CODEWITHSUNDEEP
cachingopencl
pro_newbie
pro_newbie

Reputation: 21

How does cache affect while a same kernel is being launched repeatedly

I recently start learning OpenCL and have a question about interaction between cache and kernel in OpenCL. I am writing a program to measure a latency for accessing main memory.(bypassing caches) Therefore, I am wondering whether cache memory is cleared automatically after a kernel execution is finished or it will be remained and be used while the same kernel is executed repeatedly? Thanks!

Upvotes: 1

Views: 158

Answers (2)

Melissa P
Melissa P

Reputation: 141

For AMD Radeon GCNs, L1 and L2 cache is persistent between all kernels and all different kernels. A kernel can use cached data from any other kernel. Additionally, Local Memory inside a Compute Unit is not cleared/zeroed between kernel runs (more precisely, between work-group runs). This means you have to initialize local variables. The same should apply for nVidia/CUDA devices and generic SIMD CPUs.

That being said, OpenCL does not know or define different level of caches, caches are vendor specific. Any functionality that handles or manages caching is a vendor specific extension.

To test latency, use a pseudo-random number generator in your kernel, and read random memory addresses. Use 2 kernels, the 1st one pollutes all caches, the 2nd one then does the actual latency measuring.

Upvotes: 1

Leos313
Leos313

Reputation: 5627

in OpenCL memory hierarchy there are NO "caches" (in the sense of CPU). In OpenCL there are different kind of memories that you can controll with some instructions. Here you can have a look on what I mean:

enter image description here

The fastest memories are the Private memory and the Local Memory. You can declare Variables in this memory space and controll, moving them in the way that you prefer. You should be careful because in the Local memory you can share data among "Block" and data inside the Privite is visible only by the Thread. Here you can find a lot of other informations. So if you run repeatedly a kernel you can store your variables in the memory that you prefer and you will notice that if the variables are in the privite mamory you will be realy fast in comparison with the other solutions.

Upvotes: 0

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