When should texture memory be prefered over consta

Question

Does the use of data storage in constant memory provides any benefit over texture in the Pascal architecture if the data request frequency is very high among threads (every thread pick at least one data from a specific column)?

EDIT: This is a split version of this question to improve community searching

Answer 1

If the expectations for constant memory usage are satisfied, the use of constant memory is a good idea in the general case. It is allowing your code to take advantage of an additional cache mechanism provided by the GPU hardware, and in so doing putting less pressure on the usage of texture by other parts of your code.

Since the constant memory and its cache, as the texture and surface memory and it is own cache are defined by the hardware Compute Capability, the target hardware should be accounted. Thus the option by constant memory and texture memory is dependent of the access pattern and the cache use, as the cache availability.

The constant memory performance is related to data broadcast among threads in a warp, so the maximum performance is achieved if all threads request the very same data address and the data is already on the cache. Thus, if in the same warp there are request to multiple address, the service is splitted in multiple requests, since it can retrive a single address per operation. If the number of splitted requests due to data retrieval from multiple addresses is too high, the texture and surface memory performance may superior over constant memory in this specific situation.. This information is detailed in the Cuda Programming Guide:

The constant memory space resides in device memory and is cached in the constant cache mentioned in Compute Capability 2.x.

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.

The texture memory cache is more flexible than constant memory cache. It can take advantage of readings in the same warp of address that are close together in a 2D fashion. Despite of some advantages over constant memory, in general, the texture memory should be used if the data access pattern or the data size does not follow the constant memory requirements or to make use of texture memory cache. More detailed information can be found at:

The texture and surface memory spaces reside in device memory and are cached in texture cache, so a texture fetch or surface read costs one memory read from device memory only on a cache miss, otherwise it just costs one read from texture cache. The texture cache is optimized for 2D spatial locality, so threads of the same warp that read texture or surface addresses that are close together in 2D will achieve best performance. Also, it is designed for streaming fetches with a constant latency; a cache hit reduces DRAM bandwidth demand but not fetch latency.

Reading device memory through texture or surface fetching present some benefits that can make it an advantageous alternative to reading device memory from global or constant memory:

• If the memory reads do not follow the access patterns that global or constant memory reads must follow to get good performance, higher bandwidth can be achieved providing that there is locality in the texture fetches or surface reads;
• Addressing calculations are performed outside the kernel by dedicated units;
• Packed data may be broadcast to separate variables in a single operation;
• 8-bit and 16-bit integer input data may be optionally converted to 32 bit floating-point values in the range [0.0, 1.0] or [-1.0, 1.0] (see Texture Memory).

The developer should keep in mind that exploiting of the combination of texture memory with constant memory can be a real advantage over the preference for a single one, because it may allow to take advantage of the dedicated cache from both, since both caches have higher performance than over any data retrieved outside the cache (i.e. device memory).