# How is the loss computed from the batch and round?

Posted 6 days ago
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 Hello,I am trying to just reproduce the loss computed during training a very simple network. In this case the loss is just the standard L2 loss function. I also attached the complete notebook.Start with some data to learn a sin function: trainingData = Table[x -> Sin[x], {x, 0, 2 Pi, 2 Pi/100.0}]; Next, create a very simple network: chain = NetChain[{10, Tanh, 1}]; Normally, I would train it just like this: result = NetTrain[chain, trainingData, All But to reproduce the loss, I need to know what the batch data was used at each iteration. So let's also save that: lastBatchIn = {}; lastBatchOut = {}; lastBatchLossList = None; appendBatch = Block[{}, (* Save the last two batch data *) AppendTo[lastBatchIn, Normal[#BatchData["Input"]]]; AppendTo[lastBatchOut, Normal[#BatchData["Output"]]]; If[Length[lastBatchIn] > 2, lastBatchIn = lastBatchIn[[-2 ;;]]; lastBatchOut = lastBatchOut[[-2 ;;]]; ]; (* Save the last two losses for these batches *) lastBatchLossList = #BatchLossList[[-2 ;;]]; ] &; result = NetTrain[chain, trainingData, All, TrainingProgressFunction -> appendBatch] Here I have create a function to save the #BatchData and the #BatchLossList for the last two examples. The factor 2 comes from the fact that I see it is using 2 batches for each round of training.Now to the question: The loss of the last round is reported as this: Print["Last round loss reported: ", result["RoundLoss"]] (* Last round loss reported: 6.05732*10^-6 *)  I can reproduce this from the stored loss list for each batch. Since every round contains two batches, I average the two: Print["Mean round loss recalculated: ", Mean[lastBatchLossList], " from loss of the last 2 batches: ", lastBatchLossList]; (* Mean round loss recalculated: 6.05732*10^-6 from loss of the last 2 batches: {4.61266*10^-6,7.50198*10^-6} *)  Great! Now I want to recompute it by feeding the actual batch data into the network: trained = result["TrainedNet"]; mb1 = Mean[(trained[lastBatchIn[[1]]] - lastBatchOut[[1]])^2]; mb2 = Mean[(trained[lastBatchIn[[2]]] - lastBatchOut[[2]])^2]; Print["Mean round loss recomputed: ", 0.5*(mb1 + mb2), " from last 2 batches: ", mb1, " ", mb2]; (* Mean round loss recomputed: 5.77591*10^-6 from last 2 batches: 5.35384*10^-6 6.19799*10^-6 *)  It's definitely not the same. It may be close, but I want to figure out how to get the exact same result. How can I reproduce the loss exactly with the trained network?Thanks! Attachments:
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