Tokenization is an important text preprocessing step to prepare input tokens
for deep language models. WordPiece and BPE are de facto methods employed by
important models, such as BERT and GPT. However, the impact of tokenization can
be different for morphologically rich languages, such as Turkic languages,
where many words can be generated by adding prefixes and suffixes. We compare
five tokenizers at different granularity levels, i.e. their outputs vary from
smallest pieces of characters to the surface form of words, including a
Morphological-level tokenizer. We train these tokenizers and pretrain
medium-sized language models using RoBERTa pretraining procedure on the Turkish
split of the OSCAR corpus. We then fine-tune our models on six downstream
tasks. Our experiments, supported by statistical tests, reveal that
Morphological-level tokenizer has challenging performance with de facto
tokenizers. Furthermore, we find that increasing the vocabulary size improves
the performance of Morphological and Word-level tokenizers more than that of de
facto tokenizers. The ratio of the number of vocabulary parameters to the total
number of model parameters can be empirically chosen as 20% for de facto
tokenizers and 40% for other tokenizers to obtain a reasonable trade-off
between model size and performance.

本文研究了在土耳其语 (OSCAR corpus) 的分裂数据上，比较了不同粒度级别的分词器的性能和预训练语言模型的效果，并发现单独定制的分子级别分词器具有挑战性的表现，同时也发现增加词汇量可以提高单独定制的分子级别分词器以及使用 RoBERTa 预训练的中型语言模型的性能。