1============================== 2Deadline IO scheduler tunables 3============================== 4 5This little file attempts to document how the deadline io scheduler works. 6In particular, it will clarify the meaning of the exposed tunables that may be 7of interest to power users. 8 9Selecting IO schedulers 10----------------------- 11Refer to Documentation/block/switching-sched.rst for information on 12selecting an io scheduler on a per-device basis. 13 14------------------------------------------------------------------------------ 15 16read_expire (in ms) 17----------------------- 18 19The goal of the deadline io scheduler is to attempt to guarantee a start 20service time for a request. As we focus mainly on read latencies, this is 21tunable. When a read request first enters the io scheduler, it is assigned 22a deadline that is the current time + the read_expire value in units of 23milliseconds. 24 25 26write_expire (in ms) 27----------------------- 28 29Similar to read_expire mentioned above, but for writes. 30 31 32fifo_batch (number of requests) 33------------------------------------ 34 35Requests are grouped into ``batches`` of a particular data direction (read or 36write) which are serviced in increasing sector order. To limit extra seeking, 37deadline expiries are only checked between batches. fifo_batch controls the 38maximum number of requests per batch. 39 40This parameter tunes the balance between per-request latency and aggregate 41throughput. When low latency is the primary concern, smaller is better (where 42a value of 1 yields first-come first-served behaviour). Increasing fifo_batch 43generally improves throughput, at the cost of latency variation. 44 45 46writes_starved (number of dispatches) 47-------------------------------------- 48 49When we have to move requests from the io scheduler queue to the block 50device dispatch queue, we always give a preference to reads. However, we 51don't want to starve writes indefinitely either. So writes_starved controls 52how many times we give preference to reads over writes. When that has been 53done writes_starved number of times, we dispatch some writes based on the 54same criteria as reads. 55 56 57front_merges (bool) 58---------------------- 59 60Sometimes it happens that a request enters the io scheduler that is contiguous 61with a request that is already on the queue. Either it fits in the back of that 62request, or it fits at the front. That is called either a back merge candidate 63or a front merge candidate. Due to the way files are typically laid out, 64back merges are much more common than front merges. For some work loads, you 65may even know that it is a waste of time to spend any time attempting to 66front merge requests. Setting front_merges to 0 disables this functionality. 67Front merges may still occur due to the cached last_merge hint, but since 68that comes at basically 0 cost we leave that on. We simply disable the 69rbtree front sector lookup when the io scheduler merge function is called. 70 71 72Nov 11 2002, Jens Axboe <jens.axboe@oracle.com> 73