Faster decoding for subword level Phrase-based SMT between related languages
Table 2: Translation accuracy and Relative decoding time for orthographic syllable level translation using different decoding methods and parameters. Relative decoding time is indicated as a multiple of word-level decoding time. The following methods & parameters in Moses have been experimented with: (i) normal stack decoding - vary ss: stack-size, tt: table-limit; (ii) cube pruning: vary pl:cube-pruning-pop-limit. +tuning indicates that the decoder settings mentioned on previous row were used for tuning too. Translation accuracy and decode time per sentence for word-level decoding (in milliseconds) is shown on the last line for comparison.
['[EMPTY]', '[BOLD] Translation Accuracy  [BOLD] ben-hin', '[BOLD] Translation Accuracy  [BOLD] hin-mal', '[BOLD] Translation Accuracy  [BOLD] mal-hin', '[BOLD] Translation Accuracy  [BOLD] tel-mal', '[BOLD] Relative Decoding Time  [BOLD] ben-hin', '[BOLD] Relative Decoding Time  [BOLD] hin-mal', '[BOLD] Relative Decoding Time  [BOLD] mal-hin', '[BOLD] Relative Decoding Time  [BOLD] tel-mal']
For normal stack decoding, two decoder parameters which impact the decode time are: (1) beam size of the hypothesis stack, and (2) table-limit: the number of translation options for each source phrase considered by the decoder. Since the vocabulary of the OS-level model is far less than that of the word-level model, we hypothesize that lower values for these parameters can reduce the decoding time without significantly affecting the accuracy. We can see that with a beam size of 10, the decoding time is now about 9 times that of word-level decoding. This is a 7x improvement in decoding time over the default parameters, while the translation accuracy drops by less than 1%. If a beam size of 10 is used while decoding too, the drop in translation accuracy is larger (2.5%). Using this beam size during decoding also slightly reduces the translation accuracy. On the other hand, reducing the table-limit significantly reduces the translation accuracy, while resulting in lesser gains in decoding time.