ptrendx commented on issue #9774: mx.io.ImageRecordIter does not respect dtype argument URL: https://github.com/apache/incubator-mxnet/issues/9774#issuecomment-365484238 Engine does not seem to differentiate between first layer and subsequent layers on that it considers data going into first layer as being modified by the backward pass of the network (even though it does not actually happen). This means that copying of the next batch has to wait to the end of backward, which effectively exposes the copy. Having this double-buffering scheme of either cast fp32->fp16 or just identity fp32->fp32 makes sure that the ndarray used to copy the next batch to the gpu is returned from engine before backward pass ends, which enables copy to happen while backward computation takes place.
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