Comment by joshhart
6 days ago
A single node with GPUs has a lot of FLOPs and very high memory bandwidth. When only processing a few requests at a time, the GPUs are mostly waiting on the model weights to stream from the GPU ram to the processing units. When batching requests together, they can stream a group of weights and score many requests in parallel with that group of weights. That allows them to have great efficiency.
Some of the other main tricks - compress the model to 8 bit floating point formats or even lower. This reduces the amount of data that has to stream to the compute unit, also newer GPUs can do math in 8-bit or 4-bit floating point. Mixture of expert models are another trick where for a given token, a router in the model decides which subset of the parameters are used so not all weights have to be streamed. Another one is speculative decoding, which uses a smaller model to generate many possible tokens in the future and, in parallel, checks whether some of those matched what the full model would have produced.
Add all of these up and you get efficiency! Source - was director of the inference team at Databricks
How is speculative decoding helpful if you still have to run the full model against which you check the results?
So the inference speed at low to medium usage is memory bandwidth bound, not compute bound. By “forecasting” into the future you do not increase the memory bandwidth pressure much but you use more compute. The compute is checking each potential token in parallel for several tokens forward. That compute is essentially free though because it’s not the limiting resource. Hope this makes sense, tried to keep it simple.