Why is the constant memory size limited in CUDA?

Question

According to "CUDA C Programming Guide", a constant memory access benefits only if a multiprocessor constant cache is hit (Section 5.3.2.4)¹. Otherwise there can be even more memory requests for a half-warp than in case of the coalesced global memory read. So why the constant memory size is limited to 64 KB?

One more question in order not to ask twice. As far as I understand, in the Fermi architecture the texture cache is combined with the L2 cache. Does texture usage still make sense or the global memory reads are cached in the same manner?

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¹Constant Memory (Section 5.3.2.4)

The constant memory space resides in device memory and is cached in the constant cache mentioned in Sections F.3.1 and F.4.1.

For devices of compute capability 1.x, a constant memory request for a warp is first split into two requests, one for each half-warp, that are issued independently.

A request is then split into as many separate requests as there are different memory addresses in the initial request, decreasing throughput by a factor equal to the number of separate requests.

The resulting requests are then serviced at the throughput of the constant cache in case of a cache hit, or at the throughput of device memory otherwise.

Answer 1

The constant memory size is 64 KB for compute capability 1.0-3.0 devices. The cache working set is only 8KB (see the CUDA Programming Guide v4.2 Table F-2).

Constant memory is used by the driver, compiler, and variables declared __device__ __constant__. The driver uses constant memory to communicate parameters, texture bindings, etc. The compiler uses constants in many of the instructions (see disassembly).

Variables placed in constant memory can be read and written using the host runtime functions cudaMemcpyToSymbol() and cudaMemcpyFromSymbol() (see the CUDA Programming Guide v4.2 section B.2.2). Constant memory is in device memory but is accessed through the constant cache.

On Fermi texture, constant, L1 and I-Cache are all level 1 caches in or around each SM. All level 1 caches access device memory through the L2 cache.

The 64 KB constant limit is per CUmodule which is a CUDA compilation unit. The concept of CUmodule is hidden under the CUDA runtime but accessible by the CUDA Driver API.