アプリケーションから発行されたI/OはカーネルのI/Oスケジューラでマージ（まとめる）やソート（並び替え）が行われるというのは本で読んで知っていたが、ソースコードは見たことがなかったのでちょっとだけ覗いてみた。以下は deadline の個所
- block/deadline-iosched.c
/* * See Documentation/block/deadline-iosched.txt */ static const int read_expire = HZ / 2; /* max time before a read is submitted. */ static const int write_expire = 5 * HZ; /* ditto for writes, these limits are SOFT! */ static const int writes_starved = 2; /* max times reads can starve a write */ static const int fifo_batch = 16; /* # of sequential requests treated as one by the above parameters. For throughput. */
- Documentation/block/deadline-iosched.txt
fifo_batch (number of requests) ---------- Requests are grouped into ``batches'' of a particular data direction (read or write) which are serviced in increasing sector order. To limit extra seeking, deadline expiries are only checked between batches. fifo_batch controls the maximum number of requests per batch. This parameter tunes the balance between per-request latency and aggregate throughput. When low latency is the primary concern, smaller is better (where a value of 1 yields first-come first-served behaviour). Increasing fifo_batch generally improves throughput, at the cost of latency variation.
