1Deadline IO scheduler tunables 2============================== 3 4This little file attempts to document how the deadline io scheduler works. 5In particular, it will clarify the meaning of the exposed tunables that may be 6of interest to power users. 7 8Each io queue has a set of io scheduler tunables associated with it. These 9tunables control how the io scheduler works. You can find these entries 10in: 11 12/sys/block/<device>/queue/iosched 13 14assuming that you have sysfs mounted on /sys. If you don't have sysfs mounted, 15you can do so by typing: 16 17# mount none /sys -t sysfs 18 19 20******************************************************************************** 21 22 23read_expire (in ms) 24----------- 25 26The goal of the deadline io scheduler is to attempt to guarantee a start 27service time for a request. As we focus mainly on read latencies, this is 28tunable. When a read request first enters the io scheduler, it is assigned 29a deadline that is the current time + the read_expire value in units of 30milliseconds. 31 32 33write_expire (in ms) 34----------- 35 36Similar to read_expire mentioned above, but for writes. 37 38 39fifo_batch 40---------- 41 42When a read request expires its deadline, we must move some requests from 43the sorted io scheduler list to the block device dispatch queue. fifo_batch 44controls how many requests we move, based on the cost of each request. A 45request is either qualified as a seek or a stream. The io scheduler knows 46the last request that was serviced by the drive (or will be serviced right 47before this one). See seek_cost and stream_unit. 48 49 50write_starved (number of dispatches) 51------------- 52 53When we have to move requests from the io scheduler queue to the block 54device dispatch queue, we always give a preference to reads. However, we 55don't want to starve writes indefinitely either. So writes_starved controls 56how many times we give preference to reads over writes. When that has been 57done writes_starved number of times, we dispatch some writes based on the 58same criteria as reads. 59 60 61front_merges (bool) 62------------ 63 64Sometimes it happens that a request enters the io scheduler that is contigious 65with a request that is already on the queue. Either it fits in the back of that 66request, or it fits at the front. That is called either a back merge candidate 67or a front merge candidate. Due to the way files are typically laid out, 68back merges are much more common than front merges. For some work loads, you 69may even know that it is a waste of time to spend any time attempting to 70front merge requests. Setting front_merges to 0 disables this functionality. 71Front merges may still occur due to the cached last_merge hint, but since 72that comes at basically 0 cost we leave that on. We simply disable the 73rbtree front sector lookup when the io scheduler merge function is called. 74 75 76Nov 11 2002, Jens Axboe <axboe@suse.de> 77