by 3 for a RGB (or 4 for a RGBA) dataset.
+For example, a band of a dataset at level 21 with a GoogleMaps tiling requires 2097152x2097152 tiles of 256x256 pixels. This means that GDAL will try to allocate an array of 32768x32768 = 1 billion elements (32768 = 2097152 / 64). The size of this array is 4 GB on a 32-bit build, so it cannot be allocated at all. And it is 8 GB on a 64-bit build (even if this is generally only virtual memory reservation but not actually allocation of physical pages of memory, due to over-commit mechanism of the operating system). At dataset closing, this means that those 1 billion cells will have to be explored to discover remaining cached blocks. In reality, all above figures must be multiplied by 3 for a RGB (or 4 for a RGBA) dataset.
 
 In the hash set implementation, memory allocation depends directly on the number of cached blocks. Typically with the default GDAL_CACHEMAX size of 40 MB, only 640 blocks of 256x256 pixels can be simultaneously cached (for all datasets).