What is coherent memory on GPU?

Question

I have stumbled not once into a term \"non coherent\" and \"coherent\" memory in the

tech papers related to graphics programming.I have been searching for a simple and

Answer 1

If memory is coherent then all threads accessing that memory must agree on the state of the memory at all times, e.g.: if thread 0 reads memory location A and thread 1 reads the same location at the same time, both threads should always read the same value.

But if memory is not coherent then threads A and B might read back different values. Thread 0 could think that location A contains a 1, while thread thinks that that location contains a 2. The different threads would have an incoherent view of the memory.

Coherence is hard to achieve with a high number of cores. Often every core must be aware of memory accesses from all other cores. So if you have 4 cores in a quad core CPU, coherence is not that hard to achieve as every core must be informed about the memory accesses addresses of 3 other cores, but in a GPU with 16 cores, every core must be made aware of the memory accesses by 15 other cores. The cores exchange data about the content of their cache using so called "cache coherence protocols".

This is why GPUs often only support limited forms of coherency. If some memory locations are read only or are only accessed by a single thread, then no coherence is required. If caches are small and coherence is not always required but only at specific instructions of the program, then it is possible to achieve correct behavior of the program using cache flushes before or after specific memory accesses.

If your hardware offers both coherent and non-coherent memory types, then you can expect that non-coherent memory will be faster, but if you try to run parallel algorithms using this memory they will fail in really weird ways.