AMD Radeon HD5870 memory capacity and performance

Memory capacity and performance

AMD is very clear about the memory capacity and performance details in their OpenCL programming guide. The figure below showcases these hardware characteristics of the Radeon HD5870:

OpenCL Memory Type	Hardware Resource	Size/CU	Size/GPU	Peak Read Bandwidth / Stream Core
Private	GPRs	256KB	5MB	48 bytes/cycle
Local	LDS	32KB	640KB	8 bytes/cycle
Constant	Direct-addressed constant	-	48KB	16 bytes/cycle
	Same-indexed constant	-	-	4 bytes/cycle
	Varying-indexed constant	-	-	~0.6 bytes/cycle
Images	L1 Cache	8KB	160KB	4 bytes/cycle
	L2 Cache	-	512KB	~1.6 bytes/cycle
Global	Global Memory	-	1GB	~0.6 bytes/cycle

GPRs – General Purpose Registers
LDS – Local Data Store
Direct-addressed constant – a constant accessed using a constant address.
Same-indexed constant – a varying-indexed constant where each processing element accesses the same index.
Varying-indexed constant – a varying-indexed constant where the processing elements access different indices.

Of course, consider this data for fetches that are properly aligned. In case of unaligned data access the actual throughput can be much lower. In order to be able to reach the peak bandwidth we have to align our data usually to multiples of 4, 8 or 16 bytes (depending on actual hardware).

As it can be seen, constant storage can also fall into three different access performance categories so do buffers and images. While actual numbers differ on various platforms, the guidelines apply to most of modern GPUs: use a particular addressing method wisely and take in consideration access locality in order to get optimum performance.