alt Hacker News“ Each PIM block only has fast access to its locally attached DRAM bank. All other input data has to be brought in through the DRAM chip’s comparatively constrained external interface. PIM blocks can’t directly exchange data with each other, so the host has to move data using regular DRAM reads and writes if one PIM block needs to use results generated by another.”
So how big are these banks? If you can’t fit the weights of a layer into one bank then presumably you lose a lot of the speed gains.
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For GEMV you lose nothing.
The reason is quite simple. You can split the matrix along both dimensions so you just tile it into 64x64 or whatever fits into the bank and just fill it up. The biggest problem is load balancing the tiles across all banks for maximum parallelism.
For GEMM I believe there is no point in doing PIM, you are better off with a GPU or NPU.
Not necessarily. If your weights have to go across 2 banks, you just have to split and transfer the input and output vectors, which are much smaller.