Question about SSP gem5 implementation

[K16DIABLO]

Hello,

I'm testing few kernels on this framework and I have questions about your design.

1. It seems StreamingEngine issues too many memory requests per cycle.

• src/cpu/gem_forge/accelerator/stream/stream_engine.cc:
  StreamEngine::issueElements() module issues memory requests in for all readyElements (e.g., 16), however, issuing that many requests are impossible.

2. When loading data from FIFO, checkValueReady is not correct

• src/cpu/gem_forge/accelerator/stream/stream_engine.cc:
  StreamElement.isValueReady is used to check whether value is loaded to FIFO, and this state is marked True after memory response. This model only shows the ready state of first value on FIFO. In my opinion, this model should check the valid of each FIFO entry.

[seanzw]

Hi Sungjun,

Thanks for checking out the code. I hope I can answer your questions:

1. Indeed the current implementation of StreamEngine issues all ready elements. But these requests are regulated by the GemForgeDcachePortImpl to ensure that each cycle only a limited number of memory requests can go through (if I remember correctly). A possible fix could add a break in the StreamEngine to check if it issues too many requests and break out loop.
2. I am not sure I understood your question here. The element is marked value ready after it received all the memory responses in tryMarkValueReady. Can you elaborate more here?

[K16DIABLO]

1. Thank you for the comment. I'll try to fix.
2. Once the memory response received, handlePacketResponse -> setValue -> tryMarkValueReady marks the isValueReady true. If I understood the implementation correctly, isValueReady indicates that pseudo register has a valid value. Once the pseudo register is stepped (or consumed by stream.load), isValueReady should be marked as false, but I cannot find when does isValueReady marked as false. Without marking isValueReady false after data consumed, execution pipeline may consume invalid value.

[seanzw]

I see -- after the pseudo register is stepped, it will pointed to next StreamElement, which essentially has its own isValueReady. I hope that answers your question. Let me know if you have more questions.